Anatomical and diffusional determinants inside leaves explain the difference in photosynthetic capacity between Cypripedium and Paphiopedilum, Orchidaceae

Rapid loss of plastid ndh genes in slipper orchids (Cypripedioideae, Orchidaceae)

Introduction

The eleven plastid ndh genes encode for subunits of the ndh (NAD(P)H dehydrogenase-like) complex, which mediates electron flow in photosystem I. The loss of ndh genes in plants was observed in many different lineages of Viridiplantae. In lineages of Orchidaceae, the loss of ndh genes was often associated with myco-heterotrophy. However, in previous studies on this topic only a few slipper orchids were included. Our study aimed to analyze the loss of ndh genes within Cypripedioideae, a subfamily that is assumed to be fully autotroph.

Methods

Based on a comprehensive sampling of 100 published plastomes representing 60% of Cypripedioideae species, the phylogenetic relationships were revealed on three levels. For family and subfamily levels, 57 and 66 plastid genes, respectively, were extracted and concatenated in Geneious, while for the genus-level phylogeny, complete plastomes were used to calculate a maximum likelihood tree. Additionally, divergence time estimates were performed to illuminate the evolutionary timeframe of the gene loss. The prevalence, pseudogenization and loss of ndh genes were assessed and visualized along the phylogenetic trees.

Results

The results confirmed the four analyzed genera of Cypripedioideae to be monophyletic and could increase the resolution within the genera compared to previous studies. The diversification of the subfamily started at about 30 Ma with genus Paphiopedilum displaying the most recent diversification starting at about 11 Ma and showing most speciation events around 4 Ma. The rapid loss of plastid ndh genes within the subfamily Cypripedioideae, particularly in the genera Mexipedium, Phragmipedium and Paphiopedilum could be illustrated. Furthermore, the results illustrated that Cypripedioideae are in an early stage of plastid degradation.

Discussion and conclusions

Recent studies showed that partial myco-heterotrophy (mixotrophy) is far more common in plant lineages than originally assumed. Based on our findings, we suggest that the possibility of a mixotrophic lifestyle within (sub-)tropical slipper orchids should be reevaluated. Further research regarding the reasons behind plastid gene loss in slipper orchids could provide a better understanding of the ecological evolution of Cypripedioideae.

Different Leaf Strategies Between Lithophytic and Terrestrial Orchids in a Subtropical Karst Forest

The leaf economic spectrum framework explains how plants optimize leaf traits for productivity, distribution, and stress tolerance. Orchids in Southwestern China's karst forests, especially lithophytic species, are challenged by prolonged drought and limited light availability. This study investigated different leaf strategies between lithophytic and terrestrial orchids under the harsh karst environment. We measured key leaf traits, including photosynthesis, structure, biomechanics, nitrogen allocation, and water relations, in twenty-two lithophytic and six terrestrial orchids in a subtropical karst forest. After accounting for phylogenetic influences, we found that lithophytic orchids had a higher leaf mass per area, cuticle thickness, and biomechanical resistance (Fp) but a lower maximum photosynthetic rate (Amax-mass), nitrogen allocation to photosynthesis (NT), and saturated water content (SWC) than terrestrial orchids. These results suggest that lithophytic orchids prioritize structural investment and stress tolerance over photosynthetic efficiency. Across species, NT correlated positively with Amax-mass and negatively with Fp, highlighting nitrogen allocation as a key mechanism in leaf cost-benefit strategies. Additionally, SWC emerged as a critical driver of variation in multiple traits, supporting its integration into the leaf economic spectrum for orchids in karst ecosystems. This study offers new insights into orchid adaptation in subtropical karst environments, with implications for plant resilience under changing climates.

Leaf dry mass per unit area and leaf pigments underlying the higher stomatal conductance of deciduous species relative to evergreen species in Dendrobium

BACKGROUND

Leaf stomatal conductance is an important indicator of photosynthetic capacity. However, stomatal conductance is poorly quantified and rarely explored in the context of the leaf functional traits for epiphytes, particularly when it comes to herbaceous species with different leaf habits (deciduous vs. deciduous species). Here, we investigated leaf stomatal conductance, leaf dry mass per unit area, leaf thickness, stomatal density, abaxial epidermal cell size and pigment contents in 23 Dendrobium evergreen and deciduous species from a greenhouse. Our main objectives were to compare differences in all measured traits between evergreen and deciduous species, and to determine the relationships of leaf stomatal conductance with leaf functional traits and leaf pigments.

