Chloroplast CO2 Concentration Research Articles

Vapour pressure deficit affects crop water productivity, yield, and quality in tomatoes

A high atmospheric vapour pressure deficit (VPD) hinders calcium absorption in tomatoes (Solanum lycopersicum L.), which severely reduces tomato yield, crop water productivity (WPcrop), and quality. Although reducing the VPD is effective for increasing tomato yield and water productivity, regulating the VPD is costly. Therefore, this study aimed to explore the effects of VPD regulation (high VPD=2.22 kPa, low VPD=0.95 kPa) during different growth stages on tomato WPcrop, yield, and quality to identify a VPD management method that saves costs and enhances the above parameters. The results showed that the yield was significantly positively correlated with fruit calcium absorption. Decreasing the VPD during the fruit expansion stage and increasing it during the flowering stage significantly increased calcium accumulation in the fruits, thereby improving tomato WPcrop, yield, and quality. Additionally, a high VPD during the flowering stage increased stomatal conductance and mesophyll conductance, thereby increasing intercellular and chloroplast CO2 concentrations and enhancing plant photosynthetic capacity, and reduced the stem water potential, leaf water potential, and leaf relative water content, thereby increasing the water potential difference, leaf hydraulic conductance, and calcium absorption. By integrating game theory and the technique for order of preference by similarity to ideal solution method, a comprehensive analysis of tomato appearance, nutrients, and flavour quality revealed that growing plants under a high VPD during the seedling, flowering, and fruit ripening stages and a low VPD during the fruit expansion stage not only enhanced the comprehensive tomato quality value but also maximally reduced the humidification cost. Conclusively, controlling the environmental VPD at 2.22 kPa during the seedling, flowering, and fruit ripening stages and at 0.95 kPa during the fruit expansion stage can effectively increase tomato WPcrop, yield, and quality. This study provides a theoretical basis for the environmental regulation of high-quality, efficient, and water-conserving cultivation of greenhouse tomatoes.

Halotolerant PGPB Staphylococcus sciuri ET101 protects photosynthesis through activation of redox dissipation pathways in Lycopersicon esculentum

Photosynthesis is known to be seriously affected by salt stress. The stress induced membrane damage leads to disrupted photosynthetic components causing imbalance between production and utilization of ATP/NADPH with generation of ROS leading to photoinhibition and photodamage. In the current study, role of halotolerant plant growth promoting bacteria (PGPB) Staphylococcus sciuri ET101 in protection of photosynthesis in tomato plants during salinity stress was evaluated by analysing changes in antioxidant defense and activation of redox dissipation pathways. Inoculation of S. sciuri ET101 significantly enhanced the growth of tomato plants with significantly higher photosynthetic rates (PN) under normal and salinity stress conditions. Further, increased membrane stability, soluble sugar accumulation and significant decrease in malondialdehyde (MDA) content in leaves of ET101 inoculated tomato plants under normal and salinity were observed along with increased expression of antioxidant genes for efficient ROS detoxification and suppression of oxidative damage. Additionally, salinity induced decrease in rate of photosynthesis (PN) due to lowered chloroplastic CO2 concentration (Cc) attributed by low mesophyll conductance (gm) in uninoculated plants was alleviated by ET101 inoculation showing significantly higher carboxylation rate (Vcmax), RuBP generation (Jmax) and increased photorespiration (PR). The genes involved in photorespiratory process, cyclic electron flow (CEF), and alternative oxidase (AOX) pathway of mitochondrial respiration were abundantly expressed in leaves of ET101 inoculated plants indicating their involvement in protecting photosynthesis from salt stress induced photoinhibition. Collectively, our results indicated that S. sciuri ET101 has the potential in protecting photosynthesis of tomato plants under salinity stress through activation of redox dissipation pathways.

