Carboxylation Capacity Research Articles

Hemp (Cannabis sativa L.) leaf photosynthesis in relation to nitrogen content and temperature: implications for hemp as a bio‐economically sustainable crop

AbstractHemp (Cannabis sativa L.) may be a suitable crop for the bio‐economy as it requires low inputs while producing a high and valuable biomass yield. With the aim of understanding the physiological basis of hemp's high resource‐use efficiency and yield potential, photosynthesis was analysed on leaves exposed to a range of nitrogen and temperature levels. Light‐saturated net photosynthesis rate (Amax) increased with an increase in leaf nitrogen up to 31.2 ± 1.9 μmol m−2 s−1 at 25 °C. The Amax initially increased with an increase in leaf temperature (TL), levelled off at 25–35 °C and decreased when TL became higher than 35 °C. Based on a C3 leaf photosynthesis model, we estimated mesophyll conductance (gm), efficiency of converting incident irradiance into linear electron transport under limiting light (κ2LL), linear electron transport capacity (Jmax), Rubisco carboxylation capacity (Vcmax), triose phosphate utilization capacity (Tp) and day respiration (Rd), using data obtained from gas exchange and chlorophyll fluorescence measurements at different leaf positions and various levels of incident irradiance, CO2 and O2. The effects of leaf nitrogen and temperature on photosynthesis parameters were consistent at different leaf positions and among different growth environments except for κ2LL, which was higher for plants grown in the glasshouse than for those grown outdoors. Model analysis showed that compared with cotton and kenaf, hemp has higher photosynthetic capacity when leaf nitrogen is <2.0 g N m−2. The high photosynthetic capacity measured in this study, especially at low nitrogen level, provides additional evidence that hemp can be grown as a sustainable bioenergy crop over a wide range of climatic and agronomic conditions.

Acclimation of light and dark respiration to experimental and seasonal warming are mediated by changes in leaf nitrogen in Eucalyptus globulus.

Quantifying the adjustments of leaf respiration in response to seasonal temperature variation and climate warming is crucial because carbon loss from vegetation is a large but uncertain part of the global carbon cycle. We grew fast-growing Eucalyptus globulus Labill. trees exposed to +3 °C warming and elevated CO2 in 10-m tall whole-tree chambers and measured the temperature responses of leaf mitochondrial respiration, both in light (RLight) and in darkness (RDark), over a 20-40 °C temperature range and during two different seasons. RLight was assessed using the Laisk method. Respiration rates measured at a standard temperature (25 °C - R25) were higher in warm-grown trees and in the warm season, related to higher total leaf nitrogen (N) investment with higher temperatures (both experimental and seasonal), indicating that leaf N concentrations modulated the respiratory capacity to changes in temperature. Once differences in leaf N were accounted for, there were no differences in R25 but the Q10 (i.e., short-term temperature sensitivity) was higher in late summer compared with early spring. The variation in RLight between experimental treatments and seasons was positively correlated with carboxylation capacity and photorespiration. RLight was less responsive to short-term changes in temperature than RDark, as shown by a lower Q10 in RLight compared with RDark. The overall light inhibition of R was ∼40%. Our results highlight the dynamic nature of leaf respiration to temperature variation and that the responses of RLight do not simply mirror those of RDark. Therefore, it is important not to assume that RLight is the same as RDark in ecosystem models, as doing so may lead to large errors in predicting plant CO2 release and productivity.

Coordinated modifications in mesophyll conductance, photosynthetic potentials and leaf nitrogen contribute to explain the large variation in foliage net assimilation rates across Quercus ilex provenances.

