Mesophyll Conductance Research Articles

Functional plasticity of Capsicum annuum var. glabriusculum through multiple traits

The diversity of functional traits still has not been studied enough in model plant species, even less so in little-known species. This experiment was carried out under the extreme heat of Sonoran Desert, using shading nets and under conditions where the availability of water and nutrients was not a stress factor. We evaluated how the low, intermediate and high sunlight regimes impact survival and promote multiple alterations on phenological and ecophysiological response of cultivated Capsicum annuum var. glabriusculum plants. Extremely warm temperatures promoted a high heat sum in degrees days throughout plants development. Most plants grown in high sunlight regimes did not survive; under intermediate sunlight regimes survival was high and plants developed vegetative and reproductively; but under low sunlight regimes plants survival was high; however, they developed just vegetatively. Photosynthetic response to light suggests that plants are physiologically acclimated to low and intermediate irradiance, whereas the CO2 assimilation curves suggest contrasting photosynthetic capacity traits. Under the intermediate sunlight regimes, plants strengthened their performance through multiple functional traits (e.g. CO2 and water diffusion traits, photosynthetic capacity, respiration, among others). Consequently, their biomass gain was faster and proportionally higher by 76 % with an investment of 14 % in fruits development. The principal components analysis extracted the main explanatory functional traits: photosynthetic nitrogen allocation, stomatal limitation, mesophyll conductance, Rubisco maximum carboxylation velocity, among others. In conclusion, phenological response and multiple functional traits determine plants acclimation to sunlight regimes and extremely warm temperatures in short term.

Leaf morpho-physiological comparison between native and non-native plant species in a Mediterranean island

Islands tend to be more prone to plant invasions than mainland regions, with the Mediterranean ones not being an exception. So far, a large number of studies on comparing leaf morphological and physiological traits between native and non-native plants in Mediterranean environments have been performed, although none of them on Mediterranean islands. To fill this gap, this study focuses on 14 plant species grown in a controlled growth chamber in the absence of stress. The goal was (1) to differentiate leaf morpho-physiological traits between native and non-native plants on a Mediterranean island and (2) to deepen in the underlying causes of the differential photosynthetic traits displayed by non-native species. Results showed that in Mediterranean islands, non-native plant species show on average larger values of net CO2 assimilation, stomatal conductance (gm), photosynthetic nitrogen-use efficiency, among others, and lower leaf mass per area (LMA) and leaf thickness, compared to the native species. Among the assessed traits, this study reports for the first time larger gm, and lower mesophyll conductance limitation in non-native species, which seems to be linked to their lower LMA. These novel traits need to be added to the ‘leaf physiological trait invasive syndrome’. It was also found that on a Mediterranean island, native and non-native species are placed on opposite sides of the leaf economics spectrum, with non-native species being placed on the ‘‘fast-return’’ end. In conclusion, this study demonstrates that non-native species inhabiting a Mediterranean island possess distinct leaf morphological and physiological traits compared to co-occurring native species, at least during the favorable growth season, which increases the chances of a successful invasion.

Cell-level anatomy explains leaf age-dependent declines in mesophyll conductance and photosynthetic capacity in the evergreen Mediterranean oak Quercus ilex subsp. rotundifolia.

Leaves of Mediterranean evergreen tree species experience a reduction in net CO2 assimilation (AN) and mesophyll conductance to CO2 (gm) during aging and senescence, which would be influenced by changes in leaf anatomical traits at cell level. Anatomical modifications can be accompanied by the dismantling of photosynthetic apparatus associated to leaf senescence, manifested through changes at the biochemical level (i.e., lower nitrogen investment in photosynthetic machinery). However, the role of changes in leaf anatomy at cell level and nitrogen content in gm and AN decline experienced by old non-senescent leaves of evergreen trees with long leaf lifespan is far from being elucidated. We evaluated age-dependent changes in morphological, anatomical, chemical and photosynthetic traits in Quercus ilex subsp. rotundifolia Lam., an evergreen oak with high leaf longevity. All photosynthetic traits decreased with increasing leaf age. The relative change in cell wall thickness (Tcw) was less than in chloroplast surface area exposed to intercellular air space (Sc/S), and Sc/S was a key anatomical trait explaining variations in gm and AN among different age classes. The reduction of Sc/S was related to ultrastructural changes in chloroplasts associated to leaf aging, with a concomitant reduction in cytoplasmic nitrogen. Changes in leaf anatomy and biochemistry were responsible for the age-dependent modifications in gm and AN. These findings revealed a gradual physiological deterioration related to the dismantling of the photosynthetic apparatus in older leaves of Q. ilex subsp. rotundifolia.

