Zhang2018 PDF
Zhang2018 PDF
Zhang2018 PDF
Shi-Bao Zhang, Ying-Jie Yang, Jia-Wei Li, Jiao Qin, Wei Zhang, Wei Huang, Hong Hu
PII: S2468-2659(18)30055-6
DOI: 10.1016/j.pld.2018.06.003
Reference: PLD 111
Please cite this article as: Zhang, S.-B., Yang, Y.-J., Li, J.-W., Qin, J., Zhang, W., Huang, W., Hu, H.,
Physiological Diversity of Orchids, Plant Diversity (2018), doi: 10.1016/j.pld.2018.06.003.
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6 University of Chinese Academy of Sciences, Beijing 100049, China
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8 Corresponding author.
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9 E-mail address: sbzhang@mail.kib.ac.cn
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11 These two authors contributed equally to this paper.
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13 ABSTRACT
14 The Orchidaceae is a diverse and wide spread family of flowering plants that are of great
15 value in ornamental, medical, conservation, and evolutionary research. The broad diversity in
16 morphology, growth form, life history, and habitat mean that the members of Orchidaceae
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18 succulent leaves with thick cell walls, cuticles, and sunken stomata, whereas terrestrial
19 orchids possess rhizomes, corms, or tubers. Most orchids have a long juvenile period, slow
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20 growth rate, and low photosynthetic capacity. This reduced photosynthetic potential can be
21 largely explained by CO2 diffusional conductance and leaf internal structure. The amount of
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22 light required for plant survival depends upon nutritional mode, growth form, and habitat.
23 Most orchids can adapt to their light environments through morphological and physiological
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adjustments but are sensitive to sudden changes in irradiance. Orchids that originate from
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25 warm regions are susceptible to chilling temperatures, whereas alpine members are vulnerable
26 to high temperatures. For epiphytic orchids, rapid water uptake by the velamen radicum,
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27 water storage in their pseudobulbs and leaves, slow water loss, and Crassulacean Acid
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28 Metabolism contribute to plant-water balance and tolerance to drought stress. The presence of
29 the velamen radicum and mycorrhizal fungi may compensate for the lack of root hairs,
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30 helping with quick absorbance of nutrients from the atmosphere. Under cultivation conditions,
31 the form and concentration of nitrogen affect orchid growth and flowering. However, the
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32 limitations of nitrogen and phosphorous on epiphytic orchids in the wild, which require these
33 plants to depend on mycorrhizal fungi for nutrients throughout the entire life cycle, are not
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34 clearly understood. Because they lack endosperm, seed germination depends upon obtaining
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35 nutrients via mycorrhizal fungi. Adult plants of some autotrophic orchids also gain carbon,
36 nitrogen, phosphorus, and other elements from their mycorrhizal partners. Future studies
37 should examine the mechanisms that determine slow growth and flower induction, the
38 physiological causes of variations in flowering behavior and floral lifespan, the effects of
40 in orchid cultivation.
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42 mycorrhiza
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44 1. Introduction
45 Orchidaceae is one of the largest and most diverse families of flowering plants, with more
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46 than 28,000 accepted species spanning 763 genera (Christenhusz and Byng, 2016). Those
47 species are absent only from polar and desert regions but are particularly abundant in the wet
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48 tropics worldwide (Chase, 2005). However, many orchids are locally distributed and
49 generally rare (Waterman and Bidartondo, 2008). China, with its small tropical area and large
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50 desert region, has relatively few orchids (Luo et al., 2003). The most recent common ancestor
51 of extant orchids lived in the Late Cretaceous, and the dramatic radiation of orchids began
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shortly after the mass extinctions at the Cretaceous-Tertiary boundary (65.5 Myr ago)
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53 (Ramírez et al., 2007). Orchidaceae appears to have undergone one significant acceleration of
54 net species diversification in the orchidoids, and two accelerations in the upper epidendroids
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55 (Givnish et al., 2015). This rapid speciation and high species diversity is likely linked to the
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58 2004; Silvera et al., 2009; Givnish et al., 2015). Due to the important ecological and
59 evolutionary significance of orchids, research has long been conducted on their pollination
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60 biology and associations with mycorrhizal fungi (Waterman and Bidartondo, 2008; Fay and
61 Chase, 2009).
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62 As fascinating and highly popular plants, orchids are valued for their exquisite flowers and
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63 long floral lifespan. These plants exhibit great diversity in floral form, size, color, fragrance,
64 and texture (Fig. 1). Commercial production has greatly expanded and become a very
66 Phalaenopsis, are cultivated for the enjoyment of their flowers (Hew and Yong, 2004). In fact,
67 some members of Cymbidium have been cultivated in China for more than 1000 years (Chen
68 and Luo, 2003; Luo et al., 2003). Orchids currently account for a significant share of the
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69 world's flower trade, with annual sales of more than $4 billion (U.S.). Some plants are also
70 used as food and traditional medicine in many countries (Arditti, 1992). For example, the
71 dried seed pods of vanilla (especially Vanilla planifolia) are commercially important as a
72 flavoring in baking, as well as for perfume manufacturing and aromatherapy (Lubinsky et al.,
73 2008). Gastrodia elata is one of three orchids listed in the earliest known Chinese Materia
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74 Medica, and is used for treating headaches, dizziness, tetanus, and epilepsy (Tsaia et al., 2011).
75 However, because they are economically valuable to floral and pharmaceutical industries, and
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76 have suffered great losses in habitat, many species are becoming rare (Luo et al., 2003; Liu et
77 al., 2015). All known orchid species are protected by the Convention on International Trade in
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78 Endangered Species of Wild Fauna and Flora (Luo et al., 2003). In addition, their life histories,
79 including interactions with mycorrhizal fungi, specialized pollinators, and host trees, make
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many orchid species particularly vulnerable to environmental changes and human disturbance
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81 (Fay and Chase, 2009). Therefore, the physiology of orchids requires further study, which is
82 important for appropriately utilizing and conserving species resources while increasing our
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83 understanding of the evolution of orchid species diversity (Hew and Yong, 2004; Zhang et al.,
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84 2015b, c).
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86 Figure 1. Flowers of nine Paphiopedilum species. a, P. charlesworthii; b, P. armeniacum; c,
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89 Orchids have complex life histories and diversified adaptation strategies; consequently,
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90 researchers have paid much more attention to orchid pollination and orchid-mycorrhizal fungi
91 interactions than to orchid physiology. Here, we review advances in orchid biology made in
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93 extremes; as well as strategies for nutrient acquisition and utilization. Our objective is to
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94 summarize the main findings on orchids, which may provide guidance for future
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95 investigations.