RESULTS

The results showed that the evergreen species of Dendrobium had thicker leaves and higher leaf dry mass per unit area, whereas deciduous species had higher leaf stomatal conductance and higher leaf chlorophyll contents. Leaf stomatal conductance had a negative correlation with leaf thickness, and dry mass per unit area, but a positive correlation with leaf pigment contents. There was a negative correlation between pigment contents and leaf dry mass per unit area.

CONCLUSION

The results reveal the clear differences in leaf stomatal conductance, leaf functional traits and leaf pigments between deciduous and evergreen Dendrobium species, with the form groups showing trait values indicative of less investments in structural components and of more investments in photosynthetic carbon gain. Furthermore, leaf dry mass per unit area and leaf pigments play an important role in shaping leaf stomatal conductance.

Variation in Photosynthetic Efficiency under Fluctuating Light between Rose Cultivars and its Potential for Improving Dynamic Photosynthesis

Photosynthetic efficiency under both steady-state and fluctuating light can significantly affect plant growth under naturally fluctuating light conditions. However, the difference in photosynthetic performance between different rose genotypes is little known. This study compared the photosynthetic performance under steady-state and fluctuating light in two modern rose cultivars (Rose hybrida), "Orange Reeva" and "Gelato", and an old Chinese rose plant Rosa chinensis cultivar, "Slater's crimson China". The light and CO₂ response curves indicated that they showed similar photosynthetic capacity under steady state. The light-saturated steady-state photosynthesis in these three rose genotypes was mainly limited by biochemistry (60%) rather than diffusional conductance. Under fluctuating light conditions (alternated between 100 and 1500 μmol photons m^-2 m^-1 every 5 min), stomatal conductance gradually decreased in these three rose genotypes, while mesophyll conductance (g_m) was maintained stable in Orange Reeva and Gelato but decreased by 23% in R. chinensis, resulting in a stronger loss of CO₂ assimilation under high-light phases in R. chinensis (25%) than in Orange Reeva and Gelato (13%). As a result, the variation in photosynthetic efficiency under fluctuating light among rose cultivars was tightly related to g_m. These results highlight the importance of g_m in dynamic photosynthesis and provide new traits for improving photosynthetic efficiency in rose cultivars.

Photosynthetic Induction Under Fluctuating Light Is Affected by Leaf Nitrogen Content in Tomato

The response of photosynthetic CO₂ assimilation to changes of illumination affects plant growth and crop productivity under natural fluctuating light conditions. However, the effects of nitrogen (N) supply on photosynthetic physiology after transition from low to high light are seldom studied. To elucidate this, we measured gas exchange and chlorophyll fluorescence under fluctuating light in tomato (Solanum lycopersicum) seedlings grown with different N conditions. After transition from low to high light, the induction speeds of net CO₂ assimilation (A _N ), stomatal conductance (g _s ), and mesophyll conductance (g _m ) delayed with the decline in leaf N content. The time to reach 90% of maximum A _N , g _s and g _m was negatively correlated with leaf N content. This delayed photosynthetic induction in plants grown under low N concentration was mainly caused by the slow induction response of g _m rather than that of g _s . Furthermore, the photosynthetic induction upon transfer from low to high light was hardly limited by photosynthetic electron flow. These results indicate that decreased leaf N content declines carbon gain under fluctuating light in tomato. Increasing the induction kinetics of g _m has the potential to enhance the carbon gain of field crops grown in infertile soil.

Leaf physiological and anatomical responses of two sympatric Paphiopedilum species to temperature

Paphiopedilum dianthum and P. micranthum are two endangered orchid species, with high ornamental and conservation values. They are sympatric species, but their leaf anatomical traits and flowering period have significant differences. However, it is unclear whether the differences in leaf structure of the two species will affect their adaptabilities to temperature. Here, we investigated the leaf photosynthetic, anatomical, and flowering traits of these two species at three sites with different temperatures (Kunming, 16.7 ± 0.2 °C; Puer, 17.7 ± 0.2 °C; Menglun, 23.3 ± 0.2 °C) in southwest China. Compared with those at Puer and Kunming, the values of light-saturated photosynthetic rate (Pmax), stomatal conductance (gs), leaf thickness (LT), and stomatal density (SD) in both species were lower at Menglun. The values of Pmax, gs, LT, adaxial cuticle thickness (CTad) and SD in P. dianthum were higher than those of P. micranthum at the three sites. Compared with P. dianthum, there were no flowering plants of P. micranthum at Menglun. These results indicated that both species were less resistance to high temperature, and P. dianthum had a stronger adaptability to high-temperature than P. micranthum. Our findings can provide valuable information for the conservation and cultivation of Paphiopedilum species.

