Stomatal Limitation Of Photosynthesis Research Articles

Improved stomatal opening enhances photosynthetic rate and biomass production in fluctuating light.

It has been reported that stomatal conductance often limits the steady-state photosynthetic rate. On the other hand, the stomatal limitation of photosynthesis in fluctuating light remains largely unknown, although in nature light fluctuates due to changes in sun position, cloud cover, and the overshadowing canopy. In this study, we analysed three mutant lines of Arabidopsis with increased stomatal conductance to examine to what extent stomatal opening limits photosynthesis in fluctuating light. The slac1 (slow anion channel-associated 1) and ost1 (open stomata 1) mutants with stay-open stomata, and the PATROL1 (proton ATPase translocation control 1) overexpression line with faster stomatal opening responses exhibited higher photosynthetic rates and plant growth in fluctuating light than the wild-type, whereas these four lines showed similar photosynthetic rates and plant growth in constant light. The slac1 and ost1 mutants tended to keep their stomata open in fluctuating light, resulting in lower water-use efficiency (WUE) than the wild-type. However, the PATROL1 overexpression line closed stomata when needed and opened stomata immediately upon irradiation, resulting in similar WUE to the wild-type. The present study clearly shows that there is room to optimize stomatal responses, leading to greater photosynthesis and biomass accumulation in fluctuating light in nature.

Historical changes in the stomatal limitation of photosynthesis: empirical support for an optimality principle.

The ratio of leaf internal (ci ) to ambient (ca ) partial pressure of CO2 , defined here as χ, is an index of adjustments in both leaf stomatal conductance and photosynthetic rate to environmental conditions. Measurements and proxies of this ratio can be used to constrain vegetation model uncertainties for predicting terrestrial carbon uptake and water use. We test a theory based on the least-cost optimality hypothesis for modelling historical changes in χ over the 1951-2014 period, across different tree species and environmental conditions, as reconstructed from stable carbon isotopic measurements across a global network of 103 absolutely dated tree-ring chronologies. The theory predicts optimal χ as a function of air temperature, vapour pressure deficit, ca and atmospheric pressure. The theoretical model predicts 39% of the variance in χ values across sites and years, but underestimates the intersite variability in the reconstructed χ trends, resulting in only 8% of the variance in χ trends across years explained by the model. Overall, our results support theoretical predictions that variations in χ are tightly regulated by the four environmental drivers. They also suggest that explicitly accounting for the effects of plant-available soil water and other site-specific characteristics might improve the predictions.

Mild water stress-induced priming enhance tolerance to Rosellinia necatrix in susceptible avocado rootstocks

BackgroundWhite root rot (WRR) disease caused by Rosellinia necatrix is one of the most important threats affecting avocado orchards in temperate regions. The eradication of WRR is a difficult task and environmentally friendly control methods are needed to lessen its impact. Priming plants with a stressor (biotic or abiotic) can be a strategy to enhance plant defense/tolerance against future stress episodes but, despite the known underlying common mechanisms, few studies use abiotic-priming for improving tolerance to forthcoming biotic-stress and vice versa (‘cross-factor priming’). To assess whether cross-factor priming can be a potential method for enhancing avocado tolerance to WRR disease, ‘Dusa’ avocado rootstocks, susceptible to R. necatrix, were subjected to two levels of water stress (mild-WS and severe-WS) and, after drought-recovery, inoculated with R. necatrix. Physiological response and expression of plant defense related genes after drought-priming as well as the disease progression were evaluated.ResultsWater-stressed avocado plants showed lower water potential and stomatal limitations of photosynthesis compared to control plants. In addition, NPQ and qN values increased, indicating the activation of energy dissipating mechanisms closely related to the relief of oxidative stress. This response was proportional to the severity of the water stress and was accompanied by the deregulation of pathogen defense-related genes in the roots. After re-watering, leaf photosynthesis and plant water status recovered rapidly in both treatments, but roots of mild-WS primed plants showed a higher number of overexpressed genes related with plant defense than severe-WS primed plants. Disease progression after inoculating primed plants with R. necatrix was significantly delayed in mild-WS primed plants.ConclusionsThese findings demonstrate that mild-WS can induce a primed state in the WRR susceptible avocado rootstock ‘Dusa’ and reveal that ‘cross-factor priming’ with water stress (abiotic stressor) is effective for increasing avocado tolerance against R. necatrix (biotic stressor), underpinning that plant responses against biotic and abiotic stress rely on common mechanisms. Potential applications of these results may involve an enhancement of WRR tolerance of current avocado groves and optimization of water use via low frequency deficit irrigation strategies.