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98 Orchids are mostly long-lived, evergreen or deciduous herbs. Some individual plants, such as
99 those of Cypripedium calceolus, can live 30 to 100 years (Kull, 1999). The pre-flowering
100 vegetative period for most species usually lasts for four to seven years, but can be even longer
101 (Kull, 1999; Wang et al., 2010). This longevity may be attributed to their inherently slow
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102 growth and reduced photosynthetic capacity (Schmidt and Zotz, 2002; Shefferson, 2006).
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104 Figure 2. Root anatomy of Dendrobium officinale. a, longitudinal section; b, cross section;
105 c, fluorescence microstructure.
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107 Orchids usually grow according to one of two patterns. For monopodial orchids, the stem
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108 emerges from a single bud, elongating and producing leaves from the apex each year.
109 Sympodial orchids develop a series of adjacent shoots that continue to grow until they bloom
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110 and are finally replaced (Arditti, 1992; Sailo et al., 2014). The orchid life form can be
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111 terrestrial, epiphytic, lithophytic, or saprophytic. The epiphytic orchids, living in tree canopies
112 or on rocks, exhibit many differences from soil-grown terrestrial orchids in their roots, stems,
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113 and leaves. For example, the roots of terrestrial orchids are frequently ground-dwelling, thick,
114 and fleshy, with a storage function. Epiphytic orchids have modified aerial roots that are
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115 sometimes more than 1 m long. They also feature a velamen that consists of dead cells (Fig.
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116 2). This velamen covers the entire root except the tip, and functions in rapidly absorbing
117 moisture and nutrients from the surrounding humid atmosphere (Benzing, 1990; Zotz and
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120 Figure 3. Leaf epidermal structures of Paphiopedilum species. a, P. malipoense; b, P.
121 micranthum; c, P. armeniacum; d, P. emersonii; e, P. hangianum; f, P. concolor; g, P.
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124 Like most monocots, orchids generally have simple leaves with parallel veins, although
125 some species in the subfamily Vanilloideae show a reticulate venation. However, epiphytic
126 orchids are characterized by thick and succulent leaves with thick cell walls, cuticles, and a
127 small substomatal chamber, and they have smaller stomata than terrestrial species (Table 1;
128 Arditti, 1992; Guan et al., 2011; Sailo et al., 2014). The stomata are slightly sunken into the
129 leaf epidermis in Paphiopedilum (Fig. 3) but extrude outside the leaf surface in Cypripedium
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130 (Guan et al., 2011). One or more internodes of the stems from some orchids (e.g., Cymbidium,
131 Cattleya, or Dendrobium) thicken to form pseudobulbs that can store nutrients and water
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134 Figure 4. Seed anatomies of eight Paphiopedilum species. A, P. malipoense; B, P.
135 armeniacum; C, P. micranthum; D, P. bellatulum; E, P. emersonii; F, P. concolor; G, P.
136 rhizomatosum; H, P. dianthum.
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139 Table 1. Differences in functional traits between epiphytic and terrestrial species within the
140 family Orchidaceae.
Functional traits Function Difference between two life forms
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Leaf
Leaf mass per unit area Water availability and Epiphytic > Terrestrial
energy exchange
Leaf thickness Water availability Epiphytic > Terrestrial
Leaf epidermal thickness Water conservation Epiphytic > Terrestrial
Degree of leaf succulence Water conservation Epiphytic > Terrestrial
Saturated water content Water conservation Epiphytic > Terrestrial
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Epidermal conductance Water loss Epiphytic < Terrestrial
Water loss rate Water balance Epiphytic > Terrestrial
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Vessel diameter Water transport Epiphytic > Terrestrial
Crassulacean acid metabolism Water utilization Occurs only in epiphytic species
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Pseudobulb
Relative water content Water conservation Epiphytic > Terrestrial
Ratio of leaf area to pseudobulb Water balance Epiphytic > Terrestrial
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dry weight
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Area of water storage cell Water storage Epiphytic > Terrestrial
Root
Velamen radicum Water and nutrient Very common in epiphytic orchids
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142 Although orchid flowers are bilaterally symmetric, the inferior ovary or pedicel usually
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143 rotates 180 degrees so that the labellum goes on the lower part of the flower to form a
144 platform for pollinators. In Paphiopedilum, the shape of the lip staminode and petal, as well
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145 as the width of the petal, are phylogenetically conserved, while flower color is significantly
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146 convergent among species (Zhang et al., 2016). The seeds are numerous and extremely small
147 (Arditti, 1992), and exhibit various anatomies (Fig. 4). Compared with terrestrial species,
148 epiphytic Paphiopedilum species have larger embryos and a smaller percentage of air space.
149 Those larger embryos may ensure more successful seedling establishment while the higher
150 amount of air space in terrestrial species may increase their seed buoyancy and enable them to
151 disperse over longer distances (Zhang et al., 2015a). However, due to the lack of endosperm,
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152 most orchids are thought to begin their life cycles aided by mycorrhizal fungi that provide the
153 seeds with the nutrients necessary for germination. Thus, orchid species are
155 When plants achieve a certain size after a specific period of vegetative growth, they bloom
156 under suitable temperature and light conditions. For example, most Phalaenopsis species and
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157 hybrids must be exposed to relatively cool temperatures, i.e., < 28 ºC, to trigger elongation of
158 the spike (Lee and Lin, 1984). Cypripedium flavum, an alpine orchid, requires two years from
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159 floral bud formation to flowering. In the first year, two new buds are formed at the lateral base
160 of the two-year-old bud. They then differentiate seven to nine spires before becoming
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161 dormant in winter. In the second year, one of those younger buds develops into a floral bud
162 that then produces flowers and fruits in the third year (Weng et al., 2002). These observations
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are evidence of significant differences in life histories and flowering strategies among
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164 orchids.
