Elevated aerosol enhances plant water-use efficiency by increasing carbon uptake while reducing water loss.

Aerosols could significantly influence ecosystem carbon and water fluxes, potentially altering their interconnected dynamics, typically characterized by water-use efficiency (WUE). However, our understanding of the underlying ecophysiological mechanisms remains limited due to insufficient field observations. We conducted 4-yr measurements of leaf photosynthesis and transpiration, as well as 3-yr measurements of stem growth (SG) and sap flow of poplar trees exposed to natural aerosol fluctuation, to elucidate aerosol's impact on plant WUE. We found that aerosol improved sun leaf WUE mainly because a sharp decline in photosynthetically active radiation (PAR) inhibited its transpiration, while photosynthesis was less affected, as the negative effect induced by declined PAR was offset by the positive effect induced by low leaf vapor pressure deficit (VPDleaf). Conversely, diffuse radiation fertilization (DRF) effect stimulated shade leaf photosynthesis with minimal impact on transpiration, leading to an improved WUE. The responses were further verified by a strong DRF on SG and a decrease in sap flow due to the suppresses in total radiation and VPD. Our field observations indicate that, contrary to the commonly assumed coupling response, carbon uptake and water use exhibited dissimilar reactions to aerosol pollution, ultimately enhancing WUE at the leaf and canopy level.

The combined effect of diffuse radiation and leaf wetness on functional traits and transpiration efficiency on a cloud forest species.

Cloud forests are unique biomes that thrive in foggy environments for a substantial part of the season. Fog in cloud forests plays two critical roles: it reduces incoming radiation and creates a humid environment, leading to the wetting of the canopy. This paper aims to investigate the combined effect of both radiation and wetness on Myrica faya Wilbur-a cloud forest species present in subtropical regions-both directly in plants and through simulations. Experiments consisted of a controlled environment with two levels of radiation and leaf wetness: low radiation/wet conditions, and high radiation/no-wetness; and three treatments: continuous low radiation and wetness, continuous high radiation and no wetness and alternate high low radiation and alternate wetness. The results revealed that a combination of low radiation and leaf wetness significantly improves leaf stomata conductance and increases the specific leaf area (SLA). Changes in SLA were driven by leaf size changes. However, the minimum leaf conductance (gmin) did not respond to any of the treatments. The simulations focused on exploring the impact of radiation and canopy wetness on transpiration efficiency (TE), i.e. the ratio between photosynthesis (An) and transpiration (Tc). The simulations demonstrated that TE increased exponentially as the canopy was gradually wetted, regardless of the radiation environment. This increase in TE results from Tc approaching zero while An maintains positive values. Overall, this study provides an integrated understanding of how fog alters M. faya functioning and, potentially, other cloud forest tree species.

Thermal Analysis of Stomatal Response under Salinity and High Light.

A non-destructive thermal imaging method was used to study the stomatal response of salt-treated Arabidopsis thaliana plants to excessive light. The plants were exposed to different levels of salt concentrations (0, 75, 150, and 220 mM NaCl). Time-dependent thermograms showed the changes in the temperature distribution over the lamina and provided new insights into the acute light-induced temporary response of Arabidopsis under short-term salinity. The initial response of plants, which was associated with stomatal aperture, revealed an exponential growth in temperature kinetics. Using a single-exponential function, we estimated the time constants of thermal courses of plants exposed to acute high light. The saline-induced impairment in stomatal movement caused the reduced stomatal conductance and transpiration rate. Limited transpiration of NaCl-treated plants resulted in an increased rosette temperature and decreased thermal time constants as compared to the controls. The net CO2 assimilation rate decreased for plants exposed to 220 mM NaCl; in the case of 75 mM NaCl treatment, an increase was observed. A significant decline in the maximal quantum yield of photosystem II under excessive light was noticeable for the control and NaCl-treated plants. This study provides evidence that thermal imaging as a highly sensitive technique may be useful for analyzing the stomatal aperture and movement under dynamic environmental conditions.

Increased stomatal conductance induces rapid changes to photosynthetic rate in response to naturally fluctuating light conditions in rice.