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.

Anatomical and physiological adjustments of pubescent oak (Quercus pubescens Willd.) from two adjacent sub-Mediterranean ecosites

Patterns of drought severity in natural ecosystems depend on ecosite characteristics, particularly on local soil water availability and microclimate (evaporative demand). At the sites with less favourable conditions, the plasticity of fundamental drought adaptive traits can be exceeded under severe drought, even in well adapted species.In our study, anatomical and physiological adjustments to drought were studied in pubescent oak (Quercus pubescens Willd.) growing on adjacent sub-Mediterranean plots (ecosites) differing in bedrock and soil type (limestone with rendzic leptosol, low water retention - L; transition from limestone to flysch with eutric cambisol, high water retention - F).In both years of the study, 2015 and 2016, in the part of the season with sufficient water supply, sap flow density (SFD) was higher in oaks growing on L than those growing on F. With decreasing soil water content (SWC) in July, SFD was reduced on both plots, most in trees growing on limestone. In the period of severe drought, SFD in trees growing on L was decoupled from the changes of VPD, due to increased employment of water conservation strategies, such as decreases in stomatal conductance. This was further translated into stomatal limitation of photosynthesis and contributed to reduced radial stem growth.The seasonal dynamics of traits indicate that Q. pubescens trees use a predisposed anatomical strategy (average size of vessels in different years), adjusted to average ecosite conditions. At the same time, trees respond to local conditions and interanual variability of drought by shortening/prolonging the production and growth of vessels and by adjusting leaf functioning (stomatal conductance, sap flow) in accordance with hydraulic properties.

Photosynthetic Traits of Plants and the Biochemical Profile of Tomato Fruits Are Influenced by Grafting, Salinity Stress, and Growing Season.

Changes in the photosynthetic traits of plants and metabolic composition of fruits of two tomato cultivars, grafted onto two rootstocks, grown in three salinity levels were studied in two growing periods during the season. Increased salinity stress conditions lowered water potential, stomatal conductance, and transpiration rate of grafted tomato plants, in both growing periods. Water deficit induced stomatal closure, which resulted in stomatal limitation of photosynthesis. The proline content in tomato leaves increased and was closely correlated with salinity. Some of the quality parameters of tomato fruits were affected by rootstock. The sugar/acid ratio was the highest in fruits of 'Belle'/'Maxifort' grafts. With increasing salt stress conditions from 40 to 60 mM NaCl, the lycopene content increased and ascorbic acid content decreased in fruits of 'Gardel'/'Maxifort' grafts, indicating the ability of this scion/rootstock combination to mitigate the toxicity effect of salinity stress. A higher phenolics concentration in fruits from the first growing period may be an additional indicator of stress, caused by higher temperatures and solar radiation, compared with the later period.