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166 3. Photosynthesis
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168 Photosynthesis is the main way that many orchids acquire carbon. However, saprophytic
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169 species, which comprise a small proportion of the Orchidaceae, are myco-heterotrophic
170 (Zhang et al., 2015c). With respect to photosynthetic pathways, green plants can be divided
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171 into three groups: C3, C4, or Crassulacean Acid Metabolism (CAM). Approximately 10% of
172 all orchid species in Panama and Costa Rica belong to the CAM group, being most prevalent
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173 at low elevations and within the epiphytic clade, whereas C3 photosynthesis is the ancestral
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174 state (Silvera et al., 2009). Winter et al. (1983) have proposed that Cymbidium canaliculatum
175 and C. madidum are CAM and C3 plants, respectively, based on δ13C values. However,
176 Hocking and Anderson (1986) have reported that leaf extracts from those species show
177 substantial pyruvate phosphate dikinase (PPD) activity. This enzyme is usually absent or is
178 only slightly active in the leaves of C3 and CAM plants. The synthesis of
179 phosphoenolpyruvate through the action of PPD is considered an essential adjunct to the C4
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180 pathway. Therefore, these results seem to suggest that C4 photosynthesis occurs in those two
181 Cymbidium orchids. However, one would need a complete analysis to demonstrate this,
182 perhaps by studying the transfer of label from carbon-4 of C4 acids to carbon-1 of
184 Most orchids, especially species with thin leaves, assimilate CO2 through the C3 pathway.
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185 Those plants have fewer layers of smaller mesophyll cells and a larger number of stomata
186 than the thick-leaved species. They also have high CO2 compensation points and active
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187 glycolic acid activity, all of which are characteristics of plants with high rates of
188 photorespiration. The thick-leaved orchids usually have features typical of CAM plants, such
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189 as leaf and cell succulence, diurnal fluctuations in titratable acidity and nocturnal CO2 fixation,
190 and inverted stomatal physiology. Those genera include Vanilla, Cattleya, Thunia, Coelogyne,
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Laelia, Dendrobium, Calanthe, Bulbophyllum, Aerides, Phalaenopsis, Aranda, and Aranthera
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192 (Hew and Yong, 2004; Kerbauy et al., 2012; Sailo et al., 2014). As with other CAM plants,
193 thick-leaved orchids have four typical phases of gas exchange (Hew and Yong, 2004). For
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194 example, no net gas exchange is observed in the leaves of Aranda from 9 am to 12 noon, but
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195 CO2 uptake begins after mid-day, and the rate increases with time. Those leaves also have two
196 peaks of CO2 uptake: approximately 7 pm and 3 am. In addition, distinct regions of the same
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197 plant may utilize different photosynthetic pathways and varying degrees of CAM expression
198 depending upon water availability (Rodrigues et al., 2013). Epiphytic orchids, many of which
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199 are CAM plants, grow on rock or tree trunks in tropical and subtropical forests where water
200 deficits are frequent (Silvera et al., 2005). Thus, CAM plants can adapt to drought stress and
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202 Because the ability of plants to transport water from root to leaf (hydraulic conductivity) is
203 relatively lower in orchids than in other angiosperms, orchids utilize a variety of mechanisms
204 to reduce water losses. Compared with terrestrial orchids, epiphytic CAM orchids usually
205 grow under conditions where the volume of substrates is limited because of the scouring
206 action of frequent rainfall. Furthermore, because those regions have relatively high
207 temperatures, the potential for daytime evaporation is elevated. Stomatal closure is a very
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208 efficient strategy for minimizing water losses during the daytime. At night, the relative air
209 humidity is very high there, which may lead to a low rate of evaporation and prompts those
210 orchids to open the stomata for CO2 uptake. Photosynthetic carbon gain is optimized in some
211 orchids that are facultative CAM plants but also induces C3 photosynthesis under
212 well-watered conditions (Kerbauy et al., 2012). For example, Dendrobium officinale exhibits
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213 a typical CAM pattern when the content of substrate water is diminished (Zhang et al., 2014),
214 but those plants reveal a concomitance of C3 and CAM patterns when re-watered. A shorter
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215 light‒dark cycle leads to a C3 pattern alone. Consequently, substrate moisture and the light‒
216 dark cycle are inducible factors for switching between C3 and CAM patterns in that species
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217 (Zhang et al., 2014). Thus, the CAM pathway is an important strategy by which many
218 epiphytic orchids prevent water loss and acclimate to fluctuations in water availability.
221 This capacity can be affected by stomatal conductance (gs), mesophyll conductance (gm), and
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222 biochemical factors (Grassi and Magnani, 2005). In angiosperms, biochemical limitations
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223 tend to be the main constraint (Carriqui et al., 2015). In contrast, the photosynthetic capacities
224 of Cypripedium and Paphiopedilum species are more strongly limited by gm than by
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225 biochemical factors or gs (Yang et al., 2018). However, the three deciduous Cypripedium
226 species show significantly higher photosynthetic capacities, gs, and gm than the three
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227 evergreen Paphiopedilum species (Yang et al., 2018). Higher values for gs in Cypripedium are
228 independent of stomatal density but mainly affected by a larger stomatal apparatus area and
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229 smaller pore depths. Furthermore, the low levels of gm in Paphiopedilum are caused by much
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230 thicker cell walls and a reduced surface area for mesophyll cells and chloroplasts exposed to
231 intercellular airspace per unit of leaf area. In that genus, cell wall resistance is responsible for
232 approximately 50% of total mesophyll resistance. As wall thickness increases, the
233 contribution of cell wall resistance to total resistance also rises (Terashima et al., 2011). A
234 reduction in gm increases the resistance of CO2 conductance to the chloroplasts, causing
235 chloroplast CO2 concentrations to decline, thereby restricting CO2 assimilation (Carriqui et al.,
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238
239 Figure 5. A, Light-intensity dependence of photosynthetic electron flow through PSII
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240 (ETRII); B, cyclic electron flow around PSI (CEF); and C, non-photochemical
241 quenching in PSII (NPQ) for the leaves of four Cymbidium species.
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242
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243 The light reactions of photosynthesis convert solar energy into chemical energy in the form of
244 NADPH and ATP, which are utilized for CO2 assimilation. In photosynthesis, NADPH and
245 ATP are mainly synthesized by linear electron transport from water to NADP+. The
246 production ratio of ATP/NADP by linear electron transport is approximately 1.29 (Allen,
247 2002), which cannot satisfy the ratio of 1.5 required by the Calvin-Benson cycle. Therefore,
248 supplemental mechanisms for ATP synthesis are needed. In C3 plants, the process of
249 photorespiration can increase that ratio up to 1.6. In addition, alternative electron transport,
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250 including cyclic electron flow (CEF) around photosystem I (PSI) and the water‒water cycle,
251 contributes to this compensation of ATP synthesis. In our recent study, four Cymbidium
252 species show little CEF activation under low light (Fig. 5B), suggesting that other
253 mechanisms, such as the malate valve and the Mehler reaction, can maintain the energy
254 balance when electron flow is low. Under intense irradiance, those four species have
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255 significant CEF activity. This is especially true for C. faberi, which has the highest electron
256 flow through PSII (ETRII) (Fig. 5A). By comparison, the species with the lowest ETRII, C.
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257 lowianum, shows the least CEF activity. These results indicate that CEF is required for energy
258 balance under high PPFD. The low levels of ETRII and CEF in C. lowianum are accompanied
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259 by a high level of non-photochemical quenching in PSII (NPQ) under more intense light (Fig.