Exploring the responses of crop photosynthesis to CO2 elevation at the molecular, physiological, and morphological levels toward increasing crop production

Exploring the impact of elevated CO2 on photosynthesis is vital for understanding plant responses to climate change. In C3 plants, elevated CO2 concentrations generally enhance CO2 assimilation by increasing chloroplast CO2 concentration. However, the underlying mechanisms are complex since photosynthesis involves multiple physiological processes operating at different time scales and varying among plant species. In this review, we focused on the responses of key photosynthetic processes in crop, including CO2 diffusion conductances such as stomatal conductance (gs), mesophyll conductance (gm), photochemical reactions, the Calvin-Benson cycle, and related metabolic pathways. Short-term exposure to elevated CO2 often decreases gs and gm while increasing the electron transport rate. However, long-term exposure to elevated CO2 can decrease photosynthetic capacity due to coordinated downregulation of multiple processes, particularly when the sink‒source ratio declines. To enhance plant productivity under elevated CO2, it is crucial to maintain or enhance sink activity and understand the CO2 response mechanisms at the molecular, physiological, and morphological levels. This review provides an update on the short- and long-term responses of gs, gm, electron transport system, and carbon assimilation metabolism to elevated CO2. Furthermore, it offers a perspective on improving crop production in the future with elevated CO2 levels.

Photosynthetic traits of Australian wild rice (Oryza australiensis) confer tolerance to extreme daytime temperatures

Key messageA wild relative of rice from the Australian savannah was compared with cultivated rice, revealing thermotolerance in growth and photosynthetic processes and a more robust carbon economy in extreme heat.Above ~ 32 °C, impaired photosynthesis compromises the productivity of rice. We compared leaf tissues from heat-tolerant wild rice (Oryza australiensis) with temperate-adapted O. sativa after sustained exposure to heat, as well as diurnal heat shock. Leaf elongation and shoot biomass in O. australiensis were unimpaired at 45 °C, and soluble sugar concentrations trebled during 10 h of a 45 °C shock treatment. By contrast, 45 °C slowed growth strongly in O. sativa. Chloroplastic CO2 concentrations eliminated CO2 supply to chloroplasts as the basis of differential heat tolerance. This directed our attention to carboxylation and the abundance of the heat-sensitive chaperone Rubisco activase (Rca) in each species. Surprisingly, O. australiensis leaves at 45 °C had 50% less Rca per unit Rubisco, even though CO2 assimilation was faster than at 30 °C. By contrast, Rca per unit Rubisco doubled in O. sativa at 45 °C while CO2 assimilation was slower, reflecting its inferior Rca thermostability. Plants grown at 45 °C were simultaneously exposed to 700 ppm CO2 to enhance the CO2 supply to Rubisco. Growth at 45 °C responded to CO2 enrichment in O. australiensis but not O. sativa, reflecting more robust carboxylation capacity and thermal tolerance in the wild rice relative.

Physiological responses of young oil palm (Elaeis guineensis Jacq.) plants to repetitive water deficit events

Stomatal and non-stomatal limitations to photosynthesis and acclimation of the photosynthetic machinery to repetitive water deficit (WD) events were investigated in oil palms (Elaeis guineensis Jacq.). For this, well-watered plants (control treatment) were compared with plants subjected to one, two, and three WD events imposed by withholding irrigation until their predawn leaf water potential (Ψpd) reached around −2.5 MPa. Treatment comparisons were performed over 28 days. The Ψpd decreased equally between WD treatments. Decreases in net CO2 assimilation rate (A) were similar between stressed plants until day 7. In the following days, the A was higher in plants thrice stressed than in those stressed once. Stomatal conductance decreased similarly between WD treatments, but mesophyll conductance to CO2 was lower in plants subjected to a single WD. Chloroplast CO2 concentration decreased similarly between plants subjected to one and three WD events until day 14, but it was lower in the former on subsequent days. Plants subjected to a single WD event showed lower Ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco) activity, maximum rate of electron transport (Jmax), and higher rates of photorespiration and dark respiration than other treatments. Plants subjected to one WD event also showed lower maximum quantum efficiency of photosystem II (PSII) photochemistry and PSII maximum efficiency concomitantly with higher malondialdehyde content. The results support that repetitive WD events induce the acclimation of the photosynthetic machinery in oil palm through adjustments in carboxylase Rubisco activity, Jmax, photorespiration, and respiration rates, as well as attenuating oxidative damages to PSII and membrane lipids.