Acclimation of mesophyll conductance and anatomy to light during leaf aging in Arabidopsis thaliana

Mesophyll conductance (g_m ), a key limitation to photosynthesis, is strongly driven by leaf anatomy, which is in turn influenced by environmental growth conditions and ontogeny. However, studies examining the combined environment × age effect on both leaf anatomy and photosynthesis are scarce, and none have been carried out in short-lived plants. Here, we studied the variation of photosynthesis and leaf anatomy in the model species Arabidopsis thaliana (Col-0) grown under three different light intensities at two different leaf ages. We found that light × age interaction was significant for photosynthesis but not for anatomical characteristics. Increasing growth light intensities resulted in increases in leaf mass per area, thickness, number of palisade cell layers, and chloroplast area lining to intercellular airspace. Low and moderate-but not high-light intensity had a significant effect on all photosynthetic characteristics. Leaf aging was associated with increases in cell wall thickness (T_cw ) in all light treatments and in increases in leaf thickness in plants grown under low and moderate light intensities. However, g_m did not vary with leaf aging, and photosynthesis only decreased with leaf age under moderate and high light, suggesting a compensatory effect between increased T_cw and decreased chloroplast thickness on the total CO₂ diffusion resistance.

Floral Longevity of Paphiopedilum and Cypripedium Is Associated With Floral Morphology

Floral longevity (FL) is an important trait influencing plant reproductive success by affecting the chance of insect pollination. However, it is still unclear which factors affect FL, and whether FL is evolutionarily associated with structural traits. Since construction costs and water loss by transpiration play a role in leaf longevity, we speculated that floral structures may affect the maintenance and loss of water in flowers and, therefore, FL. Here, we investigated the slipper orchid Paphiopedilum and Cypripedium, which are closely related, but strongly differ in their FL. To understand the evolutionary association of floral anatomical traits with FL, we used a phylogenetic independent comparative method to examine the relationships between 30 floral anatomical traits and FL in 18 species of Paphiopedilum and Cypripedium. Compared with Paphiopedilum species, Cypripedium species have lower values for floral traits related to drought tolerance and water retention capacity. Long FL was basically accompanied by the thicker epidermal and endodermal tissues of the floral stem, the thicker adaxial and abaxial epidermis of the flower, and low floral vein and stomatal densities. Vein density of the dorsal sepals and synsepals was negatively correlated with stomatal density. Our results supported the hypothesis that there was a correlation between FL and floral anatomical traits in slipper orchids. The ability to retain water in the flowers was associated with FL. These findings provide a new insight into the evolutionary association of floral traits with transpirational water loss for orchids under natural selection.

The Light Dependence of Mesophyll Conductance and Relative Limitations on Photosynthesis in Evergreen Sclerophyllous Rhododendron Species

Mesophyll conductance (g_m) limits CO₂ diffusion from sub-stomatal internal cavities to the sites of RuBP carboxylation. However, the response of g_m to light intensity remains controversial. Furthermore, little is known about the light response of relative mesophyll conductance limitation (l_m) and its effect on photosynthesis. In this study, we measured chlorophyll fluorescence and gas exchange in nine evergreen sclerophyllous Rhododendron species. g_m was maintained stable across light intensities from 300 to 1500 μmol photons m^-2 s^-1 in all these species, indicating that g_m did not respond to the change in illumination in them. With an increase in light intensity, l_m gradually increased, making g_m the major limiting factor for area-based photosynthesis (A_N) under saturating light. A strong negative relationship between l_m and A_N was found at 300 μmol photons m^-2 s^-1 but disappeared at 1500 μmol photons m^-2 s^-1, suggesting an important role for l_m in determining A_N at sub-saturating light. Furthermore, the light-dependent increase in l_m led to a decrease in chloroplast CO₂ concentration (C_c), inducing the gradual increase of photorespiration. A higher l_m under saturating light made A_N more limited by RuBP carboxylation. These results indicate that the light response of l_m plays significant roles in determining C_c, photorespiration, and the rate-limiting step of A_N.

Photosynthesis and photosynthetic efficiencies along the terrestrial plant's phylogeny: lessons for improving crop photosynthesis

Photosynthesis is the basis of all life on Earth. Surprisingly, until very recently, data on photosynthesis, photosynthetic efficiencies, and photosynthesis limitations in terrestrial land plants other than spermatophytes were very scarce. Here we provide an updated data compilation showing that maximum photosynthesis rates (expressed either on an area or dry mass basis) progressively scale along the land plant's phylogeny, from lowest values in bryophytes to largest in angiosperms. Unexpectedly, both photosynthetic water (WUE) and nitrogen (PNUE) use efficiencies also scale positively through the phylogeny, for which it has been commonly reported that these two efficiencies tend to trade-off between them when comparing different genotypes or a single species subject to different environmental conditions. After providing experimental evidence that these observed trends are mostly due to an increased mesophyll conductance to CO₂ - associated with specific anatomical changes - along the phylogeny, we discuss how these findings on a large phylogenetic scale can provide useful information to address potential photosynthetic improvements in crops in the near future.