260 5C). Because NPQ activation is based on lumenal acidification, which is dependent upon
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photosynthetic electron flow and functioning of chloroplast ATP synthase, activity by that
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262 enzyme in C. lowianum is the lowest among the four species.
263 The restriction on CO2 assimilation can increase the production of reactive oxygen species
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264 (ROS) that cause photoinhibition. Under such conditions, photorespiration and CEF are
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265 important for alleviating photoinhibition, and proton motive force (pmf) must also be rapidly
266 formed to protect PSI and PSII against excess light energy (Huang et al., 2015). The
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267 generation of ∆pH eases PSII photoinhibition by activating NPQ and stabilizing the
268 oxygen-evolving complex (Huang et al., 2016). Meanwhile, lumenal acidification slows
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269 electron transfer from PSII to PSI via Cyt b6/f (Tikkanen and Aro, 2014), thereby preventing
270 the over-reduction of PSI reaction centers and diminishing the production of superoxide and
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271 singlet oxygen within the thylakoid membrane to protect PSI activity (Kanazawa et al., 2017).
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272 When the leaves of Bletilla striata are transferred from darkness to light, CEF stimulation
273 plus the low activity of chloroplastic ATP synthase contributes to rapid formation of high pmf
274 (Huang et al., 2018). During photosynthetic induction, the performance of CEF is finely
275 regulated to coordinate the activity of chloroplastic ATP synthase, optimizing photosynthesis
277
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279 In orchids, a universal pattern of light requirement exists for individual species. Because they
280 live in forests, the photosynthesis and growth of most orchids require a low level of irradiance
281 (Zhang et al., 2007; Chang et al., 2011). However, specific light requirements for each species
282 may depend on nutritional mode, life form, developmental stage, and habitat.
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283 Myco-heterotrophic orchid species are usually light-independent because they acquire
284 carbon through heterotrophic exploitation of mycorrhizal fungi rather than through
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285 photosynthesis. Even though such plants harbor a certain amount of photosynthetic pigment,
286 e.g., chlorophyll (Chl) a and xanthophylls, they are photochemically ineffective (Cameron et
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287 al., 2009). Nevertheless, for species that are partially myco-heterotrophic (PMH), their
288 reliance upon nearby fungi is governed by light availability, i.e., low levels lead to strong
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myco-heterotrophy while higher irradiance drives orchids toward autotrophy (Preiss et al.,
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290 2010). Species with different life forms also exhibit different requirements for light. For
291 example, Cymbidium tracyanum, occurring in the tree canopy as an epiphyte, is more tolerant
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292 of intense irradiance than the closely related C. sinense found on shady forest floors (Kuang
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293 and Zhang, 2015). When light intensity exceeds the amount necessary for photosynthesis, e.g.,
294 after seasonal leaf-shedding by the host tree in a tropical dry forest, epiphytic orchids in the
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295 newly exposed canopy show considerable photoprotective plasticity to cope with such stress
297 heterogeneity in light environments, orchids may display a strategy for light interception that
298 is commonly observed for plants that typically grow in low-light environments
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300 Light can inhibit the seed germination of many terrestrial, and even some epiphytic, species
301 (Rasmussen et al., 2015). Thus, in vitro germination for most orchids are conducted in the
302 dark (Huang and Hu, 2001). Germination under darkness may help avoid seedling desiccation
303 that might be a fatal consequence of exposure to naturally high light. Orchids may also have
304 different light requirements at various developmental stages after germination. For
305 Phalaenopsis hybrids, a relatively lower light intensity helps seeding whereas stronger light
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306 promotes seedling growth, with an even higher intensity recommended for the induction of
308 Orchid growth is also affected by light quality. Stomatal opening and photosynthesis by
309 Cypripedium flavum is highly induced by mixed blue and red light rather than by pure blue or
310 red light. However, because guard cell chloroplasts are lacking in the closely related
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311 Paphiopedilum species, stomatal opening is stimulated by specific blue light during
312 photosynthetic induction (Zhang et al., 2011). Red or far-red light usually promotes the
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313 vegetative growth of seedlings in flask or greenhouse cultivation, whereas blue light elevates
314 Chl production (Islam et al., 1999; Lee et al., 2017). In a natural habitat, orchids growing
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315 beneath a canopy are inescapably subjected to a reduction and alteration of light quality due
316 to reflection and selective absorption within the upper canopy. When compared with a more
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open habitat, the lower canopy generally has a higher level of green light but lower levels of
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318 both red light and its proportion to far-red light (Caldwell and Pearcy, 1994). The
319 photosynthetic apparatus of an orchid growing on a forest floor can acclimate to such light
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320 environments by various means, such as modifying levels of Chl and the ratio of Chl a to b to
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321 maintain coordination between PSII and PSI (Zhang et al., 2007).
322 Like other plants, orchids can be classified as short-day, long-day, or day-neutral (Hew and
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323 Yong, 2004). The impact of photoperiod on orchid vegetative growth is species-specific, and
324 species and hybrids within a genus may have different responses. Some orchids require a
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325 short day for flower initiation, whereas others, such as species within Cymbidium and
327 of the influence of growth temperature (Lopez and Runkle, 2005). The practice of night
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328 interruption can be used to stimulate flowering by long-day orchids and improve flower
329 quality in commercial cultivation (Kim et al., 2011). Although a relatively high light intensity
330 can increase photosynthesis during that interruption, the photosynthesis rate and PSII activity
331 in those plants during the daytime may decline due to a leaf-nitrogen deficiency. Therefore,
332 night interruption accompanied by additional fertilization is recommended (Kim et al., 2015).
333 Most orchids can adapt, in a species-specific manner, to a broad range of light
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334 environments in different habitats. In a dry forest, wide fluctuations in irradiance levels are
335 generally caused by the phenology of the individual host tree. Orchids from such a habitat
336 may demonstrate higher plasticity than those from a more humid forest (Rosa-Manzano et al.,
337 2017). Some species are capable of adjusting, via morphological and physiological changes,
338 to a wide range of environments. For example, Cypripedium guttatum, the only species in that
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339 genus occurring in both the Old and the New World, is found in both open and shady habitats
340 where irradiance can vary from 22 to 76% of full sunlight (Zhang et al., 2007). Tropical
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341 orchids that are adapted to full sunlight also do well after being subjected to 75% shading
342 (Pires et al., 2012). However, when compared with plants in other herbaceous families,
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343 orchids have a very low rate of leaf turnover, which might put them at risk if they encounter a
344 sudden change in growth irradiance because turnover rate plays an important role in light
345
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acclimation (Ishii and Ohsugi, 2011). One extreme example is Pleione aurita, an orchid that
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346 produces only one leaf per growing season. As a result, the mature leaf of P. aurita cannot
347 photosynthesize optimally under new lighting conditions, due to this structural restriction.