Variation of photosynthesis during plant evolution and domestication: implications for improving crop photosynthesis.

Studies investigating the mechanisms underlying the variation of photosynthesis along plant phylogeny and especially during domestication are of great importance, and may provide new insights to further improve crop photosynthesis. In the present study, we compiled a database including 542 sets of data of leaf gas exchange parameters and leaf structural and chemical traits in ferns and fern allies, gymnosperms, non-crop angiosperms, and crops. We found that photosynthesis was dramatically improved from ferns and fern allies to non-crop angiosperms, and further increased in crops. The improvement of photosynthesis during phylogeny and domestication was related to increases in carbon dioxide diffusional capacities and, to a lesser extent, biochemical capacity. Cell wall thickness rather than chloroplast surface area facing intercellular airspaces drives the variation of mesophyll conductance. The variation of the maximum carboxylation rate was not related to leaf nitrogen content. The slope of the relationship between mass-based photosynthesis and nitrogen was lower in crops than in non-crop angiosperms. These findings suggest that the manipulation of cell wall thickness is the most promising approach to further improve crop photosynthesis, and that an increase of leaf nitrogen will be less efficient in improving photosynthesis in crops than in non-crop angiosperms.

The response of mesophyll conductance to ozone-induced oxidative stress is genotype-dependent in poplar

The CO2 diffusion conductance within the leaf mesophyll (gm) is considered a major limiting factor of photosynthesis. However, the effects of the major secondary air pollutant ozone (O3) on gm have been poorly investigated. Eight genotypes of the economically important tree species Populus × canadensis Moench were exposed to 120 ppb O3 for 21 d. gm showed a genotype-dependent response to O3-induced oxidative stress and was a major limiting factor of net assimilation rate (Anet), ahead of stomatal conductance to CO2 (gsc) and of the maximum carboxylation capacity of the Rubisco enzyme (Vcmax) in half of the tested genotypes. Increased leaf dry mass per area (LMA) and decreased chlorophyll content were linked to the observed gm decrease, but this relationship did not entirely explain the different genotypic gm responses. Moreover, the oxidative stress defence metabolites ascorbate and glutathione were not related to O3 tolerance of gm. However, malondialdehyde probably mitigated the observed gm decrease in some genotypes due to its oxidative stress signalling function. The large variation of gm suggests different regulation mechanisms amongst poplar genotypes under oxidative stress.

A plasma membrane aquaporin is potentially involved in moderating water stress and photosynthetic depression by sustaining water transport and CO2 uptake under high VPD stress conditions in Solanum lycopersicum (tomato)

The molecular mechanism underlying the plant response to high vapor pressure deficit (VPD) stress remains unclear. In the present study, the roles of the plasma membrane aquaporin SlPIP1;2 in moderating plant water stress and photosynthetic depression in response to a series of VPD stress conditions are investigated. Plant water stress and photosynthetic depression are gradually exacerbated as VPD increases. The effects of SlPIP1;2 in mediating plant water status and photosynthesis are gradually enhanced as VPD increases. SlPIP1;2 exerts no significant effect on plant water status and photosynthesis under low VPD conditions. Overexpression of SlPIP1;2 moderates plant water stress by enhancing root water uptake under high VPD stress conditions, sustaining stomatal and mesophyll conductance for photosynthetic CO2 uptake and transport. Consequently, the relatively high CO2 availability inside chloroplasts in SlPIP1;2-overexpressing plants efficiently moderates photosynthetic depression under high VPD stress conditions. The present study demonstrates that SlPIP1;2 is potentially involved in moderating water stress and photosynthetic depression by sustaining water transport and CO2 uptake under high VPD stress conditions.

