•We examine the coordination between WWC and photorespiration.•WWC positively responds to the change in photorespiration.•WWC provides extra ATP when photorespiration is high.•HN-plants enhance the activity WWC to maintain high rates of primary metabolism.Photosynthetic electron transport produces ATP and NADPH, which are used by the primary metabolism. The production and consumption of ATP and NADPH must be balanced to maintain steady-state rates of CO2 assimilation and photorespiration. It has been indicated that the water–water cycle (WWC) is indispensable for driving photosynthesis via increasing ATP/NADPH production. However, the relationship between the WWC and photorespiration is little known. We tested the hypothesis that the WWC responds to change in photorespiration by balancing ATP/NADPH ratio. Measurements of gas exchange and chlorophyll fluorescence were conducted in tobacco plants supplied with high (HN-plants) or low nitrogen concentration (LN-plants). The WWC was activated under high light but not low light in both HN-plants and LN-plants. HN-plants had significantly higher capacities of the WWC and photorespiration than LN-plants. Under high light, the relative high WWC activation in HN-plants was accompanied with relative low levels of NPQ compared LN-plants, suggesting that the main role of the WWC under high light was to favor ATP synthesis but not to activate NPQ. Interestingly, the activation of WWC was positively correlated to the electron flow devoted to RuBP oxygenation, indicating that the WWC plays an important role in energy balancing when photorespiration is high. We conclude that the WWC is an important flexible mechanism to optimize the stoichiometry of the ATP/NADPH ratio responding to change in photorespiration. Furthermore, HN-plants enhance the WWC activity to maintain higher rates of CO2 assimilation and photorespiration.

Modifications in leaf anatomy of tobacco plants induced greater leaf water transport capacity, meeting greater transpirational demands and acclimating to warmer temperatures with a higher vapor pressure deficit.Temperature is one of the most important environmental factors affecting photosynthesis and growth of plants. However, it is not clear how it may alter leaf hydraulic architecture. We grew plants of tobacco (Nicotiana tabacum) ‘k326’ in separate glasshouse rooms set to different day/night temperature conditions: low (LT 24/18 °C), medium (MT 28/22 °C), or high (HT 32/26 °C). After 40 days of such treatment, their leaf anatomies, leaf hydraulics, photosynthetic rates, and instantaneous water-use efficiency (WUEi) were measured. Compared with those under LT, plants exposed to HT or MT conditions had significantly higher values for minor vein density (MVD), stomatal density (SD), leaf area, leaf hydraulic conductance (Kleaf), and light-saturated photosynthetic rate (Asat), but lower values for leaf water potential (ψl) and WUEi. However, those parameters did not differ significantly between HT and MT conditions. Correlation analyses demonstrated that SD and Kleaf increased in parallel with MVD. Moreover, greater SD and Kleaf were partially associated with accelerated stomatal conductance. And then stomatal conductance was positively correlated with Asat. Therefore, under well-watered, fertilized conditions, when relative humidity was optimal, changes in leaf anatomy seemed to facilitate the hydraulic acclimation to higher temperatures, meeting greater transpirational demands and contributing to the maintenance of great photosynthetic rates. Because transpiration rate increased more with temperature than photosynthetic rate, WUEi reduced under warmer temperatures. Our results indicate that the modifications of leaf hydraulic architecture are important anatomical and physiological strategies for tobacco plants acclimating to warmer temperatures under a higher vapor pressure deficit.