Elevated efficiency of C3 photosynthesis in bamboo grasses: A possible consequence of enhanced refixation of photorespired CO2

AbstractBamboos are productive grasses that currently yield a high‐quality wood and potentially an abundance of lignocellulose for bioenergy. All are C3 grasses of warm habitats, where they are prone to significant photorespiratory inhibition and competitive suppression by C4 grasses. Here, we investigate whether three bamboo species from the Brazilian Cerrado (Dendrocalamus asper, Guadua angustifolia, and Guadua magna) exhibit unique adaptations that suppress photorespiratory costs and enhance photosynthetic efficiency. We evaluated photosynthetic efficiency of the bamboos and rice (Oryza sativa) by measuring C*, the CO2 compensation point in the absence of mitochondrial respiration. At 25℃, C* averaged 2.81 Pa in each of the bamboo species, which is closer to a C2 plant (2.71 Pa) than the C3 plant rice (3.31 Pa). Assuming a chloroplast CO2 concentration of 200 µmol mol−1, this represents an 18% lower cost of apparent photorespiration in bamboo than rice. Light and transmission electronic microscopy of the bamboo leaves exhibited few organelles in the bundle and mestome sheath cells, and mesophyll (M) cells are deeply lobed with 99% of the cell periphery adjacent to intercellular air space covered by chloroplast and stromules. The chloroplast layer in bamboo M cells is thick, with mitochondria adjacent to or engulfed by chloroplasts. This arrangement slows CO2 efflux and facilitates refixation of photorespired CO2, which could explain the low C* in the bamboos. The bamboos also had higher water use efficiency than rice, which may reflect efficient refixation of photorespired CO2.

Higher Atmospheric CO2 Levels Favor C3 Plants Over C4 Plants in Utilizing Ammonium as a Nitrogen Source.

Photosynthesis of wheat and maize declined when grown with NH4+ as a nitrogen (N) source at ambient CO2 concentration compared to those grown with a mixture of NO3– and NH4+, or NO3– as the sole N source. Interestingly, these N nutritional physiological responses changed when the atmospheric CO2 concentration increases. We studied the photosynthetic responses of wheat and maize growing with various N forms at three levels of growth CO2 levels. Hydroponic experiments were carried out using a C3 plant (wheat, Triticum aestivum L. cv. Chuanmai 58) and a C4 plant (maize, Zea mays L. cv. Zhongdan 808) given three types of N nutrition: sole NO3– (NN), sole NH4+ (AN) and a mixture of both NO3– and NH4+ (Mix-N). The test plants were grown using custom-built chambers where a continuous and desired atmospheric CO2 (Ca) concentration could be maintained: 280 μmol mol–1 (representing the pre-Industrial Revolution CO2 concentration of the 18th century), 400 μmol mol–1 (present level) and 550 μmol mol–1 (representing the anticipated futuristic concentration in 2050). Under AN, the decrease in net photosynthetic rate (Pn) was attributed to a reduction in the maximum RuBP-regeneration rate, which then caused reductions in the maximum Rubisco-carboxylation rates for both species. Decreases in electron transport rate, reduction of electron flux to the photosynthetic carbon [Je(PCR)] and electron flux for photorespiratory carbon oxidation [Je(PCO)] were also observed under AN for both species. However, the intercellular (Ci) and chloroplast (Cc) CO2 concentration increased with increasing atmospheric CO2 in C3 wheat but not in C4 maize, leading to a higher Je(PCR)/ Je(PCO) ratio. Interestingly, the reduction of Pn under AN was relieved in wheat through higher CO2 levels, but that was not the case in maize. In conclusion, elevating atmospheric CO2 concentration increased Ci and Cc in wheat, but not in maize, with enhanced electron fluxes towards photosynthesis, rather than photorespiration, thereby relieving the inhibition of photosynthesis under AN. Our results contributed to a better understanding of NH4+ involvement in N nutrition of crops growing under different levels of CO2.

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

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

Constant ratio of Cc to Ci under various CO2 concentrationsand light intensities, and during progressive drought, in seedlings of Japanese white birch.