The Response Mechanism and Threshold of Spring Wheat to Rapid Drought

In order to deeply understand the effect mechanism of rapid drought stress on the physiological and biochemical properties of crop leaves and determine drought thresholds, the potted spring wheat under two water treatments, adequate water supply and continuous drought stress, was researched. In the early stage of drought, the parameters of leaves decreased in the order of stomatal conductance (gs), intercellular CO2 concentration (Ci), maximum electron transfer rate (Jmax), mesophyll conductance (gm), photosynthetic rate (Pn,) leaf water content (LWC), triose phosphate utilization rate (TPU), transpiration rate (Tr), and maximum carboxylation rate (Vcmax). Photosynthesis was dominated by stomatal limitation and also limited by carboxylation and mesophyll limitation. The carboxylation limitation was mainly caused by the reduction of electron transport capacity. In the late stage of drought, stomatal limitation first decreased, and then mesophyll limitation decreased. During extreme drought, carboxylation limitation also decreased. With the decrease of relative soil moisture (RSM), except for Ci, which first decreased and then increased, other physicochemical parameters of leaves all showed an S-shaped, decreasing trend. Mild and severe drought thresholds were determined to be 56.6% and 43.6% of the RSM, respectively, according to the curve’s inflection point, corresponding to 16.6% and about 52.2% of the average initial decrease amplitude among all parameters. This will provide a reference for monitoring as well as an early warning of rapid drought in spring wheat.

Hydrometeorology for plant omics: Potential evaporation as a key index for transcriptome in rice

Transpiration is a major physiological phenomenon in plants and is induced by atmospheric transpirational demand. However, little is known about quantitative relationships between transpirational demand and multiple meteorological factors, and how plants respond to short-time (day-to-day) changes in transpirational demand at the gene and molecular levels. We used a physical-based model of heat balance and transfer at a saturated surface to quantify the sensitivity of potential evaporation (Ep; a hydrometeorological indicator of transpirational demand) to each meteorological factor under a wide range of climatic conditions. Sensitivity analysis showed that Ep varies dynamically in time and space with meteorological conditions. That is, Ep changes commensurately with solar radiation Sd, somewhat increases with wind speed WS, while somewhat decreases with increasing relative humidity Rh. The sensitivity of Ep to WS and Rh increases with decreasing Sd. The logarithm of Ep decreases approximately linearly with increasing inverse absolute temperature. The validity of use of Ep as the transpirational demand under natural field conditions was confirmed by high correlation between hourly based Ep and hourly evapotranspiration in the flooded rice paddy fields. We also examined the effects of short-term changes in Ep on transpiration (ET) and transcriptomes of rice at the heading stage by altering meteorological conditions on sunny days for four hours. ET changed commensurately with Ep, and the expression levels of numerous genes, including those encoding aquaporins in rice leaves and roots, showed a significant positive or negative correlation with ET, especially in leaves. The present study suggests that rice plants may adjust their physiological condition, such as hydraulic conductivity and mesophyll conductance, through dynamic transcriptome changes in response to transpirational demand under fluctuating meteorological conditions.

Leaf Photosynthesis and Its Temperature Response Are Different between Growth Stages and N Supplies in Rice Plants.

Leaf photosynthesis is highly correlated with CO2-diffusion capacities, which are determined by both leaf anatomical traits and environmental stimuli. In the present study, leaf photosynthetic rate (A), stomatal conductance (gs), mesophyll conductance (gm) and the related leaf anatomical traits were studied on rice plants at two growth stages and with two different N supplies, and the response of photosynthesis to temperature (T) was also studied. We found that gm was significantly higher at mid-tillering stage and at high N treatment. The larger gm was related to a larger chloroplast surface area facing intercellular air spaces and a thinner cell wall in comparison with booting stage and zero N treatment. At mid-tillering stage and at high N treatment, gm showed a stronger temperature response. The modelling of the gm-T relationships suggested that, in comparison with booting stage and zero N treatment, the stronger temperature response of gm was related to the higher activation energy of the membrane at mid-tillering stage and at high N treatment. The findings in the present study can enhance our knowledge on the physiological and environmental determinants of photosynthesis.