A close correlation between stomatal conductance and the steady-state photosynthetic rate has been observed for diverse plant species under various environmental conditions. However, it remains unclear whether stomatal conductance is a major limiting factor for the photosynthetic rate under naturally fluctuating light conditions. We analysed a SLAC1 knockout rice line to examine the role of stomatal conductance in photosynthetic responses to fluctuating light. SLAC1 encodes a stomatal anion channel that regulates stomatal closure. Long exposures to weak light before treatments with strong light increased the photosynthetic induction time required for plants to reach a steady-state photosynthetic rate and also induced stomatal limitation of photosynthesis by restricting the diffusion of CO2 into leaves. The slac1 mutant exhibited a significantly higher rate of stomatal opening after an increase in irradiance than wild-type plants, leading to a higher rate of photosynthetic induction. Under natural conditions, in which irradiance levels are highly variable, the stomata of the slac1 mutant remained open to ensure efficient photosynthetic reaction. These observations reveal that stomatal conductance is important for regulating photosynthesis in rice plants in the natural environment with fluctuating light.

Patterns and dynamics of canopy-root coupling in tropical tree saplings vary with light intensity but not with root depth.

Carbon (C) dynamics in canopy and roots influence whole-tree carbon fluxes, but little is known about canopy regulation of tree-root activity. Here, the patterns and dynamics of canopy-root C coupling are assessed in tropical trees. Large aeroponics facility was used to study the root systems of Ceiba pentandra and Khaya anthotheca saplings directly at different light intensities. In Ceiba, root respiration (Rr ) co-varied with photosynthesis (An ) in large saplings (3-to-7-m canopy-root axis) at high-light, but showed no consistent pattern at low-light. At medium-light and in small saplings (c. 1-m axis), Rr tended to decrease transiently towards midday. Proximal roots had higher Rr and nonstructural carbohydrate concentrations than distal roots, but canopy-root coupling was unaffected by root location. In medium-sized Khaya, no Rr pattern was observed, and in both species, Rr was unrelated to temperature. The early-afternoon increase in Rr suggests that canopy-root coupling is based on mass flow of newly fixed C in the phloem, whereas the early-morning rise in Rr with An indicates an additional coupling signal that travels faster than the phloem sap. In large saplings and potentially also in higher trees, light and possibly additional environmental factors control the diurnal patterns of canopy-root coupling, irrespective of root location.

Comparing two measures of leaf photorespiration rate across a wide range of light intensities.

Suppression of photorespiration by low O2 concentrations (Method 1) and simultaneous measurements of gas exchange and chlorophyll fluorescence (Method 2) are often used to estimate leaf photorespiration rate (Rp) of C3 plants. However, it is largely unknown whether Method 1 and Method 2 can be used equivalently in estimating Rp. Using a field experiment on two wheat cultivars (T. aestivum JM22 and T. aestivum Z39-118) whose leaf gas exchange and chlorophyll fluorescence at low and normal O2 concentrations (2% versus 21% O2) were simultaneously measured across a wide range of light intensities (I), this study assessed the impacts of the two measures on Rp and its response under changing irradiance conditions. All the above quantities increased with the increasing I until reaching the cultivar-specific maximum values and the corresponding saturation light intensities. However, there were significant differences between Rp estimated by Method 1 and Method 2 at the I range from 150 to 2000 µmol m-2 s-1 for T. aestivum JM22 and from 150 to 1000 µmol m-2 s-1 for T. aestivum Z39-118. These findings demonstrated that the two methods cannot be used equivalently under changing irradiance conditions.

What is the fate of xylem-transported CO2 in Kranz-type C4 plants?