Constant mesophyll conductance (gm), and two-resistance gm model (involved in resistances of cell wall and chloroplast), where gm reaches maximum under higher CO2 concentrations, cannot describe the phenomenon that gm decreases with increasing intercellular CO2 concentration (Ci) under relatively higher CO2 concentrations. Yin et al. (2020) proposed a gm model, according to which the ratio of chloroplastic CO2 concentration (Cc) to Ci is constant in the two-resistance gm model, which can describe the decreasing gm with increasing Ci. In the present study, we investigated the relationship between Cc and Ci in leaves of Japanese white birch by using simultaneous measurements of gas exchange and chlorophyll fluorescence under various CO2 concentrations, light intensities, and during progressive drought. Across the range of ambient CO2 from 50 to 1000μmolmol-1, and light intensities of 50 to 2000μmolm-2s-1, measured under well irrigation, the ratio of Cc to Ci kept constant. During the progressive drought, overestimated Ci due to stomatal patchiness and/or cuticular transpiration was empirically corrected (threshold: stomatal conductance < 0.08mol H2O m-2s-1) from the A/Ci response measured under adequate irrigation. The ratio of Cc to Ci during progressive drought (predawn leaf potential reached ≈-2MPa) also remained constant irrespective of soil drying rate in various pot sizes. The present study suggests the involvement of some physiologically regulative mechanisms to keep Cc:Ci ratio constant, which might act on gm in addition to the physical interaction of diffusive resistances in the cell components.

A Synthetic Photorespiratory Shortcut Enhances Photosynthesis to Boost Biomass and Grain Yield in Rice

Several photorespiratory bypasses have been introduced into plants and shown to improve photosynthesis by increasing chloroplastic CO2 concentrations or optimizing energy balance. We recently reported that an engineered GOC bypass could increase photosynthesis and productivity in rice. However, the grain yield of GOC plants was unstable, fluctuating in different cultivation seasons because of varying seed setting rates. In this study, we designed a synthetic photorespiratory shortcut (the GCGT bypass) consisting of genes encoding Oryza sativa glycolate oxidase and Escherichia coli catalase, glyoxylate carboligase, and tartronic semialdehyde reductase. The GCGT bypass was guided by an optimized chloroplast transit peptide that targeted rice chloroplasts and redirected 75% of carbon from glycolate metabolism to the Calvin cycle, identical to the native photorespiration pathway. GCGT transgenic plants exhibited significantly increased biomass production and grain yield, which were mainly attributed to enhanced photosynthesis due to increased chloroplastic CO2 concentrations. Despite the increases in biomass production and grain yield, GCGT transgenic plants showed a reduced seed setting rate, a phenotype previously reported for the GOC plants. Integrative transcriptomic, physiological, and biochemical assays revealed that photosynthetic carbohydrates were not transported to grains in an efficient manner, thereby reducing the seed setting rate. Taken together, our results demonstrate that the GCGT photorespiratory shortcut confers higher yield by promoting photosynthesis in rice, mainly through increasing chloroplastic CO2 concentrations.

The Kok effect revisited.

Summary The Kok effect refers to the abrupt decrease around the light compensation point in the slope of net photosynthetic rate vs irradiance. Arguably, this switch arises from light inhibition of respiration, allowing the Kok method to estimate day respiration (R d). Recent analysis suggests that increasing proportions of photorespiration (quantified as Γ*/C c, the ratio of CO2 compensation point Γ* to chloroplast CO2 concentration, C c) with irradiance explain much of the Kok effect. Also, the Kok method has been modified to account for the decrease in PSII photochemical efficiency (Φ2) with irradiance.Using a model that illustrates how varying R d, Γ*/C c, Φ2 and proportions of alternative electron transport could engender the Kok effect, we quantified the contribution of these parameters to the Kok effect measured in sunflower across various O2 and CO2 concentrations and various temperatures.Overall, the decreasing Φ2 with irradiance explained c. 12%, and the varying Γ*/C c explained c. 25%, of the Kok effect. Maximum real light inhibition of R d was much lower than the inhibition derived from the Kok method, but still increased with photorespiration.Photorespiration had a dual contribution to the Kok effect, one via the varying Γ*/C c and the other via its participation in light inhibition of R d.