Desiccation of the leaf mesophyll and its implications for CO2 diffusion and light processing.

Leaves balance CO2 and radiative absorption while maintaining water transport to maximise photosynthesis. Related species with contrasting leaf anatomy can provide insights into inherent and stress‐induced links between structure and function for commonly measured leaf traits for important crops. We used two walnut species with contrasting mesophyll anatomy to evaluate these integrated exchange processes under non‐stressed and drought conditions using a combination of light microscopy, X‐ray microCT, gas exchange, hydraulic conductance, and chlorophyll distribution profiles through leaves. Juglans regia had thicker palisade mesophyll, higher fluorescence in the palisade, and greater low‐mesophyll porosity that were associated with greater gas‐phase diffusion (g IAS), stomatal and mesophyll (g m) conductances and carboxylation capacity. More and highly‐packed mesophyll cells and bundle sheath extensions (BSEs) in Juglans microcarpa led to higher fluorescence in the spongy and in proximity to the BSEs. Both species exhibited drought‐induced reductions in mesophyll cell volume, yet the associated increases in porosity and g IAS were obscured by declines in biochemical activity that decreased g m. Inherent differences in leaf anatomy between the species were linked to differences in gas exchange, light absorption and photosynthetic capacity, and drought‐induced changes in leaf structure impacted performance via imposing species‐specific limitations to light absorption, gas exchange and hydraulics.

Desiccation tolerance in bryophytes relates to elasticity but is independent of cell wall thickness and photosynthesis.

Mosses have been found outliers of the trade‐off between photosynthesis and bulk elastic modulus described for vascular plants. Hence, potential trade‐offs among physical features of cell walls and desiccation tolerance, water relations, and photosynthesis were assessed in bryophytes and other poikilohydric species. Long‐term desiccation tolerance was quantified after variable periods of desiccation/rehydration cycles. Water relations were analyzed by pressure–volume curves. Mesophyll conductance was estimated using both CO2 curve‐fitting and anatomical methods. Cell wall elasticity was the parameter that better correlated with the desiccation tolerance index for desiccation tolerant species and was antagonistic to higher absolute values of osmotic potential. Although high values of cell wall effective porosity were estimated compared with the values assumed for vascular plants, the desiccation tolerance index negatively correlated with the porosity in desiccation tolerant bryophytes. Neither cell wall thickness nor photosynthetic capacity were correlated with the desiccation tolerance index of the studied species. The existence of a potential evolutionary trade‐off between cell wall thickness and desiccation tolerance is rejected. The photosynthetic capacity reported for bryophytes is independent of elasticity and desiccation tolerance. Furthermore, the role of cell wall thickness in limiting CO2 conductance would be overestimated under a scenario of high cell wall porosity for most bryophytes.

Effects of exogenous salicylic acid and abscisic acid on growth, photosynthesis and antioxidant system of rice