Effect of Light/Dark Cycle on Photosynthetic Pathway Switching and CO2 Absorption in Two Dendrobium Species

Many Dendrobium species are both ornamental and medicinal plants in China. Several wild species have been exploited to near extinction, and facility cultivation has become an important way to meet the great market demand. Most Dendrobium species have evolved into crassulacean acid metabolism (CAM) pathways in adapting to harsh epiphytic environment, leading to low daily net CO₂ absorption. Photosynthetic pathways of many facultative CAM plants are regulated by various environmental factors. Light/dark cycle plays an important role in regulating the photosynthetic pathway of several CAM species. The aims of this study were to investigate whether the photosynthetic pathway of Dendrobium species could be regulated between C3 and CAM by changing light/dark cycles and the daily net CO₂ absorption could be enhanced by shortening light/dark cycle. In this study, net CO₂ exchange rates of D. officinale and D. primulinum were monitored continuously during two different light/dark cycles conversion compared to Kalanchoe daigremontiana as an obligate CAM plant. The net CO₂ exchange pattern and stomatal behavior of D. officinale and D. primulinum were switched from CAM to C3-like by changing the light/dark cycle from 12/12 h to 4/4 h. However, this switching was not completely reversible. Compared to the original 12/12 h light/dark cycle, the dark, light, and daily net CO₂ exchange amount of D. officinale were significantly increased after the light/dark cycle was changed from 4/4 h to 12/12 h, but those in D. primulinum was opposite and those in K. daigremontiana was not affected. Daily net CO₂ exchange amount of D. officinale increased by 47% after the light/dark cycle was changed from 12/12 h to 4/4 h, due to the sharp increase of light net CO₂ exchange amount. However, the large decrease of dark net CO₂ exchange amount could not be offset by increased light net CO₂ exchange amount, leading to reduced daily net CO₂ exchange amount of D. primulinum. In conclusion, the 4/4 h light/dark cycle can induce the photosynthetic pathway of D. officinale and D. primulinum to C3-like, and improve the daily CO₂ absorption of D. officinale.

Physiological diversity of orchids

The Orchidaceae is a diverse and wide spread family of flowering plants that are of great value in ornamental, medical, conservation, and evolutionary research. The broad diversity in morphology, growth form, life history, and habitat mean that the members of Orchidaceae exhibit various physiological properties. Epiphytic orchids are often characterized by succulent leaves with thick cell walls, cuticles, and sunken stomata, whereas terrestrial orchids possess rhizomes, corms, or tubers. Most orchids have a long juvenile period, slow growth rate, and low photosynthetic capacity. This reduced photosynthetic potential can be largely explained by CO2 diffusional conductance and leaf internal structure. The amount of light required for plant survival depends upon nutritional mode, growth form, and habitat. Most orchids can adapt to their light environments through morphological and physiological adjustments but are sensitive to sudden changes in irradiance. Orchids that originate from warm regions are susceptible to chilling temperatures, whereas alpine members are vulnerable to high temperatures. For epiphytic orchids, rapid water uptake by the velamen radicum, water storage in their pseudobulbs and leaves, slow water loss, and Crassulacean Acid Metabolism contribute to plant-water balance and tolerance to drought stress. The presence of the velamen radicum and mycorrhizal fungi may compensate for the lack of root hairs, helping with quick absorbance of nutrients from the atmosphere. Under cultivation conditions, the form and concentration of nitrogen affect orchid growth and flowering. However, the limitations of nitrogen and phosphorous on epiphytic orchids in the wild, which require these plants to depend on mycorrhizal fungi for nutrients throughout the entire life cycle, are not clearly understood. Because they lack endosperm, seed germination depends upon obtaining nutrients via mycorrhizal fungi. Adult plants of some autotrophic orchids also gain carbon, nitrogen, phosphorus, and other elements from their mycorrhizal partners. Future studies should examine the mechanisms that determine slow growth and flower induction, the physiological causes of variations in flowering behavior and floral lifespan, the effects of nutrients and atmospheric-nitrogen deposition, and practical applications of mycorrhizal fungi in orchid cultivation.