Leaf dry mass per unit area (LMA) has been suggested to negatively affect the mesophyll conductance to CO2 (gm), the most limiting factor for photosynthesis per unit leaf area (AN) in many evergreens. Several anatomical traits (i.e., greater leaf thickness and thicker cell walls) constraining gm could explain the negative scaling of gm and AN with LMA across species. However, the Mediterranean sclerophyll Quercus ilex L. shows a major within-species variation in functional traits (greater LMA associated with higher nitrogen content and AN) that might contrast the worldwide trends. The objective of this study was to elucidate the existence of variations in other leaf anatomical parameters determining gm and/or biochemical traits improving the capacity of carboxylation (Vc,max) that could modulate the relationship of AN with LMA across this species. The results revealed that gm was the most limiting factor for AN in all the studied Q. ilex provenances from Spain and Italy. The within-species differences in gm can be partly attributed to the variation in several leaf anatomical traits, mainly cell-wall thickness (Tcw), chloroplast thickness (Tchl) and chloroplast exposed surface area facing intercellular air spaces (Sc/S). A positive scaling of gm and AN with Vc,max was also found, associated with an increased nitrogen content per area. A strong correlation of maximum photosynthetic electron transport (Jmax) with AN further indicated a coordination between the carboxylase activity and the electron transport chain. In conclusion, we have confirmed the strong ecotypic variation in the photosynthetic performance of individual provenances of Q. ilex. Thus, the within-species increases found in AN for Q. ilex with increasing foliage robustness can be explained by a synergistic effect among anatomical (at the subcellular and cellular level) and biochemical traits, which markedly improved gm and Vc,max.

Estimation of photosynthesis traits from leaf reflectance spectra: Correlation to nitrogen content as the dominant mechanism

Numerous studies have investigated reflectance-based estimations of physico-chemical leaf traits such as the contents of light absorbing pigments, leaf mass per area, or carbon and nitrogen contents. Only few studies, however, attempted to estimate leaf traits that are more directly linked to photosynthesis. We tested the feasibility of estimating two important photosynthesis traits, the maximum carboxylation capacity (Vcmax,25) and the maximum electron transport rate (Jmax,25), from in-situ leaf reflectance spectra using approaches that are applicable also on larger spatial scales. We conducted measurements of reflectance, CO2 response curves, leaf mass per area (LMA), and nitrogen content per area (Na) for 37 temperate deciduous tree species and a total of 242 leaves from widely differing light environments. Partial least squares (PLS) regression was used to estimate Vcmax,25, Jmax,25, LMA, and Na from reflectance spectra.The results showed that both Vcmax,25 and Jmax,25 can be estimated from leaf reflectance measurements with good accuracy (R2=0.64 for Vcmax,25, R2=0.70 for Jmax,25) even for a large number of tree species and varying light environments. Detailed analysis of reflectance-based PLS and linear regression models with regard to prediction performances and regression coefficients led to the conclusion that the correlation to Na was the dominating mechanism on which the Vcmax,25 and Jmax,25 PLS models were based. The PLS regression coefficients of Na,Vcmax,25 and Jmax,25 showed clear signatures of nitrogen-related absorption features.The finding that Vcmax,25 and Jmax,25 estimations from leaf reflectance are predominantly based on their relationships to Na has important implications for large scale estimations of these photosynthesis parameters. We suggest that future studies should focus more on large scale estimation of Na from remote sensing and estimate Vcmax,25 and Jmax,25 in a separate step using their respective relationships to Na.

Photo-physiological and photo-biochemical characteristics of several herbaceous and woody species based on FvCB model

To explore the photosynthetic capacity and the leaf photosynthetic apparatus for plants with different life forms, CO2 response curves of 7 woody species and 4 herbaceous species were fitted by the modified rectangular hyperbolic model and the FvCB model, and the photosynthetic parameters, including maximum net photosynthetic rate (Pn max), maximal Rubisco carboxylation rate (Vc max), maximal electron transport rate (Jmax), day respiration (Rd), and mesophyll resistance to CO2 transport (rm), were compared among different woody species, among different herbaceous species, and between woody and herbaceous life-forms, respectively. The results showed Pn max of seven woody species descended in the order of Sapium sebiferum and Boehmeria nivea ＞ Machilus pingii and Pittosporum tobira ＞ Cyclobalanopsis glauca, Castanopsis sclerophylla, and Quercus nuttallii. Vc max of S. sebiferum, B. nivea, M. pingii, and P. tobira was significantly higher than that of C. glauca and C. sclerophylla. Jmax of woody species was in descending order as S. sebiferum ＞ B. nivea and P. tobira ＞ Q. nuttallii, C. sclerophylla, and C. glauca. rm of M. pingii and C. sclerophylla was higher than that of S. sebiferum, P. tobira and B. nivea. Pn max of Phytolacca acinosa was significantly higher than that of Ageratum conyzoides and Achyranthes aspera. There was no significant difference in Vc max among 4 herbaceous species. Jmax of P. acinosa was higher than that of A. conyzoides. rm of S. nigrum and A. aspera was higher than that of A. conyzoides. Rd of P. acinosa was higher than that of A. conyzoides and A. aspera. The photosynthetic parameters (Pn max, Vc max, Jmax and rm) of woody species were significantly higher than those of herbaceous species, but no significant difference was found in Rd between woody and herbaceous species. In conclusion, the difference in photosynthetic capacity among different species and between the two plant life-forms resulted from the difference in Rubisco carboxylation capacity, electron transport capacity, and mesophyll resistance among these species.