Effect of elevated ozone, nitrogen availability and mesophyll conductance on the temperature responses of leaf photosynthetic parameters in poplar

Although ozone (O3) concentration and nitrogen (N) availability are well known to affect plant physiology, their impacts on the photosynthetic temperature response are poorly understood. We addressed this knowledge gap by exposing seedlings of hybrid poplar clone '107' (Populous euramericana cv. '74/76') to elevated O3 (E-O3) and N availability variation in a factorial experiment. E-O3 decreased light-saturated net photosynthesis (Asat), mesophyll conductance (gm) and apparent maximum rate of carboxylation (Vcmax, based on intercellular CO2 concentration) but not actual Vcmax (based on chloroplast CO2 concentration), and increased respiration in light (Rd) at 25°C. Nitrogen fertilization increased Asat, gm, Vcmax and the maximum rate of electron transport (Jmax), and reduced Rd at 25°C and the activation energy of actual Vcmax. No E-O3 or E-O3 x N interaction effects on the temperature response parameters were detected, simplifying the inclusion of O3 impacts on photosynthesis in vegetation models. gm peaked at 30°C, apparent Vcmax and Jmax at 32-33°C, while the optimum temperatures of actual Vcmax and Jmax exceeded the measured temperature range (15-35°C). Ignoring gm would, thus, have resulted in mistakenly attributing the decrease in Asat at high temperatures to reduced biochemical capacity rather than to greater diffusion limitation.

Effects of growth irradiance on photosynthesis and photorespiration of Phoebe bournei leaves

Light intensity is a major environmental factor affecting the growth and survival of trees in a forest. The effect of light reduction on photosynthesis and photorespiration of an evergreen broad-leaved tree, Phoebe bournei (Hemsley) Yang was examined with three levels of full light, 50.5% light, and 21.8% light. The results showed that shading led to significant increase in plant height and crown diameter. Light-saturated leaf photosynthetic rate (Amax), maximal carboxylation activity (Vcmax), maximum electron transfer rate (Jmax), stomatal conductance (gs), mesophyll conductance (gm) and chloroplast CO2 concentration (Cc) significantly increased in response to shade. Photorespiratory CO2 release rate (PR) was higher in plants grown under shade conditions than under full light. The relative limitations of gm (lm) was higher than the relative limitations of gs (ls) and the relative limitations of biochemical factors (lb) in leaves of P. bournei grown under full light, whereas lm was lower than ls and lb under shade. Our results suggest that increase of photosynthesis in P. bournei leaves grown under shade is associated with enhanced CO2 diffusion and biochemistry. And we propose that enhancement of the photorespiratory is essential for shade leaves to improve photosynthesis.

Associations between first true leaf physiology and seedling vigor in cotton under different field conditions

AbstractRapid development of the first true leaf has been suggested as a key driver of seedling vigor in cotton, yet studies demonstrating an association between first true leaf physiological processes and early season cotton growth are limited. It was hypothesized that both first true leaf area (FTLA) and photosynthetic rates would be positively associated with cotton (Gossypium hirsutum and G. barbadense) seedling vigor, and that photosynthetic component processes would differ in their contribution to net photosynthesis (An) of the first true leaf. To test these hypotheses, three different cotton cultivars were planted in the 2017 and 2018 seasons on three selected dates each year to generate broad differences in seedling vigor. Growth analysis was conducted to assess seedling vigor at 3 and 5 wk after planting, and a combination of dark‐ and light‐adapted gas exchange and fluorescence measurements were performed on the first true leaf, concurrent with growth assessments. Seedling vigor was much more strongly associated with FTLA than An on a per unit leaf area basis, suggesting that FTLA was a stronger indicator of seedling vigor. Correlation analysis between An and a number of component processes derived from combined gas exchange and fluorescence measurements indicated that An was strongly correlated with electron transport rates and stomatal conductance. However, we suggest that most of the variability we observed in An of the first true leaf was primarily driven by nonstomatal factors, since no significant correlations between An and chloroplast CO2 concentration or substomatal CO2 concentration was apparent.