Influence of the root endophyte Piriformospora indica on the plant water relations, gas exchange and growth of Chenopodium quinoa at limited water availability

AbstractThis study aimed to evaluate the ability of Piriformospora indica to colonize the root of Chenopodium quinoa and to verify whether this endosymbiont can improve the growth, performance and drought resistance of this species. The study delivered, for the first time, evidence for successful colonization of P. indica in quinoa. Hence, pot experiment was conducted in the greenhouse, where inoculated and non‐inoculated plants were subjected to ample (40%–50% WHC) and deficit (15%–20%WHC) irrigation treatments. Drought adversely influenced the plant growth, leading to decline the total plant biomass by 74%. This was linked to an impaired photosynthetic activity (caused by lower gs and Ci/Ca ratio; stomatal limitation of photosynthesis) and a higher risk of ROS production (enhanced ETR/Agross ratio). P. indica colonization improved quinoa plant growth, with total biomass increased by 8% (controls) and 76% (drought‐stressed plants), confirming the growth‐promoting activity of P. indica. Fungal colonization seems to diminish drought‐induced growth hindrance, likely, through an improved water balance, reflected by the higher leaf ψw and gs. Additionally, stomatal limitation of photosynthesis was alleviated (indicated by enhanced Ci/Ca ratio and Anet), so that the threat of oxidative stress was minimized (decreased ETR/Agross). These results infer that symbiosis with P. indica could negate some of the detrimental effects of drought on quinoa growth, a highly desired feature, in particular at low water availability.

Elevated atmospheric CO2 concentration leads to different salt resistance mechanisms in a C3 (Chenopodium quinoa) and a C4 (Atriplex nummularia) halophyte

This study aimed at investigating the effects of elevated atmospheric CO2 concentration on the salt resistance of the C3 halophyte Chenopodium quinoa and the C4 halophyte Atriplex nummularia. Plants were irrigated with different salinity levels according to their individual range of resistance (0, 100, 300, 500, and in the case of A. nummularia additionally 750molm−3 NaCl) under ambient and elevated (540ppm) CO2. In C. quinoa, NaCl salinity led to a decreased stomatal conductance, Ci, and net CO2 assimilation rate (stomatal limitation of photosynthesis) and consequently to a higher risk of ROS production, indicated by an increased ETR/Agross ratio. Due to its C4 metabolism, A. nummularia exhibited higher net photosynthetic rates and a lower threat of oxidative stress (lower ETR/Agross ratio), leading to a distinctly higher salt resistance.Elevated atmospheric CO2 supported the photosynthesis of both species; however, the salt resistance of quinoa stayed at a distinctly lower level than the one of A. nummularia. In C. quinoa (C3), the stomatal limitation of photosynthesis was ameliorated (indicated by increased Ci values and Anet), so that the threat of oxidative stress was reduced (decrease in ETR/Agross; direct CO2 effect). In A. nummularia (C4), CO2 enrichment did not stimulate Anet. However, the generation of ROS could be avoided by a reduction in electron transfer (indirect non-stomatal effect), resulting in a lower ETR/Agross ratio.The results imply that both species will be suited as cash crop halophytes in a future CO2-rich world.

Effects of S-Abscisic Acid and (+)-8′-Acetylene Abscisic Acid on Fruit Set and Stomatal Conductance in Apple