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348 Therefore, that sole, inefficient leaf ultimately leads to a decrease in annual carbon gain
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349 (Zhang et al., 2017). A comprehensive survey of vascular epiphytes in a lowland forest has
350 revealed that most orchid species and individual plants grow within the intermediate stratum
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351 (Zotz and Schultz, 2008), which means that they have a moderate light requirement, i.e., a
352 maximum of approximately 50% of full sunlight (Zhang et al., 2007; Zhang et al., 2017).
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353 Canopy closure caused by forest succession has an adverse impact on the reproduction of the
354 understory Cypripedium calceolus, and the practice of selective tree harvesting has been
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355 proposed to ensure that a brightly lit forest floor is available for the conservation of this rare
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356 orchid (Hurskainen et al., 2017). In contrast, epiphytic orchids on isolated trees are confronted,
357 post-logging, with a harsher microclimate characterized by more intense light and increased
358 drought conditions, and their seedling establishment is severely restricted when compared
359 with epiphytic orchids growing on trees in a closed-canopy forest (Werner and Gradstein,
360 2008). Thus, the requirements of orchids for light are highly complex.
361
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363 Plants exhibit different degrees of physiological tolerance to environmental stresses, but
364 members within the Orchidaceae can occur in habitats from tropical to temperate zones
365 (Arditti, 1992). Their growth often responds to an optimum temperature at which the rate of
366 progress toward a particular developmental event is maximal. Temperatures that exceed either
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367 end of that optimum range may have a negative effect on growth and development. For
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369 or below 15 °C (Arditti and Pridgeon, 1997).
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371 Orchids originating from tropical or sub-tropical areas tend to be sensitive to chilling stress
372 and genera such as Phalaenopsis can hardly survive in regions where severe, long-term
373
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chilling occurs naturally. Low temperatures may lead to many symptoms of stress, such as
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374 leaf-yellowing, defoliation, or a reduced rate of growth. Leaf-pitting in Phalaenopsis can be
375 induced at temperatures of 2 to 7 °C, with the amount of pitting depending upon the duration
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376 of exposure and the physiological age of the leaf tissue. At these temperatures, mature leaves
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377 are less susceptible than young leaves (Sheehan and McConnell, 1980). Anatomical studies
378 have revealed that pitting is the result of mesophyll cell collapse, which initially occurs in
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379 cells between the large vascular bundles. Severely damaged areas are characterized by
380 extensive collapse and those cells are always surrounded by hypertrophical cells (Sheehan,
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381 1983).
382 Plants may decrease their photosynthetic activities at low temperatures due to the
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383 depression of Rubisco activity and RuBP regeneration. As a result, the excess excitation
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384 energy may induce the production of a large amount of ROS, which can damage the
385 photosynthetic apparatus (Asada, 1999). Some photoprotection mechanisms are also activated
386 during periods of chilling stress, such as NPQ and CEF. Three Paphiopedilum species present
387 significant PSII photoinhibition when they are exposed to 4 °C, but their PSI activities are not
388 susceptible to combined chilling‒light stress for 8 h (Yang et al., 2017). Compared with P.
389 purpuratum, both P. armeniacum and P. micranthumare are less impaired because they have
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390 relatively higher CEF activity that alleviates PSII photoinhibition and protects PSI activity in
391 stressed leaves. Similarly, stimulation of CEF capacity is also important for easing
392 chilling-induced PSII photoinhibition in two Cymbidium species (Li and Zhang, 2016).
393 However, even the most sensitive species, P. purpuratum, is not very vulnerable to short-term
394 chilling treatment because its PSI activity remains stable. This is probably not due to CEF
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395 activation but rather to the inhibition of electron transport from PSII to PSI. The latter
396 scenario is largely responsible for preventing excess electron flow to PSI, thereby allowing
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397 the amount of active PSII to be balanced and the capacity of the PSI electron acceptors left
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399 During periods of growth and development, a trade-off usually exists among the governing
400 physiological processes. Although low temperatures may hinder plant development, a
401
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concomitant decline in the growth rate can be compensated by a longer growth period. For
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402 example, shoot growth in Cymbidium sazanami ‘Otome’ may be delayed and slowed during a
403 cold winter, causing the primary shoots to be smaller and mature later. However, the total
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404 number of primary shoots will not be affected by such conditions. Consequently, shoots can
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405 produce a similar number of leaves if the growing season is relatively long (Kako et al., 1976).
406 Furthermore, for evergreen orchids that display greater temperature homeostasis of
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407 photosynthesis, some species, such as those in Pleiones and Bletilla, employ an escape
408 strategy by up-regulating photosynthetic efficiency and fixing more carbon during the warm
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409 season, then shedding their leaves and roots, leaving a dormant pseudobulb to survive in the
412 Moderately high temperatures usually favor plant growth, but extremely high temperatures
413 may impair physiological processes. Exposure to high temperatures may cause cellular
414 membranes to weaken and ion leakage to occur, as manifested by tissue necrosis (Jones,
415 1992). For example, the optimum temperature for photosynthesis by Cypripedium flavum is
416 approximately 20 °C. When plants are transferred from their usual alpine habitat to a lower
417 elevation, their leaves exhibit decreases in rate of photosynthesis, stomatal conductance,
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418 transpiration, and carboxylation efficiency. Reduced gs values measured at that lower
419 elevation may retard the diffusion of CO2 into the leaf, which further exacerbates the
420 depression of photosynthetic capacity (Zhang et al., 2005). The optimum temperature for
421 photosynthesis is generally below 30 °C, while that for respiration occurs just below the
422 temperature at which enzymes are heat-inactivated, i.e., > 45 °C. When plants are exposed to
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423 a temperature above this photosynthetic optimum, their photosynthetic rates are depressed
424 while that of respiration continues to increase. This may lead to an imbalance between
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425 carbon-fixation and consumption, which directly affects vegetative growth and indirectly
426 influences flowering through decreases in plant size and nutrient supply (Iersel, 2003).
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427 Enhanced expression of relevant enzymes and metabolites plays an important role in
428 protecting cells against high-temperature stress (Law et al., 2001). In Phalaenopsis, the
429
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oxidative damage caused by elevated temperatures may decrease photochemical efficiency as
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430 malondialdehyde levels and lipoxygenase activity increase. Meanwhile, the activities of
432 and root; glutathione reductase in the leaf; and guaiacol peroxidase in the root are induced
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433 significantly at 40 °C when compared with 25 °C, suggesting that these enzymes have roles in
435 The optimum temperature varies among developmental processes. Some species require a
436 relatively low temperature for flower induction, and high temperature delays the development
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437 of floral buds (Sinoda et al., 1984). This temperature requirement for flower induction reflects
438 an important natural adaptation to seasonal change in the growing environment (Arditti, 1992).