High concentrations of dissolved inorganic carbon in stems of herbaceous and woody C3 plants exit leaves in the dark. In the light, C3 species use a small portion of xylem-transported CO2 for leaf photosynthesis. However, it is not known if xylem-transported CO2 will exit leaves in the dark or be used for photosynthesis in the light in Kranz-type C4 plants. Cut leaves of Amaranthus hypochondriacus were placed in one of three solutions of [NaH13 CO3 ] dissolved in KCl water to measure the efflux of xylem-transported CO2 exiting the leaf in the dark or rates of assimilation of xylem-transported CO2 * in the light, in real-time, using a tunable diode laser absorption spectroscope. In the dark, the efflux of xylem-transported CO2 increased with increasing rates of transpiration and [13 CO2 *]; however, rates of 13 Cefflux in A. hypochondriacus were lower compared to C3 species. In the light, A. hypochondriacus fixed nearly 75% of the xylem-transported CO2 supplied to the leaf. Kranz anatomy and biochemistry likely influence the efflux of xylem-transported CO2 out of cut leaves of A. hypochondriacus in the dark, as well as the use of xylem-transported CO2 * for photosynthesis in the light. Thus increasing the carbon use efficiency of Kranz-type C4 species over C3 species.

Altered stomatal dynamics induced by changes in irradiance and vapour-pressure deficit under drought: impacts on the whole-plant transpiration efficiency of poplar genotypes.

Recent findings were able to show significant variability of stomatal dynamics between species, but not much is known about factors influencing stomatal dynamics and its consequences on biomass production, transpiration and water-use efficiency (WUE). We assessed the dynamics of stomatal conductance (gs ) to a change of irradiance or vapour-pressure deficit (VPD) in two Populus euramericana and two Populus nigra genotypes grown under control and drought conditions. Our objectives were to determine the diversity of stomatal dynamics among poplar genotypes, and if soil water deficit can alter it. Physiological and morphological factors were investigated to find their potential links with stomatal morphology, WUE and its components at the whole-plant level. We found significant genotypic variability of gs dynamics to both irradiance and VPD. Genotypes with faster stomatal dynamics were correlated with higher stomatal density and smaller stomata, and the implications of these correlations are discussed. Drought slowed gs dynamics, depending on genotype and especially during stomatal closing. This finding is contrary to previous research on more drought-tolerant species. Independently of the treatment, faster stomatal dynamics were negatively correlated with daily whole-plant transpiration, presenting new evidence of a previously hypothesized contribution of stomatal dynamics to whole-plant water use.

Effects of climate warming on photosynthesis in boreal tree species depend on soil moisture.

Climate warming will influence photosynthesis via thermal effects and by altering soil moisture1-11. Both effects may be important for the vast areas of global forests that fluctuate between periods when cool temperatures limit photosynthesis and periods when soil moisture may be limiting to carbon gain4-6,9-11. Here we show that the effects of climate warming flip from positive to negative as southern boreal forests transition from rainy to modestly dry periods during the growing season. In a three-year open-air warming experiment with juveniles of 11 temperate and boreal tree species, an increase of 3.4 °C in temperature increased light-saturated net photosynthesis and leaf diffusive conductance on average on the one-third of days with the wettest soils. In all 11 species, leaf diffusive conductance and, as a result, light-saturated net photosynthesis decreased during dry spells, and did so more sharply in warmed plants than in plants at ambient temperatures. Consequently, across the 11 species, warming reduced light-saturated net photosynthesis on the two-thirds of days with driest soils. Thus, low soil moisture may reduce, or even reverse, the potential benefits of climate warming on photosynthesis in mesic, seasonally cold environments, both during drought and in regularly occurring, modestly dry periods during the growing season.

Effect of elevated CO2 and spectral quality on whole plant gas exchange patterns in tomatoes.

In controlled environment plant production facilities, elevating either light or CO2 levels generally has led to increased biomass and yield due to enhanced canopy photosynthesis. Today, advancements in light-emitting diodes (LEDs) have made this technology a viable option for both supplementary lighting in greenhouses and a sole lighting source in controlled environment chambers. Our study used tomato plants grown under both ambient CO2 (AC) and elevated CO2 (EC) conditions then exposed them to various CO2 and lighting treatments during both whole plant and leaf level measurements. Plants grown under EC reached the first flower developmental stage 8 days sooner and were approximately 15cm taller than those grown under AC. However, under AC plants had more leaf area while their dry weights were similar. Of note, under EC chlorophyll a and b were lower, as were carotenoids per unit leaf area. Whole plant analyses, under all CO2 challenges, showed that plants exposed to high-pressure sodium (HPS), red-blue LED, and red-white LED had similar photosynthesis, respiration, and daily carbon gain. Under different light qualities, day-time transpiration rates were similar among CO2 conditions. Day-time water-use efficiency (WUE) was higher in plants grown and exposed to EC. Similarly, WUE of plants grown under AC but exposed to short-term elevated CO2 conditions was higher than those grown and tested under AC during all light treatments. Under all CO2 conditions, plants exposed to red-white and red-blue LEDs had lower WUE than those exposed to HPS lighting. Assessing alterations due to CO2 and light quality on a whole plant basis, not merely on an individual leaf basis, furthers our understanding of the interactions between these two parameters during controlled environment production. Principle component analyses of both whole plant and leaf data indicates that increasing CO2 supply has a more dramatic effect on photosynthesis and WUE than on transpiration.