Photosynthetic acclimation to warming in tropical forest tree seedlings.

Tropical forests have a mitigating effect on man-made climate change by acting as a carbon sink. For that effect to continue, tropical trees will have to acclimate to rising temperatures, but it is currently unknown whether they have this capacity. We grew seedlings of three tropical tree species over a range of temperature regimes (TGrowth = 25, 30, 35 °C) and measured the temperature response of photosynthetic CO2 uptake. All species showed signs of acclimation: the temperature-response curves shifted, such that the temperature at which photosynthesis peaked (TOpt) increased with increasing TGrowth. However, although TOpt shifted, it did not reach TGrowth at high temperature, and this difference between TOpt and TGrowth increased with increasing TGrowth, indicating that plants were operating at supra-optimal temperatures for photosynthesis when grown at high temperatures. The high-temperature CO2 compensation point did not increase with TGrowth. Hence, temperature-response curves narrowed with increasing TGrowth. TOpt correlated with the ratio of the RuBP regeneration capacity over the RuBP carboxylation capacity, suggesting that at high TGrowth photosynthetic electron transport rate associated with RuBP regeneration had greater control over net photosynthesis. The results show that although photosynthesis of tropical trees can acclimate to moderate warming, carbon gain decreases with more severe warming.

Plant functional traits and canopy structure control the relationship between photosynthetic CO2 uptake and far-red sun-induced fluorescence in a Mediterranean grassland under different nutrient availability.

Sun-induced fluorescence (SIF) in the far-red region provides a new noninvasive measurement approach that has the potential to quantify dynamic changes in light-use efficiency and gross primary production (GPP). However, the mechanistic link between GPP and SIF is not completely understood. We analyzed the structural and functional factors controlling the emission of SIF at 760nm (F760 ) in a Mediterranean grassland manipulated with nutrient addition of nitrogen (N), phosphorous (P) or nitrogen-phosphorous (NP). Using the soil-canopy observation of photosynthesis and energy (SCOPE) model, we investigated how nutrient-induced changes in canopy structure (i.e. changes in plant forms abundance that influence leaf inclination distribution function, LIDF) and functional traits (e.g. N content in dry mass of leaves, N%, Chlorophyll a+b concentration (Cab) and maximum carboxylation capacity (Vcmax )) affected the observed linear relationship between F760 and GPP. We conclude that the addition of nutrients imposed a change in the abundance of different plant forms and biochemistry of the canopy that controls F760 . Changes in canopy structure mainly control the GPP-F760 relationship, with a secondary effect of Cab and Vcmax . In order to exploit F760 data to model GPP at the global/regional scale, canopy structural variability, biodiversity and functional traits are important factors that have to be considered.

Characterization of photosynthetic acclimation in Phoenix dactylifera by a modified Arrhenius equation originally developed for leaf respiration