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441 temperatures, many of them do not continue to develop any further. The percentage of the
442 plant that initiates inflorescences and develops opening flowers is greatest at a temperature of
443 14 °C to 17 °C (Blanchard, 1993). In Dendrobium, flower initiation only occurs when mature
444 pseudobulbs are exposed to temperatures of 7.5 °C to 20.0 °C. Decreasing temperatures
445 caused by cool rain may promote the flowering of these orchids in their natural habitats.
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446 Temperature signals usually affect floral development and morphogenesis by influencing
447 hormone levels (Arditti and Pridgeon, 1997). In Phalaenopsis hybrida, this temperature
448 regulation depends upon an optimal concentration of endogenous gibberellin in the tip of the
449 flowering shoot. Such shoots have a lower amount of the hormone when grown at 30/25 °C
450 (day/night) than at 25/20 °C (Su et al., 2001). These findings have been used to devise
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451 strategies for commercial production of orchids. For example, temperature manipulation is
452 used to control and synchronize flowering time for Cymbidium, Dendrobium, and
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453 Phalaenopsis (Chen et al., 1994; Hew and Yong, 2004). Future investigations to verify other
454 mechanisms that underlie temperature acclimation in orchids would be of great importance
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455 for the conservation and cultivation of rare species.
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459 with at least 70% of the species in this family being canopy-adapted, and approximately 2/3
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460 of all epiphytes being orchids (Benzing, 1990; Zhang et al., 2015c). The epiphytic orchids
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461 benefit from intense irradiance and relatively little competition but are confronted with
462 limited supplies of nutrients and, especially, water (Zotz and Hietz, 2001). To cope with such
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463 challenges, these orchids develop suites of anatomical and physiological adaptations to
464 improve the uptake and internal storage of water, as well as to reduce its loss (Table 1).
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466 Fast water uptake from the atmosphere is an important strategy for survival in the tree canopy.
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467 The velamen radicum is a unique dead structure on the root surfaces of most epiphytic orchids.
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468 One of its most important roles is the absorption of water (Benzing, 1990; Zotz and Winkler,
469 2013) after rainfall is captured and immobilized. It generally takes more than one hour for the
470 velamen radicum of many orchids to fill with water (Dycus and Knudson, 1957). However,
471 Zotz and Winkler (2013) have shown that this structure, when dry, can take up moisture
472 within seconds. The volume of the velamen radicum varies among species and is positively
473 linked with higher initial rates of uptake. However, because all velamina become saturated
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474 very rapidly, such differences may not be very important functionally.
476 The water stored within the organs is key to maintaining the whole-plant water balance during
477 periods of drought. For most orchids, the leaves and pseudobulbs act as those storage organs.
478 Values for leaf mass per unit area (LMA), leaf thickness, and saturated water content (SWC)
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479 are important functional traits when characterizing leaf water storage capacity. In Cymbidium,
480 the epiphytic species have greater ability than the terrestrial species to tolerate drought
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481 because they have higher LMA, leaf thickness, and SWC (Table 1; Zhang et al., 2015b).
482 Large leaf epidermal cells also contribute to water storage. In some orchids, the amount of
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483 water stored in those epidermal cells can account for up to 80% of the entire leaf volume
484 (Pridgeon and Stern, 1982). For Paphiopedilum species growing in karst habitats, the adaxial
485
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epidermis cells have significantly larger volume than the abaxial cells, and the leaves are
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486 thick, fleshy, and contain more water than plants of those species growing in other
488 The pseudobulb, an adaptively unique stem of many orchids, serves as a buffer against
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489 drought stress because of its ability to retain water (Ng and Hew, 2000). During a period of
490 drought, the presence of pseudobulbs may slow the usual reductions in leaf water content and
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491 water potential (He et al., 2013). We have found significant differences in water-related traits
492 and the physiological responses of pseudobulbs to drought between epiphytic and terrestrial
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493 orchids. Compared with terrestrial Cymbidium sinense, epiphytic C. tracyanum has larger
494 water storage cells and a higher relative water content (Table 1). Those features may
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495 contribute to the maintenance of normal physiological functioning for longer periods of time
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496 under a water deficit. As expected, C. tracyanum can quickly utilize water stored in the
497 pseudobulb when exposed to drought stress, and leafless pseudobulbs help sustain long-term
498 leaf photosynthesis (our unpublished data). Many epiphytic orchids buffer transpiration to
499 extend stomatal conductance and photosynthesis for more than 20 d when responding to a
500 soil-moisture deficit (Sinclair, 1983). For example, Dimerandra emarginata can maintain a
501 normal leaf water content for 23 d in the absence of rain (Zotz, 1999). Although D.
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502 chrysotoxum and D. officinale have thicker leaves and upper cuticles when compared with D.
503 chrysanthum and D. crystallinum, the latter two compensate for that by having higher SWCs
504 in their pseudobulbs (Yang et al., 2016). This indicates that the strategies for maintaining
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507 As part of their strategies for reducing water loss, the epiphytic species in Cymbidium have a
508 thicker epidermis and require more time to dry saturated leaves to 70% relative water content
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509 when compared with terrestrial species in that genus (Table 1). These traits make the
510 epiphytes more drought-tolerant (Zhang et al., 2015b). Physiological and proteomic analyses
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511 of an epiphytic and a terrestrial orchid have found that the former has greater ability to
512 maintain a carbon balance under water stress and it also responds more effectively to abscisic
513
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acid in the leaves (Li et al., 2018). The stomata of Paphiopedilum armeniacum are slightly
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514 sunken into the leaf epidermis; this specific structure may reflect an adaptation to periodic
516 Velamen radicum appears to be effective in reducing water loss (Zotz and Winkler, 2013).
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517 Orchids growing in drier habitats usually have thicker velamina (Sanford and Adanlawo,
518 1973). Water retention volume also increases with the size of that velamen radicum (Luttge,
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519 1989). Zotz and Winkler (2013) have reported that, for most orchids, water is retained for
520 more than 1 h in the velamen radicum. An epiphytic orchid, C. tracyanum, has a higher radio
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521 of velamen thickness to root thickness, along with larger-diameter xylem conduits than those
522 of terrestrial C. sinense (Table 1). These findings indicate that epiphytic orchids have greater
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523 capacity to conserve water and avoid the negative effects of drought based on their root
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525 In many orchid species, CAM plays a critical role in improving carbon gains and reducing
526 water losses (Kerbauy et al., 2012). In addition to CAM regulation in the leaves (described
527 above), CAM photosynthesis also occurs in the stems, fruits, and flowers of some orchids,
528 and in the aerial roots of epiphytic plants (Hew and Yong, 2004; Motomura et al., 2008;