The key function of plant growth regulators is to adjust the growth and development of plants to adapt to the changing environment, which plays a vital role in the growth and yield of crops. This study was aimed to evaluate the ability of foliar application of salicylic acid (SA) and abscisic acid (ABA) to regulate rice (Oryza sativa L.) growth quality at seedling stage and reproductive stage. The two rice cultivars Wanshengyoutianhong-4 (W4) and Huanghuazhan (HZ) were sprayed foliar with SA (100 mg L-1) and ABA (4 mg L-1) at two-leaf stage, and their effects on morphological growth, physiological characteristics, malondialdehyde (MDA), osmotic substances, protective enzyme activities, photosynthesis and yield components were studied. Results indicated that SA and ABA significantly increased morphological growth, osmotic substances, antioxidant enzyme activities; MDA content dropped significantly (p < 0.05) in W4. While the two regulators inhibited morphological growth and physiological characteristics in HZ. Additionally, net photosynthesis, stomatal conductance, transpiration rate, water use efficiency and apparent mesophyll conductance were improved in both of rice cultivars at heading stage as well as grain weight. In conclusion, foliar spray of SA and ABA enhanced plant growth, induced an increase in protective enzyme activities and reduced the oxidative damage of the hybrid rice cultivar but not of the conventional rice cultivar at seedling stage. Meanwhile, photosynthetic parameters and grain weight were slightly enhanced at reproductive stage. These results served as a theoretical basis and technical support for the chemical regulation and cultivation practices of rice.

Mesophyll conductance is unaffected by expression of Arabidopsis PIP1 aquaporins in the plasmalemma of Nicotiana.

In plants with C3 photosynthesis, increasing the diffusion conductance for CO2 from the substomatal cavity to chloroplast stroma (mesophyll conductance) can improve the efficiencies of both CO2 assimilation and photosynthetic water use. In the diffusion pathway from substomatal cavity to chloroplast stroma, the plasmalemma and chloroplast envelope membranes impose a considerable barrier to CO2 diffusion, limiting photosynthetic efficiency. In an attempt to improve membrane permeability to CO2, and increase photosynthesis in tobacco, we generated transgenic lines in Nicotiana tabacum L. cv Petite Havana carrying either the Arabidopsis PIP1;2 (AtPIP1;2) or PIP1;4 (AtPIP1;4) gene driven by the constitutive dual 2x35S CMV promoter. From a collection of independent T0 transgenics, two T2 lines from each gene were characterized, with western blots confirming increased total aquaporin protein abundance in the AtPIP1;2 tobacco lines. Transient expression of AtPIP1;2-mGFP6 and AtPIP1;4-mGFP6 fusions in Nicotiana benthamiana identified that both AtPIP1;2 and AtPIP1;4 localize to the plasmalemma. Despite achieving ectopic production and correct localization, gas exchange measurements combined with carbon isotope discrimination measurements detected no increase in mesophyll conductance or CO2 assimilation rate in the tobacco lines expressing AtPIP. We discuss the complexities associated with trying to enhance gm through modified aquaporin activity.

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

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

Leaf photosynthesis is positively correlated with xylem and phloem areas in leaf veins in rice (Oryza sativa) plants.

Leaf structure is an important determinant of leaf photosynthesis; however, the impacts of leaf structural traits on gas exchange parameters are still not fully understood. In the present study, 11 rice genotypes were grown in pots to investigate the influence of leaf structural traits on leaf photosynthesis and hydraulic conductance (Kleaf). In this study, leaf photosynthetic rate (A), stomatal conductance (gs), mesophyll conductance and Kleaf were measured. In addition, leaf structural traits including leaf thickness (LT), leaf mass per area and leaf xylem and phloem sizes were also measured to investigate their impacts on rice photosynthesis. We found that the total area of xylem conduits per major vein (Xmajor), leaf phloem area per minor vein (Pminor) and LT were positively correlated with Kleaf, gs and A. The path analysis suggested that, however, only Pminor had a direct impact on A; Xmajor had an indirect impact on A via gs and Pminor, while LT did not show any direct or indirect impact on A. This study highlighted the importance of manipulations in Xmajor and Pminor, two previously overlooked leaf traits, to improve leaf photosynthesis in rice plants.