Instantaneous temperature responses of leaf respiration and photosynthesis can be described by the same equation, to help understand acclimation of primary metabolism to altered growth temperature and water supply. We used a three-parameter, modified Arrhenius equation, originally developed for leaf respiration, to characterize A/T curves of Date Palm and acclimation to elevated growth temperature and water deprivation: $$A_{\text{T}} = A_{\text{ref}} \times e^{{\left[ {\frac{{E_{\text{o}} (\text{Ref}_{\text{A}} )}}{\Re } \times \left( {\frac{{T - T_{\text{ref}} }}{{T \times T_{\text{ref}} }}} \right) + \delta_{\text{A}} \times \left( {\frac{{T - T_{\text{ref}} }}{{T \times T_{\text{ref}} }}} \right)^{2} } \right]}}$$ where A ref is the net CO2-assimilation (A) at fixed reference temperature (T ref), E o(RefA) is the activation energy of A close to T ref, and δ A describes the change of E o with increasing incubation temperature (T). Similar to respiration parameters, E o(RefA) and δ A-values were strongly correlated. Symmetry of A/T curves, i.e., constancy of dE o/dT between 20–45 °C incubation temperatures, suggests close coordination of component processes underlying A. This symmetry remained at high growth temperature, despite large reductions in biochemical capacity for P i regeneration relative to carboxylation capacity (i.e., increased abundance of RubisCO activase). Acclimation to higher temperature caused pronounced reductions in physiological capacity of respiration (R Cap) (type II acclimation, determined via gas exchange measurements). Reductions in R Cap were not a result of limitations in substrate availability (i.e., pyruvate), but were related to lower abundances of mitochondrial enzymes in well-watered plants (i.e., pyruvate dehydrogenase and cytochrome oxidase). Water shortage led to sucrose accumulation, with modest reductions in mitochondrial enzyme pools. R Cap remained low when growth temperature was increased.

The inhibition of photosynthesis under water deficit conditions is more severe in flecked than uniform irradiance in rice (Oryza sativa) plants

Water deficit is considered the major environmental factor limiting leaf photosynthesis, and the physiological basis for decreased photosynthesis under water deficit has been intensively studied with steady irradiance. Leaves within a canopy experience a highly variable light environment in magnitude and time, but the effect of water deficit on photosynthesis in fluctuating irradiance is not well understood. Two rice cultivars with different drought tolerance, Champa and Yangliangyou 6 (YLY6), were hydroponically grown under well-watered, 15% (m/v) and 20% PEG (polyethylene glycol, 6000Da) induced water deficit conditions. The inhibition of steady-state photosynthesis in Champa is more severe than YLY6. The maximum Rubisco carboxylation capacity (Vcmax) and maximum electron transport capacity (Jmax) were decreased under 20% PEG treatment in Champa, whereas less or no effect was observed in YLY6. The induction state (IS%, which indicates photosynthesis capacity after exposure of low-light period) of both leaf photosynthetic rate (A) and stomatal conductance (gs) was highly correlated, and was significantly decreased under water deficit conditions in both cultivars. Water deficit had no significant effect on the time required to reach 50 or 90% of the maximum photosynthetic rate (T50%,A and T90%,A) after exposure to high-light level, but significantly led to a greater decrease in photosynthetic rate in the low-light period under flecked irradiance (Amin-fleck) relative to photosynthetic rate in the same light intensity of continuously low-light period (Ainitial). The lower IS% of A and more severe decrease in Amin-fleck relative to Ainitial will lead to a more severe decrease in integrated CO2 fixation under water deficit in flecked compared with uniform irradiance.

Individual and interactive effects of drought and heat on leaf physiology of seedlings in an economically important crop.

Heat waves in combination with drought are predicted to occur more frequently with climate warming, yet their interactive effects on crop carbon and water balance are still poorly understood. Hence, research on the capacity of crops to withstand and recover from the combined stress is urgently needed. This study investigated the effects of drought and heat wave on a crop species as well as the recovery from the combined stress. Seedlings were grown in growth chambers under two soil water conditions (i.e. well watered and drought stress) at ambient temperature (26°C) for 10 days. Afterwards, half of the seedlings were exposed to a 7-day 42°C heat wave. All the drought-stressed seedlings were then rehydrated upon relief of the heat wave. Leaf gas exchange, the maximum carboxylation capacity (V cmax), plant growth, relative chlorophyll content and leaf water potential were examined during the experimental period. The heat wave reduced leaf gas exchange rates, V cmax and relative chlorophyll content, while it had no impacts on leaf water potential. In contrast, drought stress led to greater reductions in leaf gas exchange rates, growth and water potential than heat wave alone. Seedlings underwent a greater degree of stress in the combination of drought and heat wave than under the single drought treatment. The recovery of leaf gas exchange from drought stress lagged behind the water potential recovery and was delayed by heat wave. Our results show that drought stress had a predominant role in determining plant physiological responses and the negative impacts of drought stress were exacerbated by heat wave. The greater stress in the combination of drought and heat wave translated into the slower recovery of leaf gas exchange. Therefore, drought combined with heat wave may induce greater risks on crops under future climates.