Impact of pre-anthesis photosynthetic traits on yield of wheat cultivars under in vivo condition: insight based on biochemical models

Photosynthesis could be improved by increasing photosynthetic rate per unit leaf area and by optimizing light interception and utilization through modified canopy architecture, and also by modifying photosynthetic duration. The yield potential of wheat is determined by the accumulated photosynthates over the entire growing season or the fraction of the total biomass partitioned into grain at harvest. Significant correlation between grain yield and net photosynthesis rate was found under controlled environmental condition (in vitro) but similar relationship under open field conditions is not established. However, phenotyping of photosynthetic traits based on biochemical models can provide accurate physiological information, and thus connecting those traits with yield at crop level would only make crop improvement feasible involving photosynthetic manipulations. At this perspective, the present investigation was undertaken with ten diverse wheat genotypes to characterize pre-anthesis photosynthetic traits and its possible relationship with wheat productivity under in vivo condition. Genotypes were found to have high natural variations for pre-anthesis photosynthetic traits under in vivo condition. Genotypes like DPW-621-50, K 0307, HW 2044 were having high value of Vcmax, Jmax and Cc and thus expected to have better photosynthetic efficiency under ambient atmospheric CO2 condition. Rubisco specificity factor (τ) was found high in KRL-210, HD 2009, KRL-19 and thus expected to have better carboxylase activity under normal condition. Maximum genetic distance was recorded between KRL-210 and DPW-621-50. Rubisco specificity factor (τ) was found as highly correlated with fraction of photorespiration (F) and chloroplast CO2 concentration (Cc). Positive correlation of Cc with fraction of photorespiration (F), mesophyll conductance (gm) and Rubisco specificity factor (τ) indicated high value of CO2 compensation point (Г*). However, correlation between flag leaf photosynthesis rate and grain or biological yield was found non-significant in the present wheat genotypes.

Maximizing leaf carbon gain in varying saline conditions: An optimization model with dynamic mesophyll conductance

While the adverse effects of elevated salinity levels on leaf gas exchange in many crops are not in dispute, representing such effects on leaf photosynthetic rates (A) continues to draw research attention. Here, an optimization model for stomatal conductance (gc ) that maximizes A while accounting for mesophyll conductance (gm ) was used to interpret new leaf gas exchange measurements collected for five irrigation water salinity levels. A function between chloroplastic CO2 concentration (cc ) and intercellular CO2 concentration (ci ) modified by salinity stress to estimate gm was proposed. Results showed that with increased salinity, the estimated gm and maximum photosynthetic capacity were both reduced, whereas the marginal water use efficiency λ increased linearly. Adjustments of gm , λ and photosynthetic capacity were shown to be consistent with a large corpus of drought-stress experiments. The inferred model parameters were then used to evaluate the combined effects of elevated salinity and atmospheric CO2 concentration (ca ) on leaf gas exchange. For a given salinity level, increasing ca increased A linearly, but these increases were accompanied by mild reductions in gc and transpiration. The ca level needed to ameliorate A reductions due to increased salinity is also discussed using the aforementioned model calculations.

Changes in the temperature-dependency of the photosynthetic response to chloroplast CO2 concentrations of outdoor-grown Vitis vinifera cv. Shiraz vines with a mid-season crop removal

Removing fruit from horticultural trees and vines is a common practice in source-sink studies. What effect removal of sinks as fruit growth commences has on photosynthesis is much less addressed. The objective was to remove Shiraz bunches at the rapid growth stage and compare photosynthetic performance of vines with and without fruit. Photosynthetic responses to CO2 concentrations and leaf temperatures were followed across the season to determine performance with changes in fruit sink demand. Assimilation across all temperatures peaked in spring and declined progressively throughout the season. Detaching bunches removed about 160 g dry matter from vines and this effected assimilation immediately, a decrease of about 20% which was sustained over 30 days. For fruiting vines, bunch biomass was severely impeded by a sustained heat event, reducing berry sugar content by 50% and may have impacted on the vines without bunches, thus obscuring the effects of sink removal. There was no evidence that assimilation was impaired by the heat event but the sink removal reduced RuBP carboxylation and regeneration, probably resulting from an inactivation of Rubisco. In contrast to removal of sinks in early development, the late removal appeared to inhibit photosynthetic acclimation of the Shiraz vines, reflecting maturity of the leaves.