Fruit set of apple can be reduced by cloudy weather, short-term shade treatments, or application of photosynthetic inhibitors when the young fruit are ≈8 to 15 mm in diameter, indicating that fruit are sensitive to a transient carbohydrate stress during this period. We investigated the potential for S-abscisic acid (ABA) and an ABA analog [(+)-8′-acetylene ABA] to chemically thin apple fruit by causing a stomatal limitation of photosynthesis. Stomatal conductance (gS) of ‘Imperial Gala’/M.7 was reduced by 60% 3 h after application of 250 mg·L−1 ABA or 25 mg·L−1 (+)-8′-acetylene ABA. Stomatal conductance began to recover 4 days after application but did not return to control levels until 19 days after treatment. Application of 250 mg·L−1 ABA combined with 100 mg·L−1 6-benzyladenine (6-BA) when mean fruit diameter was ≈10 mm reduced fruit set of ‘Gala’/M.7 but not ‘Pink Lady™’/M.7 or ‘Morganspur Delicious’/MM.111. Fruit set of ‘Pink Lady™’/M.7 was reduced by application of 20 mg·L−1 (+)-8′-acetylene ABA + 100 mg·L−1 6-BA at full bloom or 10 mg·L−1 (+)-8′-acetylene ABA + 100 mg·L−1 6-BA at the 10-mm fruit diameter stage. Fruit set of ‘Morganspur Delicious’/MM.111 was reduced by application of 25 mg·L−1 (+)-8′-acetylene ABA, either alone or in combination with 75 mg·L−1 6-BA, at the 10-mm fruit diameter stage. ABA and (+)-8′-acetylene ABA triggered leaf abscission at rates above 250 mg·L−1 and 25 mg·L−1, respectively. Fruit set and gS data from the present studies indicate the biological activity of (+)-8′-acetylene ABA is 10-fold higher than ABA. These results suggest that ABA and (+)-8′-acetylene ABA reduced fruit set by causing a stomatal limitation in photosynthesis that resulted in a transient carbohydrate stress. Thinning responses to ABA and (+)-8′-acetylene ABA at the concentrations used in these experiments were reduced compared with standard concentrations of currently available chemical thinning agents. However, increasing the concentration of ABA or (+)-8′-acetylene ABA to levels that would achieve comparable thinning are also likely to result in unacceptable leaf abscission.

Photosynthetic limitations imposed by self-shading in field-grown sugarcane varieties

As a result of self-shading, a large fraction of a plant's leaf area is exposed to low light levels, causing photosynthesis to occur under limiting light conditions. The aim of this study was to evaluate photosynthesis in the canopy of two sugarcane varieties grown under field conditions in view of the effects of self-shading on photosynthetic metabolism. Photosynthesis response curves to increases in CO2 partial pressure (A/Ci) and light intensity (A/Q) were obtained for leaves from the upper (leaf +2) and lower (leaf +5) canopy layers, and the leaf nitrogen and chlorophyll content was also evaluated. The IACSP93-2060 genotype, which shows higher self-shading, did not exhibit differences in photosynthetic capacity between canopy layers, presenting similarities in C4 maximum RubisCO capacity (Vmax) and A/Q-derived parameters when the canopy layers were compared. This pattern was most likely the result of light acclimation, with IACSP93-2060 showing increased photosynthetic nitrogen use efficiency in the lower canopy layer. Despite higher light availability due to a lower level of self-shading, the IACSP95-3028 genotype showed a decrease in photosynthetic capacity in the lower layer. This response was related to a decrease in Vmax, indicating a biochemical limitation of photosynthesis due to self-shading. Our data on stomatal conductance did not show any differences in the stomatal limitation of photosynthesis in both canopy layers of both varieties. In general, self-shading reduced the chlorophyll a and b and leaf N concentrations without significant changes in the PEPCase activity (given by the initial slope of the A/Ci curve) in the lower canopy layer. In addition to presenting a higher photosynthetic capacity, IACSP95-3028 also received more light in the lower canopy layer, supporting its higher biomass production compared with IACSP93-2060.

Photosynthetic traits of Siebold's beech and oak saplings grown under free air ozone exposure in northern Japan

We set up a free-air ozone (O3) exposure system for determining the photosynthetic responses of Siebold's beech (Fagus crenata) and oak (Quercus mongolica var. crispula) to O3 under field conditions. Ten-year-old saplings of beech and oak were exposed to an elevated O3 concentration (60 nmol mol−1) during daytime from 6 August to 11 November 2011. Ozone significantly reduced the net photosynthetic rate in leaves of both species in October, by 46% for beech and 15% for oak. In beech there were significant decreases in maximum rate of carboxylation, maximum rate of electron transport in photosynthesis, nitrogen content and photosynthetic nitrogen use efficiency, but not in oak. Stomatal limitation of photosynthesis was unaffected by O3. We therefore concluded photosynthesis in beech is more sensitive to O3 than that in oak, and the O3-induced reduction of photosynthetic activity in beech was due not to stomatal closure, but to biochemical limitation.