Variation among Soybean Cultivars in Mesophyll Conductance and Leaf Water Use Efficiency.

Improving water use efficiency (WUE) may prove a useful way to adapt crop species to drought. Since the recognition of the importance of mesophyll conductance to CO2 movement from inside stomatal pores to the sites of photosynthetic carboxylation, there has been interest in how much intraspecific variation in mesophyll conductance (gm) exists, and how such variation may impact leaf WUE within C3 species. In this study, the gm and leaf WUE of fifteen cultivars of soybeans grown under controlled conditions were measured under standardized environmental conditions. Leaf WUE varied by a factor of 2.6 among the cultivars, and gm varied by a factor of 8.6. However, there was no significant correlation (r = −0.047) between gm and leaf WUE. Leaf WUE was linearly related to the sub-stomatal CO2 concentration. The value of gm affected the ratio of maximum Rubisco carboxylation capacity calculated from the sub-stomatal CO2 concentration to that calculated from the CO2 concentration at the site of carboxylation. That is, variation in gm affected the efficiency of Rubisco carboxylation, but not leaf WUE. Nevertheless, there is considerable scope for genetically improving soybean leaf water use efficiency.

Reductive carboxylation is a major metabolic pathway in the retinal pigment epithelium

The retinal pigment epithelium (RPE) is a monolayer of pigmented cells that requires an active metabolism to maintain outer retinal homeostasis and compensate for oxidative stress. Using 13C metabolic flux analysis in human RPE cells, we found that RPE has an exceptionally high capacity for reductive carboxylation, a metabolic pathway that has recently garnered significant interest because of its role in cancer cell survival. The capacity for reductive carboxylation in RPE exceeds that of all other cells tested, including retina, neural tissue, glial cells, and a cancer cell line. Loss of reductive carboxylation disrupts redox balance and increases RPE sensitivity to oxidative damage, suggesting that deficiencies of reductive carboxylation may contribute to RPE cell death. Supporting reductive carboxylation by supplementation with an NAD+ precursor or its substrate α-ketoglutarate or treatment with a poly(ADP ribose) polymerase inhibitor protects reductive carboxylation and RPE viability from excessive oxidative stress. The ability of these treatments to rescue RPE could be the basis for an effective strategy to treat blinding diseases caused by RPE dysfunction.

A novel ethylene responsive factor CitERF13 plays a role in photosynthesis regulation

Ethylene responsive factors (ERFs) act as critical downstream components of the ethylene signalling pathway in regulating plant development and stress responses. However little is known about its role in regulation of photosynthesis. Here, we identified an ethylene-inducible ERF gene in citrus, CitERF13. Transient over-expression of CitERF13 in N. tabacum leaves, resulted in a significant decrease in net photosynthetic rate. Closer examination of photosynthetic activity of PSII and PSI indicated that CitERF13 overexpression led to declines of Fv/Fm, Y(II) and Y(I). However, change in NPQ was less pronounced. CitERF13 overexpression also significantly reduced Vc,max, Jmax and AQY, indicating inhibition of the Calvin cycle. The expression of photosynthesis-related genes was suppressed to a variable extent in leaf blades transiently over-expressing CitERF13. CitERF13 transient overexpression in tobacco or citrus both resulted in a decline of Chlorophyll content and CitERF13 overexpressing tobacco leaf disc was more susceptible to chlorosis in response to MV-mediated oxidative stress. The results suggest that CitERF13 is potentially involved in suppressing photosynthesis through multiple pathways, for instance, inhibiting photochemical activity of photosynthesis, CO2 carboxylation capacity and chlorophyll metabolism.