Biofertilization with photosynthetic bacteria as a new strategy for mitigating photosynthetic acclimation to elevated CO2 on cherry tomato

The stimulated effects of elevated CO2 concentration decline over time in many C3 plants. However, the phenomenon is a rare occurrence on plants hosting some symbiosis bacteria. The photosynthesis acclimation associated with an extended period of high CO2 concentration was therefore studied in cherry tomato that treated with a kind of photosynthetic bacteria. Bio-fertilization with photosynthetic bacteria, which can fix nitrogen for host crops and contains a variety of growth promoting factors, can play an important role in maintaining photosynthetic activity. In this research, we conducted a CO2 elevation experiment combined different water regimes and inoculation with photosynthetic bacterium, to evaluate the improvement of photosynthesis acclimation by using photosynthesis bacteria, and to explore associated mechanisms for artificial chamber grown cherry tomatoes. As a result of these experiments, we found that elevated CO2 had a positive effect on cherry tomato photosynthetic activity and yield of all treatment. After 55 days of CO2 injection, photosynthetic acclimation was found in the plant of uninoculated photosynthetic bacteria, which were most pronounced in high water treatments. Reduced irrigation amounts decreased the magnitude of photosynthetic acclimation, but relative fruit yield gradually decreased after achieving a maximum. In contrast, the photosynthetic bacteria inoculated plants just showed a slightly down-regulation of photosynthesis under durative high CO2 condition. Because inoculation of photosynthetic bacteria increased the photosynthetic pigment content, mesophyll conductance, maximum carboxylation rate and maximum electron transport rate. Furthermore, inoculation with the photosynthetic bacteria diminished the yield penalties caused by the reduced irrigation treatment through promoting nitrogen content and relieve starch accumulation in tomato leaves compared to those of the uninoculated control. And enhanced yield of inoculated plants grown under durative high CO2 concentration were observed. The results suggest that photosynthesis bacteria application could be a potential means to further improve the CO2 stimulation effect on cherry tomato production.

Nitrogen Fertilization Weakly Influences the Anatomy and Chemical Composition of Rice Leaves

In this paper, four rice genotypes showing different leaf mass per area (LMA) are used to explore the effects of nitrogen (N) supplies on rice leaf anatomy and leaf chemical composition as well as their impacts on leaf gas exchange parameters. The results showed that the mass-based and area-based leaf N contents as well as the net photosynthetic rate (A) under high N supply (HN) were all higher than those under a low N supply (LN). However, N supplies had no effect on stomatal conductance, mesophyll conductance, and photosynthetic N use efficiency. Moreover, N supplies had no significant effect on LMA and cell wall thickness. Leaf thickness and leaf density responses to N supplies were inconsistent in different genotypes. Except for the soluble sugar in Huanghuazhan and non-structural carbohydrates (NSC) in Sab Ini, N supplies showed no significant effects on mass-based leaf chemical components (pectic substance, hemicellulose, cellulose, lignin, total cell wall, soluble sugar, starch and NSC) content. The area-based leaf chemical components content showed significant differences between HN and LN in some occasions. The soluble sugar, NSC, hemicellulose, and lignin contents of Sab Ini under HN were higher than those under LN. The pectic substance, hemicellulose, and lignin contents of Huanghuazhan under LN were higher than those under HN. The cellulose and cell wall contents of Yongyou 12 under LN were higher than those under HN. Therefore, we conclude that nitrogen fertilization weakly influences the anatomy and chemical composition of rice leaves with a few exceptions.

Topographic Controls on Stomatal and Mesophyll Limitations to Photosynthesis in Two Subalpine Conifers