Effects of Long-term Exposure to Black Carbon Particles on Growth and Gas Exchange Rates of Fagus crenata, Castanopsis sieboldii, Larix kaempferi and Cryptomeria japonica Seedlings

To clarify the effects of black carbon (BC) particles on growth and gas exchange rates of Asian forest tree species, the seedlings of Fagus crenata, Castanopsis sieboldii, Larix kaempferi and Cryptomeria japonica were exposed to BC particles with sub-micron size for two growing seasons from 1 June 2009 to 11 November 2010. The BC particles deposited after the exposure to BC were observed on the foliar surface of the 4 tree species. At the end of the experiment, the amount of BC accumulated on the foliar surface after the exposure to BC aerosols were 0.13, 0.69, 0.32 and 0.58 mg C <TEX>$m^{-2}$</TEX> total leaf area in F. crenata, C. sieboldii, L. kaempferi and C. japonica seedlings, respectively. In August 2010, the exposure to BC particles did not significantly affect net photosynthetic rate under any light intensity, stomatal diffusive conductance to water vapour (<TEX>$g_s$</TEX>), stomatal limitation of photosynthesis, response of <TEX>$g_s$</TEX> to increase in vapour pressure deficit and leaf temperature under light saturated condition in the leaves or needles of the seedlings. These results suggest that the BC particles deposited on the foliar surface did not reduce net photosynthesis by shading, did not increase leaf temperature by absorption of irradiation light, and did not induce plugging of stomata in the leaves or needles of the seedlings. There were no significant effects of BC particles on the increments of plant height and stem base diameter during the experimental period and the whole-plant dry mass at the end of the experiment. These results indicate that the exposure to BC particles with sub-micron size for two growing seasons did not significantly affect the growth and leaf or needle gas exchange rates of F. crenata, C. sieboldii, L. kaempferi and C. japonica seedlings.

Developing a mathematical model for variation of physiological responses of Jatropha curcas leaves depending on leaf positions under soil flooding

The effect of soil flooding on photosynthesis, transpiration and stomatal conductance of Jatropha curcas seedlings were studied under natural environmental variables. Soil flooding reduced photosynthesis (P N), transpiration (E) and stomatal conductance (gs) in response to leaf positions of Jatropha curcas plants. Based on the results, we conclude that decrease in stomatal opening and stomatal limitation of photosynthesis, followed by decrease in individual leaf area are the main causes of reductions in carbon uptake of flooded seedlings. A mathematical relationship was successfully developed to describe photosynthesis, transpiration and stomatal response of Jatropha under soil flooding stress.

Adaptive Value and Costs of Physiological Plasticity to Soil Moisture Limitation in Recombinant Inbred Lines ofAvena barbata

Costs are hypothesized to constrain the evolution of adaptive phenotypic plasticity, but they have been difficult to quantify because strong selection should eliminate costly genotypes from natural populations. However, recent studies suggest that crosses between natural populations can recover these genotypes. We determined the adaptive value and costs of, as well as the genetic variation for, physiological and morphological plasticity to soil water limitation in Avena barbata recombinant inbred lines (RILs) created by crossing mesic and xeric ecotypes. All traits were plastic, and plasticity in stomatal limitation of photosynthesis and photosynthetic rate before and at reproduction was adaptive. However, we detected a significant cost of plasticity only for stomatal conductance at reproduction, and the mean cost for all traits of A. barbata RILs was at least 50% smaller than costs previously estimated using RILs. In addition, heritabilities for plasticity were <0.1 and were significant only for photosynthesis at reproduction and leaf mass per unit area. Our results suggest that costs are less likely to constrain the evolution of adaptive plasticity in A. barbata than genetic variation for plasticity.