Paphiopedilum and Cypripedium are closely related in phylogeny, but have contrasting leaf traits and habitats. To understand the divergence in leaf traits of Paphiopedilum and Cypripedium and their adaptive significance, we analyzed the leaf anatomical structures, leaf dry mass per area (LMA), leaf lifespan (LL), leaf nitrogen concentration (Nmass), leaf phosphorus concentration (Pmass), mass-based light-saturated photosynthetic rate (Amass), water use efficiency (WUE), photosynthetic nitrogen use efficiency (PNUE) and leaf construction cost (CC) for six species. Compared with Cypripedium, Paphiopedilum was characterized by drought tolerance derived from its leaf anatomical structures, including fleshy leaves, thick surface cuticles, huge adaxial epidermis cells, lower total stoma area, and sunken stomata. The special leaf structures of Paphiopedilum were accompanied by longer LL; higher LMA, WUE, and CC; and lower Nmass, Pmass, Amass, and PNUE compared with Cypripedium. Leaf traits in Paphiopedilum helped it adapt to arid and nutrient-poor karst habitats. However, the leaf traits of Cypripedium reflect adaptations to an environment characterized by rich soil, abundant soil water, and significant seasonal fluctuations in temperature and precipitation. The present results contribute to our understanding of the divergent adaptation of leaf traits in slipper orchids, which is beneficial for the conservation of endangered orchids.

Mycorrhizal fungi of six endangered species, Paphiopedilum micranthum, Paphiopedilum armeniacum, Paphiopedilum dianthum, Cypripedium flavum, Cypripedium guttatum, and Cypripedium tibeticum, from two closely related genera in the Orchidaceae from Southwestern China, were characterized using the nuclear internal transcribed spacer (ITS) and part of the large subunit gene of mitochondrial rDNA (mtLSU) sequences. The most frequently detected fungi belonged to the Tulasnellaceae. These fungi were represented by 25 ITS sequence types and clustered into seven major clades in the phylogenetic analysis of 5.8S sequences. Species of Paphiopedilum and Cypripedium shared no fungal ITS sequence types in common, but their fungal taxa sometimes occurred in the same major clade of the 5.8S phylogenetic tree. Although it had several associated fungal ITS sequence types in a studied plot, each orchid species had in general only a single dominant type. The fungal sequence type spectra of different species of Paphiopedilum from similar habitats sometimes overlapped; however, the dominant sequence types differed among the species and so did the sequence-type spectra within Cypripedium. Orchids of P. micranthum and P. armeniacum transplanted from the field and grown in two greenhouses had a greater number of mycorrhizal associations than those sampled directly from the field. Root specimens from P. micranthum taken from the greenhouses were preferably associated with mycobionts of the Tulasnella calospora complex, while those from the field had mycorrhizal associations of other tulasnelloid taxa. Such plasticity in mycorrhizal associations makes ex situ conservation or even propagation by means of mycorrhization of axenically grown seedlings possible.

The photosynthetic performance and related leaf traits of Incarvillea delavayi Bur. et Franch were studied at different water regimes to assess its capacity for photosynthetic acclimation to water stress. The initial response of I. delavayi to water stress was the closure of stomata, which resulted in down-regulation of photosynthesis. The stomatal limitation (SL) represented the main component to photosynthetic limitations but non-stomatal limitation (NSL) increased quickly with the increasing water stress, and had similar magnitude to SL under severe water stress (soil moisture 25–30 % of field capacity). Chlorophyll (Chl) a fluorescence parameters characterizing photosystem (PS) 2 photochemical efficiency (ΦPS2), electron transport rate (J) and photochemical quenching (qP) decreased with the increasing water stress, indicating impaired photosynthetic apparatus. However, the water-stressed plants had a increased mesophyll CO2 diffusional conductance, Chl a/b ratio, leaf nitrogen partitioning in RuBPCO and bioenergetics in later grown parts, indicating that I. delavay had a substantial physiological plasticity and showed a good tolerance to water stress.

To understand the ecophysiological adaptation of Lilium “Oriental Hybrids”, which are grown for their commercial bulbs, the gas exchange, leaf N and chlorophyll content of the three varieties were investigated in the central areas of the Yunnan Province. Among the three varieties, light-saturated photosynthetic rate at ambient CO2 (Amax) of Tiber was the highest, while that of Siberia was the lowest. The difference in the Amax was related to the carboxylation efficiency (CE), leaf mass per unit area and leaf N content per mass, which indicated that their photosynthetic capacity was influenced by the activity and/or the quantity of Rubisco. The three varieties had lower photosynthetic saturation points and photosynthetic compensation points, but the photosynthetic rates were not decreased up to 2000 μmol·m−2·s−1 of the light intensity. This indicates that the three varieties had broad adaptability to light intensity. There were significant differences in the photosynthetic optimum temperature among the three varieties. Siberia had the highest photosynthetic optimum temperature (25.5°C–34.9°C), and is likely to grow well in warm areas. Sorbonne had the lowest photosynthetic optimum temperature (19.3°C–25.6°C), and its growth is favored in cool areas. Tiber can maintain a high photosynthetic rate within a wide range of temperature. Therefore, Tiber is the most suitable variety for the climate in the central areas of the Yunnan Province, China.