Photorespiration is the major alternative electron sink under high light in alpine evergreen sclerophyllous Rhododendron species

Owing to the high leaf mass per area, alpine evergreen sclerophyllous Rhododendron have low values of mesophyll conductance (gm). The resulting low chloroplast CO2 concentration aggravates photorespiration, which requires a higher ATP/NADPH ratio. However, the significance of photorespiration and underlying mechanisms of energy balance in these species are little known. In this study, eight alpine evergreen sclerophyllous Rhododendron species grown in a common garden were tested for their gm, electron flow to photorespiration, and energy balancing. Under saturating light, gm was the most limiting factor for net photosynthesis (AN) in all species, and the species differences in AN were primarily driven by gm rather than stomatal conductance. The total electron flow through photosystem II (ETRII) nearly equaled the electron transport required for Rubisco carboxylation and oxygenation. Furthermore, blocking electron flow to photosystem I with appropriate inhibitors showed that electron flow to plastic terminal oxidase was not observed. As a result, these studied species showed little alternative electron flow mediated by water-water cycle or plastic terminal oxidase. By comparison, the ratio of electron transport consumed by photorespiration to ETRII (JPR/ETRII), ranging from 43%∼55%, was negatively correlated to AN and gm. Furthermore, the increased ATP production required by enhanced photorespiration was regulated by cyclic electron flow around photosystem I. These results indicate that photorespiration is the major electron sink for dissipation of excess excitation energy in the alpine evergreen sclerophyllous Rhododendron species. The coordination of gm, photorespiration and cyclic electron flow is important for sustaining leaf photosynthesis.

Stomatal anatomy coordinates leaf size with Rubisco kinetics in the Balearic Limonium

Abstract Trait integration arises through both selection on functional coordination and shared developmental pathways. Different anatomical components must both work well and develop together to generate individuals with the appropriate physiology to survive and reproduce in their environment. In this study, we used a common garden experiment and Bayesian multilevel models to test whether stomatal anatomy coordinates leaf gas exchange, Rubisco kinetics, and leaf size across 10 closely related species of Limonium from the Balearic Islands. The results indicate that the anatomical determinants of maximum stomatal conductance, stomatal density and size, were functionally coordinated with Rubisco kinetics – species whose stomatal anatomy was correlated with low stomatal conductance have evolved Rubisco enzymes better adapted to low operational chloroplastic CO2 concentrations. Lower stomatal density was associated with greater leaf size, which can be explained by a greater proportion of pavement cells in large-leaved species. These results suggest that both selection for functional coordination (stomata and Rubisco kinetics) and shared development pathways (stomatal density and leaf area) likely shape patterns of trait integration between species.

Stomatal and non-stomatal limitations at different leaf temperatures to the photosynthetic process during the post-harvest period for Vitis vinifera cv. Chardonnay vines.

ABSTRACTPhotosynthesis in Vitis vinifera cv. Chardonnay leaves was measured during the post-harvest period in relation to leaf temperatures and chloroplast CO2 concentrations to evaluate the limitations to photosynthesis once the fruit sink was removed. Photosynthesis was optimal at 30°C and rates were higher at high compared to low temperatures, consistent with the current climate. Stomatal limitation of photosynthesis ranged from 20% at low temperatures to 45% at high temperatures. RuBP carboxylation (Vcmax) and regeneration (Jmax) were optimal at 39 and 36°C, but their capacities were low below 25°C accounting for low assimilation rates. Chloroplast CO2 concentration transitioning between RuBP regeneration and RuBP carboxylation-limited assimilation, increased steeply with increasing temperature. Assimilation of Chardonnay leaves was limited by RuBP regeneration below 35°C and by RuBP carboxylation above 35°C, consistent with increased photorespiration at high temperatures. Results provide a quantitative explanation for the photosynthetic performance of Chardonnay vines in the post-harvest period.