Diurnal Course of Gas Exchange and Water Use Efficiency of Platycladus orientalis in the Semiarid Region of the Loess Plateau

The daily gas exchange, stomatal conductance, and water use efficiency of Platycladus orientalis in the test field of Fangshan county of Shanxi Province in the semiarid region of the Loess Plateau were measured with portable Li-6200 gas analysis system in natural conditions. The results showed that the diurnal course of net photosynthetic rate displayed a two-peak pattern, that of stomatal conductance displayed a hollow pattern, and that of transpiration rate displayed a signal-peak pattern. Water use efficiency culminated in the early morning. On the basis of two criteria of stomatal limitation of photosynthesis suggested by Farquhar and Sharkey, the predominant limiting factor of photosynthesis was the stomatal conductance of stomatal limitation in the morning (10:00–12:00). However, the midday depression of photosynthesis at noon (12:00–14:00) and the decrease in photosynthesis in the afternoon (16:00–18:00) were the results of nonstomatal limitation, such as low carboxylation capacity of the leaf mesophyll.

CO2 DIFFUSIONAL AND STOMATAL LIMITATIONS OF PHOTOSYNTHESIS IN AMOMUM VILLOSUM

Photosynthetic rate is a function of not only the CO_2 concentration gradient between the outside and inside of the leaf, but also the CO_2 diffusional resistance. It is accepted that stomatal resistance is the greatest factor that controls CO_2 diffusional resistance and is thus a crucial factor influencing photosynthesis. Hence, adequate analysis of CO_2 diffusional resistance is necessary for understanding photosynthesis. Intercellular CO_2 concentrations (C_i) are often utilized to calculate stomatal limitation (L_s). This traditional analysis is unreliable when plants are under stress, because C_i cannot be accurately measured under such conditions. Here we introduced the concept of diffusional limitation and introduced a new method to calculate the diffusional limitation value (L_d) without using C_i. In addition, C_i, estimated indirectly through chlorophyll fluorescence parameters (C_i′), was used to calculate a new stomatal limitation value (L_s′) for plants grown under 40% relative soil moisture (RSM). We compared the L_s′, L_s and L_d for Amomum villosum grown under both 100% and 40% RSM. Photosynthetic rates (P_n) and stomatal conductance (G_s) decreased after noon in both RSM treatments, and P_n and G_s were both higher in 100% RSM than in 40% RSM. Under 100% RSM, L_s did not increase after noontime in A. villosum, indicating stomatal limitation of photosynthesis did not increase, whereas L_d increased indicating the diffusional limitation of photosynthesis increased due to the relatively high mesophyll resistance. Under 40% RSM, L_s′ increased sooner than L_d after noon, indicating stomatal resistance was the dominant factor controlling diffusional resistance. In contrast, L_s, calculated using the traditional stomatal limitation method, did not increase under 40% RSM, which might lead us to the wrong conclusion that stomatal limitation was not a factor. The estimated value, C_i′, was lower than C_i, indicating that gas exchange system was overestimated using C_i under conditions of water stress. Our results suggest that the traditional method is unreliable under soil water stress conditions, and the two new methods presented are more reliable. Furthermore, mesophyll resistance can be estimated indirectly through the joint analysis of diffusional and stomatal limitation.

Control of stomatal aperture and carbon uptake by deficit irrigation in two grapevine cultivars