Rice (Oryza sativa L.) with reduced chlorophyll content exhibit higher photosynthetic rate and efficiency, improved canopy light distribution, and greater yields than normally pigmented plants

The net photosynthesis and photosynthetic N use efficiency (PNUE) of a rice mutant with reduced chlorophyll synthesis were much higher than those of its wild type (WT). To understand the physiological mechanism, we conducted a model analysis and pot and field experiments under different N rates. Both the model analysis and the experiments showed that low chlorophyll content did not impede the development of chloroplast in the mutant; the reduced chlorophyll content and high expression levels of chlorophyll binding protein genes suggested the smaller light-harvesting chlorophyll antenna size, and higher expression levels of thylakoid membrane protein genes indicated high potential for chloroplast electron transport in the mutant; the small antenna size avoided excessive light absorption, and increased PSII efficiency, resulting in high electron transport rate in the mutant. The higher Rubisco content in the mutant ensured a higher Rubisco capacity for carboxylation. The high CO2 conductivity of stomata and mesophyll ensured availability of CO2 at the site of carboxylation. All these contributed to the increased photosynthetic rate in the mutant. Leaf N content was found higher in the mutant than in the WT, however, the extent to which the photosynthetic rate was increased was much larger than that of leaf N content, leading to improved PNUE in mutant. Furthermore vertical light distribution was improved in the canopy of the mutant, as greater transmittance of sunlight within the canopy was observed in the density experiment in the field. The mutant showed yield advantages considering its shorter growth duration, indicating the higher photosynthetic capacity and improved canopy light distribution with reduced chlorophyll synthesis. The mutant plant could potentially be used to improve photosynthetic efficiency.

Corrigendum.

New PhytologistVolume 212, Issue 3 p. 792-792 CorrigendumFree Access Corrigendum This article corrects the following: A test of the ‘one-point method’ for estimating maximum carboxylation capacity from field-measured, light-saturated photosynthesis Martin G. De Kauwe, Yan-Shih Lin, Ian J. Wright, Belinda E. Medlyn, Kristine Y. Crous, David S. Ellsworth, Vincent Maire, I. Colin Prentice, Owen K. Atkin, Alistair Rogers, Ülo Niinemets, Shawn P. Serbin, Patrick Meir, Johan Uddling, Henrique F. Togashi, Lasse Tarvainen, Lasantha K. Weerasinghe, Bradley J. Evans, F. Yoko Ishida, Tomas F. Domingues, Volume 210Issue 3New Phytologist pages: 1130-1144 First Published online: December 31, 2015 First published: 02 September 2016 https://doi.org/10.1111/nph.14172Citations: 4 Author for correspondence: Martin G. De Kauwe Tel: +61 2 9850 9256 Email: mdekauwe@gmail.com AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat New Phytologist 210 (2016), 1130–1144 Since its publication, the authors of De Kauwe et al. (2016) have drawn to our attention that there is an error in Eqn 3 in their article. The correct Eqn 3 is shown below. We apologize to our readers for this mistake. Reference De Kauwe MG, Lin Y-S, Wright IJ, Medlyn BE, Crous KY, Ellsworth DS, Maire V, Prentice IC, Atkin OK, Rogers A et al. 2016. A test of the ‘one-point method’ for estimating maximum carboxylation capacity from field-measured, light-saturated photosynthesis. New Phytologist 210: 1130– 1144. (Eqn 3) Citing Literature Volume212, Issue3November 2016Pages 792-792 ReferencesRelatedInformation

Differential effects of ozone on photosynthesis of winter wheat among cultivars depend on antioxidative enzymes rather than stomatal conductance