529 Kerbauy et al., 2012; Rodrigues et al., 2013). Although pseudobulbs lack stomata, Hew et al.
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530 (1998) have discovered chlorophylls and PEPC and Rubisco activities in the pseudobulbs of
531 Oncidium goldiana. This implies that, in some cases, pseudobulbs may be capable of some
532 CAM activity (Winter et al., 1983; Hew et al., 1998; Rodrigues et al., 2013). This process
533 recycles the respiratory CO2 generated by the voluminous underlying parenchyma of
534 pseudobulbs (Ng and Hew, 2000). Pseudobulbs alone are apparently unable to assimilate
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535 carbon via CAM because the stomata are absent. When the entire shoot of Laelia anceps is
536 illuminated during the light period, the leaf assimilates carbon then as well as during the next
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537 dark period. However, when those pseudobulbs are exposed to darkness during the light
538 period, the leaf assimilates carbon only at night (Ando and Ogawa, 1987). This suggests that
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539 the pseudobulbs influence the uptake of CO2 by the leaf under both light and dark conditions.
540
543 Nutrients are important factors that control plant growth and development. For example,
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544 nitrogen-deficiency can decrease protein synthesis, growth rates, and productivity (Amâncio
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545 and Stulen, 2004). Terrestrial orchids obtain nutrients mainly from the soil, while sources for
546 epiphytic orchids can also include atmospheric dry/wet depositions, solid substrates (such as
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547 bark or litter), and nitrogen fixation by microorganisms (Benzing, 1990; Reich et al., 2003).
548 For two bromeliad species, atmospheric-nitrogen provides as much as 77 to 80% of that
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549 element to small individuals when compared with soil-derived nitrogen, which contributes 64
550 to 72% of leaf-nitrogen to large plants (Reich et al., 2003). Dischidia major derives 39% of its
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551 carbon from ant-related respiration, and 29% of its nitrogen supply comes from the debris
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553 Roots are the main organ for absorbing nutrients. Plants can regulate nutrient acquisitions
554 by altering root architecture and morphology. Increasing the root biomass, specific root length,
555 and number of fine roots can improve the absorption of nitrogen and other nutrients
556 (López-Bucio et al., 2003). Orchid roots are usually thick and succulent, produce a large
557 biomass, but have few root hairs (Hew and Yong, 2004). The presence of fungal mycelia can
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558 increase the absorbing surface area of those roots (Dearnaley and Cameron, 2017). The
559 velamen radicum may also compensate for the lack of root hairs and help epiphytic orchids
560 quickly absorb mineral nutrients from fog and rainwater (Zotz and Winkler, 2013). Elevating
561 nitrogen concentration decreases the number of root cells and the thickness of velamen
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563 The most common forms of nitrogen absorbed by plants are nitrate nitrogen (NO3-) and
564 ammonium nitrogen (NH4+). High activities by nitrate reductase (NR) and glutamine
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565 synthetase (GS) can enhance the assimilation of NO3- and NH4+. The uptake of NO3- is
566 highest at the root tip, and obviously decreases with increasing distance from the root tip due
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567 to the presence of fibrous layers in the older root tissue that can hinder the process. In contrast,
568 NH4+ can be taken up by any part of the root, but it mainly occurs at the mature zone (Colmer
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and Bloom, 1998). Epiphytic and terrestrial orchids can absorb both NO3- and NH4+, but the
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570 absorption rate for the former is higher in terrestrial orchids while that of the latter is higher in
571 epiphytic orchids. Both NR and GS are present in orchid roots and leaves, although the
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572 former enzyme is more active in the roots and less so in the leaves. The reverse is true for GS
(Hew et al., 1993; Hew and Yong, 2004). When provided with 75% or 100% NH4+, plants of
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573
574 Phalaenopsis are smaller and tend to show a decrease in the width of the top leaf and less
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575 whole-plant leaf spread as the level of NO3- declines from 100% to 0%. Spiking is delayed
576 and the spiking rate is reduced when those plants receive more than 50% NH4+. As the ratio
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577 between NO3- and NH4-rises, flowers become increasingly larger (Wang and Chang, 2017).
578 Flower buds and flowers of Cymbidium sinense form normally when plants are treated with
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579 NO3- at 1 or 10 mmol L-1, but no flower buds form regardless of the level of NH4+ treatment
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580 (Pan and Chen, 1994). These results suggest that orchids have a preference for nitrogen
581 forms.
582 Although the effects of nitrogen on orchid growth under cultivation conditions have been
583 confirmed by some studies, the demand for that nutrient is relatively low (Mou et al., 2012;
584 Wang and Chang, 2017). Within a certain range, increasing the nitrogen concentration can
585 promote vegetative growth, and increase the number of flowers produced by genera such as
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586 Phalaenopsis, Cattleya, and Dendrobium. However, flowering is delayed when the nitrogen
587 concentration is too high (Bichselet al., 2008; Wang and Chang, 2017). The demand for
588 nitrogen by plants of Paphiopedilum armeniacum is higher at the vegetative growth stage but
589 lower at the reproductive stage. When abundant nitrogen is available, this orchid is
590 propagated primarily by seeds. However, asexual reproduction dominates when nitrogen
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591 supply is limited (Mou et al., 2012). Therefore, these reports demonstrate that the effect of
592 nitrogen on orchid growth varies by species and developmental stage. Whereas reports have
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593 shown how low nutrient availability affects plants in epiphytic habitats, the conclusions on
594 how the relative limitations of nitrogen and phosphorus influence epiphytes growing in the
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595 wild remain ambiguous (Zotz and Hietz, 2001; Wanek and Zotz, 2011).
597
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levels of nitrogen compounds in the atmosphere may affect species diversity and ecosystem
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598 functioning due to acid rain, eutrophication, and direct toxic effects (Amâncio and Stulen,
599 2004). Because they are largely dependent on atmospheric sources for nutrients, epiphytes are
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600 more susceptible to such depositions, especially those of NHX compounds. The δ15N values
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601 measured from the leaves, pseudobulbs, and roots of Laelia speciosa are higher at sites
602 exposed to industrial and vehicular activities than in oak forests (Díaz-Álvarez et al., 2016).
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603 Performance by those plants is optimal at doses of up to 20 kg N ha yr-1, but toxic effects are
604 observed at doses of 40 and 80 kg N ha yr-1 (Díaz-Álvarez et al., 2015). However, few studies
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605 have examined the responses of orchids to nitrogen-deposition and it remains unclear whether
606 the continuous rise in those depositions is beneficial or harmful to epiphytic orchids.