Improvements in water use efficiency of crops are essential under the scenarios of water scarcity predicted by global change models for the Mediterranean region. In recent years, deficit irrigation, including partial root drying (PRD), has been proposed as an irrigation technique to improve water use efficiency and standardize grapevine yield and quality. The objective of this study was to evaluate the impact of deficit irrigation on photosynthetic responses of field grown grapevines of cv. Moscatel and Castelão. The treatments were: full irrigation (FI), corresponding to 100% ETc; non-irrigated, but rain fed (NI) and partial root zone drying (PRD) and deficit irrigation (DI), both corresponding to an irrigation of 50% ETc. While in the DI treatment water was applied to both sides of the root system, in the PRD treatment, water was supplied to only one side of the root system, alternating sides periodically. In both cultivars, PRD and DI vines showed intermediate pre-dawn leaf water potential (ψpd) values (around −0.4 MPa) while FI vines, showed ψpd around −0.2 MPa during the growing season. NI showed the lowest ψpd, reaching −0.6 MPa in Moscatel and −0.8 MPa in Castelão, at the end of growing season. Water status of PRD vines remained closer to FI than DI, especially at midday. Photosynthetic rates and fluorescence parameters of the deficit irrigation treatments (PRD and DI) did not show significant differences from FI vines, for most of the season. In Moscatel, although PRD generally showed gs, relative stomatal limitation (RSL) and intrinsic water use efficiency (A/gs) values closer to NI than DI, most of the differences between PRD and DI were not statistically significant. Our results showed a stability of the photosynthetic machinery in grapevines under low-to-moderate water availabilities, as demonstrated by the maintenance of the activity of three Calvin Cycle enzymes and of the Vcmax values. However, a decline in Jmax was observed in NI vines, that can be a result either of a decrease in ATP production or, alternatively, of decreased mesophyll conductance to CO2 diffusion. In general terms, stomatal limitation of photosynthesis is likely to be dominant in non-irrigated plants. Deficit irrigation had no negative impact on CO2 assimilation, despite less water application than in full-irrigation. Differences among varieties may be related to differences in sensitivity of stomata, shoot growth and/or the interaction between rootstock and cultivar to soil water availability.

Does higher yield potential improve barley performance in Mediterranean conditions?: A case study

Barley is one of the most widely cultivated crops in rainfed areas of the Mediterranean, where drought is the main factor that limits yield. Knowledge of the physiological traits responsible for adaptation of barley cultivars to Mediterranean environments may be relevant for future breeding strategies. Yield potential versus drought tolerance is an open debate. Here, we studied two barley cultivars (Graphic and Kym), of similar time to anthesis and crop duration, that are widely cultivated in the western Mediterranean. Grain yield was evaluated in 41 field trials and ranged (averaged for the 16–32 cultivars assayed in each trial) from 0.7 to 9.1 Mg ha −1. Yield components and carbon isotope discrimination (Δ 13C) of grains was analysed in another two trials. Graphic production was greater than Kym in all conditions. This greater yield was sustained mainly by more ears per unit ground area, which may be attributable to higher growth potential during tillering. Moreover, Graphic showed greater Δ 13C of kernels, indicating improved water status even at the end of the crop cycle. To examine differences in early growth, these cultivars were grown in optimal conditions and then photosynthetic activity and biomass analysed at the end of tillering. Graphic showed greater above-ground and root biomass as well as total leaf area per plant and per tiller than Kym, and also tended to have more tillers per plant, but its shoot-to-root biomass ratio was lower. Nitrogen content per unit leaf area was correlated negatively with plant and with tiller leaf area and positively with the shoot-to-root biomass ratio. Photosynthetic rate per unit leaf area was lower in Graphic and positively related to a lower nitrogen content, whereas stomatal limitation of photosynthesis and water use efficiency was similar in the two cultivars. Ribulose 1,5-biphosphate regeneration capacity contributed to the lower photosynthetic rate of Graphic. Moreover, quantum yield of photosystem II (PSII) electron transport was also lower in Graphic than Kym, which suggests that mechanisms other than leaf structure also contributed to the higher photosynthetic capacity of the former. Nevertheless, as result of differences in leaf area, total plant photosynthesis was greater in Graphic. The results indicate that the higher yield of Graphic under a wide range of Mediterranean conditions may be sustained by increased plant growth and total photosynthesis during tillering, although the photosynthetic capacity per unit leaf area is lower than that of Kym. Graphic has a more extensive root system than Kym, subsequently improving its water status in later stages of the crop cycle. Nitrogen content per unit leaf area is a good indicator of the growth and photosynthetic activity of barley plants in the early stages of the crop cycle.