Five modern cultivars of winter wheat (Triticum aestivum L.): Yangmai16 (Y16), Yangmai 15 (Y15), Yangfumai 2 (Y2), Yannong 19 (Y19) and Jiaxing 002 (J2) were investigated to determine the impacts of elevated ozone concentration (E-O3) on photosynthesis-related parameters and the antioxidant system under fully open-air field conditions in China. The plants were exposed to E-O3 at 1.5 times the ambient ozone concentration (A-O3) from the initiation of tillering to final harvest. Pigments, gas exchange rates, chlorophyll a fluorescence, antioxidants contents, antioxidative enzyme activity and lipid oxidation were measured in three replicated plots throughout flag leaf development. Results showed that significant O3 effects on most variables were only found during the mid-grain filling stage. Across five cultivars, E-O3 significantly accelerated leaf senescence, as indicated by increased lipid oxidation as well as faster declines in pigment amounts and photosynthetic rates. The lower photosynthetic rates were mainly due to non-stomatal factors, e.g. lower maximum carboxylation capacity and electron transport rates. There were strong interactions between O3 and cultivar in photosynthetic pigments, light-saturated photosynthesis rate and chlorophyll a fluorescence with O3-sensitive (Y19, Y2 and Y15) and O3-tolerant (J2, Y16) cultivars being clearly differentiated in their responses to E-O3. E-O3 significantly influenced the antioxidative enzymes but not antioxidant contents. Significant interactions between O3 and cultivar were found in antioxidative enzymes, such as SOD and CAT, but not in stomatal conductance (gs). Therefore, it can be concluded that antioxidative enzymes rather than gs or antioxidants are responsible for the differential responses to E-O3 among cultivars. These findings provide important information for the development of accurate modeling O3 effects on crops, especially with respect to the developmental stage when O3 damage to photosynthesis becomes manifest.

Older Thinopyrum intermedium (Poaceae) plants exhibit superior photosynthetic tolerance to cold stress and greater increases in two photosynthetic enzymes under freezing stress compared with young plants.

Effects of plant age on resource acquisition and stress tolerance processes is a largely unstudied subject in herbaceous perennials. In a field experiment, we compared rates of photosynthesis (A), ribulose-1,5-bisphosphate (RuBP) carboxylation capacity (V Cmax), maximum electron transport rate (J max), and triose phosphate utilization (TPU), as well as concentrations of Rubisco and sucrose-phosphate synthase (SPS) in 5-year-old and 2-year-old intermediate wheatgrass (Thinopyrum intermedium) under both optimal growing conditions and cold stress in early spring and autumn. This species is a relative of wheat undergoing domestication. An additional experiment compared photosynthetic rates in different cohorts at mid-season and under colder conditions. We hypothesized that photosynthetic capacity in older plants would be lower under favorable conditions but higher under cold stress. Our hypothesis was generally supported. Under cold stress, 5-year-old plants exhibited higher A, TPU, and temperature-adjusted V Cmax than younger plants, as well as 50% more SPS and 37% more Rubisco. In contrast, at mid-season, photosynthetic capacities in older plants were lower than in younger plants in one experiment, and similar in the other, independent of differences in water status. Both cohorts increased A, temperature-adjusted TPU and J max, [Rubisco], and [SPS] under cold stress, but changes were greater in older plants. Photosynthetic differences were largest at 1.2 ºC in very early spring, where older plants had 200% higher A and maintained up to 17% of their peak photosynthetic capacity. We find evidence of increased cold tolerance in older cohorts of wheatgrass, consistent with a growing body of research in woody perennials.

Comparison of solar-induced chlorophyll fluorescence, light-use efficiency, and process-based GPP models in maize.

Accurately quantifying cropland gross primary production (GPP) is of great importance to monitor cropland status and carbon budgets. Satellite-based light-use efficiency (LUE) models and process-based terrestrial biosphere models (TBMs) have been widely used to quantify cropland GPP at different scales in past decades. However, model estimates of GPP are still subject to large uncertainties, especially for croplands. More recently, space-borne solar-induced chlorophyll fluorescence (SIF) has shown the ability to monitor photosynthesis from space, providing new insights into actual photosynthesis monitoring. In this study, we examined the potential of SIF data to describe maize phenology and evaluated three GPP modeling approaches (space-borne SIF retrievals, a LUE-based vegetation photosynthesis model [VPM], and a process-based soil canopy observation of photochemistry and energy flux [SCOPE] model constrained by SIF) at a maize (Zea mays L.) site in Mead, Nebraska, USA. The result shows that SIF captured the seasonal variations (particularly during the early and late growing season) of tower-derived GPP (GPP_EC) much better than did satellite-based vegetation indices (enhanced vegetation index [EVI] and land surface water index [LSWI]). Consequently, SIF was strongly correlated with GPP_EC than were EVI and LSWI. Evaluation of GPP estimates against GPP_EC during the growing season demonstrated that all three modeling approaches provided reasonable estimates of maize GPP, with Pearson's correlation coefficients (r) of 0.97, 0.94, and 0.93 for the SCOPE, VPM, and SIF models, respectively. The SCOPE model provided the best simulation of maize GPP when SIF observations were incorporated through optimizing the key parameter of maximum carboxylation capacity (Vcmax). Our results illustrate the potential of SIF data to offer an additional way to investigate the seasonality of photosynthetic activity, to constrain process-based models for improving GPP estimates, and to reasonably estimate GPP by integrating SIF and GPP_EC data without dependency on climate inputs and satellite-based vegetation indices.