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608 Mycorrhizal fungi play an important role in the life history of orchids. The success of seed
609 germination depends upon the nutrients supplied by fungal symbionts. At the adult stage,
610 some orchids produce green leaves and become putatively autotrophic. However, many
611 achlorophyllous species remain fully MH. Aphyllorchis and Gastrodia are the largest genera
612 of full myco-heterotrophs (Merckx, 2013). In contrast, PMH orchids, commonly terrestrial,
613 obtain nutrients via their own photosynthesis and their mycobionts, and they include many
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614 green-leaved species (Gebauer and Meyer, 2003; Preiss et al., 2010). Occasionally, however,
615 achlorophyllous MH variants are found in some PMH species, such as Epipactis and
617 Compared with autotrophic plants, the adaptation and evolution of MH and PMH plants are
618 usually accompanied by changes in morphology and genetics, including reduced leaf size and
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619 the loss of expression by photosynthesis-related genes (Barrett et al., 2014). Most MH orchids
620 grow in deeply shaded habitats and are mainly colonized by the specialized ectomycorrhizal
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621 fungi of neighboring trees (Merckx, 2013; Selosse et al., 2004). A common tripartite
622 mycorrhizal network – autrotropic trees, fungi, and MH plants – is vital to those MH species
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623 in acquiring nutrition and is critically important during the early stages of forest succession
624 and tree recruitment (Selosse et al., 2006). However, only a few studies have focused on
625
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nutrient flow back to the fungal partner from MH plants (Cameron et al., 2006). Some MH
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626 orchids associate with litter- and wood-decaying fungi, including Eulophia zollingeri and
628 The PMH plants are part of a transition away from autotrophy to MH status, usually
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629 dwelling in habitats with higher irradiance where they remain chlorophyllous (Preiss et al.,
630 2010; Merckx, 2013). These plants display various levels of specificity to their fungal hosts.
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631 Members of Tulasnellaceae and Ceratobasidiaceae are very common in the mycorrhizae of
632 PMH orchids (Hock, 2012), but mutualisms also exist between Atractiellomycetes and
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633 orchids in tropical regions (Kottke et al., 2010). As an exception, some perennial green
634 species of Cephalanthera and Epipactis that are albino specimens can survive up to 14 years.
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635 They may share similar mycobionts with the green individuals but are usually maladapted due
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636 to low fitness during the vegetative or reproductive phases (Gonneau et al., 2014).
637 The transfer of nutrients from mycorrhizal fungi to their symbiotic orchids has already been
638 demonstrated (Cameron et al., 2006; Dearnaley and Cameron, 2017). For some autotrophic
639 orchids, the adult plants still obtain carbon, nitrogen, phosphorus, and other nutrients through
640 mycorrhizal fungi (Zimmer et al., 2007). Organic matter, such as bark, can be decomposed by
642 plants have higher levels of δ15N and nitrogen than autotrophic plants. When the orchid root
643 cells digest the fungal mycelium, those resulting compounds are then incorporated into the
645 Elemental studies utilizing radioisotope and stable isotope tracing have expanded our
646 understanding about the source-sink relationships among plants, fungi, and the environments
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647 (Cameron et al., 2006; Mayor et al., 2009). A field survey in 1960 is the first to confirm that
648 Monotropa hypopitys is nourished by neighboring trees through fungal mycelia, based on
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14 32
649 traces of C-labeled glucose and P-labeled phosphate (Björkman, 1960). The transfer of
650 carbon and nitrogen from trees or substrates to orchids (MH, PMH, or putatively autotrophic)
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651 through fungi has been documented by the application of radiocarbon and stable isotope
652 methods (McKendrick et al., 2000). The enrichment of 13C in MH plants can be explained by
653
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these species tapping into a carbon source that is an alternative to the atmospheric-CO2
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654 utilized for photosynthesis by autotrophic plants. Meanwhile, the N enrichment in MH
15
655 plants is probably due to receiving compounds enriched in N when compared with
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656 surrounding autotrophic plants that share the same mycorrhizal fungi (Merckx, 2013). Many
fungi are enriched in the heavy isotopes 13C and 15N in comparison to autotrophic plants from
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657
658 the same habitat due to their specific physiology (Mayor et al., 2009). However, enrichment
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659 of heavy isotopes in fungi is not uniform but, instead, is specific to certain functional and
661
662 8. Perspectives
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663 A good understanding of orchid physiology is essential for orchid conservation and utilization.
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664 However, because of their long life-history and slow growth rate, only the physiology of a
665 small portion of the species within Orchidaceae has been studied. Furthermore, we summarize
666 some important questions that remain unanswered and must be addressed in future research.
667 (1) Orchids usually have a long vegetative phase, a slow growth rate, and low
668 photosynthetic potential. Shortening the period of vegetative growth is an important concern
669 for orchid breeders and growers. Improving our knowledge about the mechanisms underlying
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670 slow growth and low photosynthetic rates is still a long-term task. In addition, while some
671 orchids bloom continuously throughout the year, others require more than two years of
672 recovery before re-blooming. This raises questions about how the costs of construction and
673 maintenance (i.e., respiration), as well as water supply, affect such flowering behavior and
674 floral lifespan. However, information is still lacking about the mechanism of flower induction
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675 and floral organ development in most orchids. More extensive research is needed to develop a
676 commercially viable method for controlling flowering in economically important orchids such
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677 as Paphiopedilum, Oncidium, and Dendrobium (Hew and Yong, 2004).
678 (2) The effects of nitrogen on orchid growth under cultivation conditions have been
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679 confirmed but it is still unclear how limited supplies of nitrogen and phosphorus might affect
680 development for wild plants of epiphytic orchids (Zotz and Hietz, 2001; Wanek and Zotz,
681
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2011). Moreover, because the epiphytic forms rely heavily upon atmospheric sources of
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682 nutrients, they are more susceptible to nitrogen deposition. The potential trade-off between
683 benefiting and harming those plants as levels of atmospheric-nitrogen continue to rise is
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685 (3) New information about the correlations between mycorrhizal fungi and orchid plants
686 has expanded our understanding about the symbiotic relationships among Gastrodia elata,
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687 Armillaria mellea, and Mycena osmundicola, and results from those studies have been used to
688 promote the artificial cultivation of Gastrodia. However, mycorrhizal fungi are still rarely
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690 (4) Genome sequencing has been completed for several orchids, including Phalaenopsis
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691 equestris (Cai et al., 2014), and new techniques are being widely used for molecular ecology,
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692 stable isotopes, and computer visualization in plant sciences. These tools will provide new
694
695 Acknowledgements
696 This work is financially supported by the National Natural Science Foundation of China
697 (31670342, 31370362) and the Natural Science Foundation of Yunnan Province
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698 (2013FA044).
699
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