Photosynthesis, leaf structure, nitrogen content and nitrogen allocation in photosynthetic functions of Cypripedium flavum were studied in a naturally varying light regime. Light-saturated leaf net photosynthetic rate (Amax) was strongly correlated with leaf dry mass per area (LMA), mesophyll conductance (gm) and area-based leaf nitrogen content (Narea), with all variables increasing with increasing irradiance. Such coordinate variation of all these parameters illustrates the plastic response of leaf structure to high light (HL). Leaf Narea was greater under HL than in low light (LL). The fractions of leaf nitrogen partitioning in carboxylation (PR) and bioenergetics (PB) were positively related to LMA. In contrast, PR and PB decreased with increasing mass-based leaf nitrogen content (Nmass). However, no correlation was found between leaf nitrogen investment in light harvesting (PL) and either LMA or Nmass. Like maximum rate of carboxylation (Vcmax) and electron transport (Jmax), the Jmax/Vcmax ratio, which was strongly correlated to LMA, also increased significantly with irradiance. Under HL, leaf maximum photosynthetic nitrogen efficiency (ANUE) and intrinsic water use efficiency (WUE) were greater than in LL conditions, despite a small difference in WUE. This suggests that a functional balance in the photosynthetic machinery favors leaf photosynthetic plasticity of C. flavum in response to different light conditions. Given an ample soil nitrogen supply, C. flavum may offset its susceptibility to HL by efficient nitrogen use and higher stomatal and mesophyll conductance against photoinhibition so as to keep leaf photosynthesis positive.

Photosynthetic rate, chlorophyll fluorescence, leaf nitrogen and chlorophyll content of Cypripedium flavum were studied at different leaf ages. The photosynthetic capacity changed significantly with leaf age. Net photosynthesis and chlorophyll content peaked when leaf age was 60 days, decreasing at 30, 90 and 120 days. Stomatal conductance showed the highest value at 60 days, while mesophyll conductance decreased with increasing leaf age. Both leaf nitrogen content per unit area and leaf nitrogen content per unit mass decreased with increasing leaf age. The age-dependent variation in photosynthetic capacity could be linked to the changes in biochemical efficiency, leaf nitrogen content and CO2 diffusion limitation.

Cypripedium guttatum can be found both in open and shady habitats. Photosynthetic acclimation of C. guttatum to different light availabilities was detected using measurements of chlorophyll fluorescence, photosynthesis and leaf traits. When growing under low light conditions, C. guttatum exhibited a greater efficiency in photochemical utilization of absorbed light energy, and a lower ability for non-photochemical dissipation of excess light energy, as compared to the plants growing under high light conditions. Under intermediate light conditions, C. guttatum exhibited higher photosynthetic capacity (Amax) than those under both low light or high light conditions. The differences in Amax among three light environments was linked to the differences in biochemical efficiency, leaf N content (LNC) and leaf dry mass per unit area (LMA), but not to the differences of chlorophyll content. However, there were no significant differences in the light compensation points (LCP) and light saturation points (LSP) for photosynthesis for the plants growing under the three light conditions. These results indicate that the photosynthetic capacity of C. guttatum leaves allows for flexible and reversible responses to different irradiance levels. Photosynthetic acclimation in C. guttatum was affected by biochemical changes, the changes in LMA and ratio of Chl a/b. Successful acclimation of C. guttatum to a broad range of light levels likely allows for its wide geographical distribution. A level of about 45% sunlight appears to be optimal for photosynthesis.