Premise of research. Leaf stomatal and mesophyll conductances limit photosynthesis and influence water use efficiency. Few studies have quantified the relative limitations imposed by these CO2 diffusion pathways on photosynthesis in mature conifer trees under natural conditions. Here, we report observations of stomatal and mesophyll conductance changes during seasonal drying across contrasting topographic positions in two Rocky Mountain conifers. We predicted that topographic controls on soil water availability and energy balance would determine limitations to photosynthesis by mesophyll conductance across conifer species with contrasting patterns of stomatal and hydraulic traits. Methodology. Concurrent measurements of leaf gas exchange and carbon isotope discrimination were used to estimate stomatal (gs) and mesophyll (gm) conductance in branches of an isohydric species, lodgepole pine (Pinus contorta), and an anisohydric species, Engelmann spruce (Picea engelmannii), in the central Rocky Mountains. Quantitative limitation analysis of photosynthesis (A) was then performed using data from CO2 response curves. Pivotal results. Stomatal conductance imposed greater limitations on photosynthesis (42%–67%) than mesophyll conductance (5%–17%), but no significant differences in gm were observed between the two conifer species. At the mesic lower hillslope position, A, gs, and gm increased during the growing season despite declines in soil moisture. In contrast, at the drier upper hillslope position, declines in soil moisture and increases in air temperature during the growing season are correlated with reductions in gs but not with A or gm. Conclusions. Adjustments in gm played a potentially important role in sustaining photosynthesis and improving plant water use efficiency when stomatal conductance decreased with water limitation during the growing season at the research site. Sustained gm with seasonal drought may be an important mechanism allowing conifers to survive and maintain competitive dominance in low-resource habitats.

Scaling Up from Leaf to Whole-Plant Level for Water Use Efficiency Estimates Based on Stomatal and Mesophyll Behaviour in Platycladus orientalis

Prediction of whole-plant short-term water use efficiency (WUEs,P) is essential to indicate plant performance and facilitate comparison across different temporal and spatial scales. In this study, an isotope model was scaled up from the leaf to the whole-plant level, in order to simulate the variation in WUEs,P in response to different CO2 concentrations (Ca; 400, 600, and 800 μmol·mol−1) and soil water content (SWC; 35–100% of field capacity). For WUEs,P modelling, leaf gas exchange information, plant respiration, and “unproductive” water loss were taken into account. Specifically, in shaping the expression of the WUEs,P, we emphasized the role of both stomatal (gsw) and mesophyll conductance (gm). Simulations were compared with the measured results to check the model’s applicability. The verification showed that estimates of gsw from the coupled photosynthesis (Pn,L)-gsw model accounting for the effect of soil water stress slightly outperformed the model neglecting the soil water status effect. The established coupled Pn,L-gm model also proved more effective in estimating gm than the previously proposed model. Introducing the two diffusion control functions into the whole-plant model, the developed model for WUEs,P effectively captured its response pattern to different Ca and SWC conditions. Overall, this study confirmed that the accurate estimation of WUEs,P requires an improved predictive accuracy of gsw and gm. These results have important implications for predicting how plants respond to climate change.

Species-specific variation of photosynthesis and mesophyll conductance to ozone and drought in three Mediterranean oaks.

Mesophyll conductance (g mCO2) is one of the most important components in plant photosynthesis. Tropospheric ozone (O3) and drought impair physiological processes, causing damage to photosynthetic systems. However, the combined effects of O3 and drought on g mCO2 are still largely unclear. We investigated leaf gas exchange during mid‐summer in three Mediterranean oaks exposed to O3 (ambient [35.2 nmol mol−1 as daily mean]; 1.4 × ambient) and water treatments (WW [well‐watered] and WD [water‐deficit]). We also examined if leaf traits (leaf mass per area [LMA], foliar abscisic acid concentration [ABA]) could influence the diffusion of CO2 inside a leaf. The combination of O3 and WD significantly decreased net photosynthetic rate (P N) regardless of the species. The reduction of photosynthesis was associated with a decrease in g mCO2 and stomatal conductance (g sCO2) in evergreen Quercus ilex, while the two deciduous oaks (Q. pubescens, Q. robur) also showed a reduction of the maximum rate of carboxylation (V cmax) and maximum electron transport rate (J max) with decreased diffusive conductance parameters. The reduction of g mCO2 was correlated with increased [ABA] in the three oaks, whereas there was a negative correlation between g mCO2 with LMA in Q. pubescens. Interestingly, two deciduous oaks showed a weak or no significant correlation between g sCO2 and ABA under high O3 and WD due to impaired stomatal physiological behaviour, indicating that the reduction of P N was related to g mCO2 rather than g sCO2. The results suggest that g mCO2 plays an important role in plant carbon gain under concurrent increases in the severity of drought and O3 pollution.