Photosynthetic Responses of a Temperate Liana to Xylella fastidiosa Infection and Water Stress

Abstract Xylella fastidiosa is a xylem‐limited bacterial plant pathogen that causes bacterial leaf scorch in its hosts. Our previous work showed that water stress enhances leaf scorch symptom severity and progression along the stem of a liana, Parthenocissus quinquefolia, infected by X. fastidiosa. This paper explores the photosynthetic gas exchange responses of P. quinquefolia, with the aim to elucidate mechanisms behind disease expression and its interaction with water stress. We used a 2 × 2‐complete factorial design, repeated over two growing seasons, with high and low soil moisture levels and infected and non‐infected plants. In both years, low soil moisture levels reduced leaf water potentials, net photosynthesis and stomatal conductance at all leaf positions, while X. fastidiosa‐infection reduced these parameters at basally located leaves only. Intercellular CO2 concentrations were reduced in apical leaves, but increased at the most basal leaf location, implicating a non‐stomatal reduction of photosynthesis in leaves showing the greatest disease development. This result was supported by measured reductions in photosynthetic rates of basal leaves at high CO2 concentrations, where stomatal limitation was eliminated. Repeated measurements over the summer of 2000 showed that the effects of water stress and infection were progressive over time, reaching their greatest extent in September. By reducing stomatal conductances at moderate levels of water stress, P. quinquefolia maintained relatively high leaf water potentials and delayed the onset of photosynthetic damage due to pathogen and drought‐induced water stress. In addition, chlorophyll fluorescence measurements showed that P. quinquefolia has an efficient means of dissipating excess light energy that protects the photosynthetic machinery of leaves from irreversible photoinhibitory damage that may occur during stress‐induced stomatal limitation of photosynthesis. However, severe stress induced by disease and drought eventually led to non‐stomatal decreases in photosynthesis associated with leaf senescence.

Leaf gas exchange, chlorophyll fluorescence and growth responses of Genipa americana seedlings to soil flooding

Effects of soil flooding on photosynthesis and growth of Genipa americana L. seedlings, a neotropical fruit-tree species used in gallery forest restoration programs, were studied under glasshouse conditions. Despite the high survival rate and wide distribution in flood-prone habitats of the neotropics, previous studies demonstrated that growth of G. americana is reduced under soil flooding. Using leaf gas exchange and chlorophyll fluorescence measurements, we tested the hypothesis that stomatal limitation of photosynthesis is the main factor that reduces carbon uptake and growth rates of G. americana seedlings. Throughout a 63-day flooding period, the survival rates were 100%. The maximum values of the net photosynthetic rate ( A) and stomatal conductance to water vapor ( g s) of control seedlings were 9.86 μmol CO 2 m −2 s −1 and 0.525 mol H 2O m −2 s −1, respectively. The earliest effects of flooding were significant decreases in g s and A, development of hypertrophied lenticels and decrease in the dry weight of roots. A strong effect of the leaf-to-air vapor pressure deficit (LAVPD) on g s and A were observed that was enhanced under flooded conditions. Between 14 and 63 days after flooding, significant reductions in g s (31.7% of control) and A (52.9% of control) were observed followed by significant increments in non-photochemical quenching ( q N) (187.5% of control). During the same period, there were no differences among treatments for the ratio between variable to initial fluorescence ( F v/ F 0), the maximum quantum efficiency of the photosystem II ( F v/ F m) and photochemical quenching ( q P), indicating that there was no damage to the photosynthetic apparatus. Based on the results, we conclude that decreases in stomatal opening and stomatal limitation of photosynthesis, followed by decrease in individual leaf area are the main causes of reductions in carbon uptake and whole plant biomass of flooded seedlings.