Investigating the European beech (Fagus sylvatica L.) leaf characteristics along the vertical canopy profile: leaf structure, photosynthetic capacity, light energy dissipation and photoprotection mechanisms.

Forest functionality and productivity are directly related to canopy light interception and can be affected by potential damage from high irradiance. However, the mechanisms by which leaves adapt to the variable light environments along the multilayer canopy profile are still poorly known. We explored the leaf morphophysiological and metabolic responses to the natural light gradient in a pure European beech (Fagus sylvatica L.) forest at three different canopy heights (top, middle and bottom). Structural adjustment through light-dependent modifications in leaf mass per area was the reason for most of the variations in photosynthetic capacity. The different leaf morphology along the canopy influenced nitrogen (N) partitioning, water- and photosynthetic N-use efficiency, chlorophyll (Chl) fluorescence and quali-quantitative contents of photosynthetic pigments. The Chl a to Chl b ratio and the pool of xanthophyll-cycle pigments (VAZ) increased at the highest irradiance, as well as lutein and β-carotene. The total pool of ascorbate and phenols was higher in leaves of the top and middle canopy layers when compared with the bottom layer, where the ascorbate peroxidase was relatively more activated. The non-photochemical quenching was strongly and positively related to the VAZ/(Chl a + b) ratio, while Chl a/Chl b was related to the photochemical efficiency of photosystem II. Along the multilayer canopy profile, the high energy dissipation capacity of leaves was correlated to an elevated redox potential of antioxidants. The middle layer gave the most relevant contribution to leaf area index and carboxylation capacity of the canopy. In conclusion, a complex interplay among structural, physiological and biochemical traits drives the dynamic leaf acclimation to the natural gradients of variable light environments along the tree canopy profile. The relevant differences observed in leaf traits within the canopy positions of the beech forest should be considered for improving estimation of carbon fluxes in multilayer canopy models of temperate forests.

Light dependence of carboxylation capacity for C<sub>3</sub> photosynthesis models

C3 photosynthesis at high light is often modeled by assuming limitation by the maximum capacity of Rubisco carboxylation (VCmax) at low CO2 concentrations, by electron transport capacity (Jmax) at higher CO2 concentrations, and sometimes by triose-phosphate utilization rate at the highest CO2 concentrations. Net photosynthetic rate (P N) at lower light is often modeled simply by assuming that it becomes limited by electron transport (J). However, it is known that Rubisco can become deactivated at less than saturating light, and it is possible that P N at low light could be limited by the rate of Rubisco carboxylation (V C) rather than J. This could have important consequences for responses of P N to CO2 and temperature at low light. In this work, P N responses to CO2 concentration of common bean, quinoa, and soybean leaves measured over a wide range of temperatures and PPFDs were compared with rates modeled assuming either VC or J limitation at limiting light. In all cases, observed rates of P N were better predicted by assuming limitation by VC rather than J at limiting light both below and above the current ambient CO2. One manifestation of this plant response was that the relative stimulation of P N with increasing the ambient CO2 concentration from 380 to 570 µmol mol-1 did not decrease at less than saturating PPFDs. The ratio of VC to VCmax at each lower PPFD varied linearly with the ratio of P N at low PPFD to P N at high PPFD measured at 380 µmol(CO2) mol-1 in all cases. This modification of the standard C3 biochemical model was much better at reproducing observed responses of light-limited P N to CO2 concentrations from pre-industrial to projected future atmospheric concentrations.