Decoding the gene coexpression network underlying the ability of Gevuina avellana to live in diverse light conditions.

Gevuina avellana (Proteaceae) is a typical tree from the South American temperate rainforest. Although this species mostly regenerates in shaded understories, it exhibits an exceptional ecological breadth, being able to live under a wide range of light conditions. Here we studied the genetic basis that underlies physiological acclimation of the photosynthetic responses of G. avellana under contrasting light conditions. We analyzed carbon assimilation and light energy used for photochemical processes in plants acclimated to contrasting light conditions. Also, we used a transcriptional profile of leaf primordia from G. avellana saplings growing under different light environments in their natural habitat, to identify the gene coexpression network underpinning photosynthetic performance and light-related processes. The photosynthetic parameters revealed optimal performance regardless of light conditions. Strikingly, the mechanism involved in dissipation of excess light energy showed no significant differences between high- and low-light-acclimated plants. The gene coexpression network defined a community structure consistent with the photochemical responses, including genes involved mainly in assembly and functioning of photosystems, photoprotection, and retrograde signaling. This ecophysiological genomics approach improves our understanding of the intraspecific variability that allows G. avellana to have optimal photochemical and photoprotective mechanisms in the diverse light habitats it encounters in nature.

Functional relationship of AtABCG21 and AtABCG22 in stomatal regulation.

Stomatal regulation is important for water transpiration from plants. Stomatal opening and closing are controlled by many transporter proteins in guard cells. AtABCG22 is a member of the ATP-binding cassette (ABC) transporters and is a stomatal regulator; however, the function of AtABCG22 has not yet been determined fully, although a mutant phenotype included a significant effect on stomatal status. Here, we further investigated the function of the AtABCG22 gene and its functional relationships with other subfamily genes. Among close family members, we found a functional relationship of stomatal phenotypes with AtABCG21, which is also expressed specifically in guard cells. Based on an analysis of double mutants, adding the atabcg21 mutation to atabcg22 mutant partially suppressed the open-stomata phenotype of atabcg22. Multiple-mutant analyses indicated that this suppression was independent of abscisic acid signaling in guard cells. We also found that atabcg22 mutant showed a unique time course-dependent phenotype, being defective in maintenance of stomatal status after initial stomatal opening elicited by light signaling. The function of AtABCG22 and its relationship with AtABCG21 in stomatal regulation are considered.

Elevated ozone reduces photosynthetic carbon gain by accelerating leaf senescence of inbred and hybrid maize in a genotype-specific manner.

Exposure to elevated tropospheric ozone concentration ([O3 ]) accelerates leaf senescence in many C3 crops. However, the effects of elevated [O3 ] on C4 crops including maize (Zea mays L.) are poorly understood in terms of physiological mechanism and genetic variation in sensitivity. Using free air gas concentration enrichment, we investigated the photosynthetic response of 18 diverse maize inbred and hybrid lines to season-long exposure to elevated [O3 ] (~100 nl L-1 ) in the field. Gas exchange was measured on the leaf subtending the ear throughout the grain filling period. On average over the lifetime of the leaf, elevated [O3 ] led to reductions in photosynthetic CO2 assimilation of both inbred (-22%) and hybrid (-33%) genotypes. There was significant variation among both inbred and hybrid lines in the sensitivity of photosynthesis to elevated [O3 ], with some lines showing no change in photosynthesis at elevated [O3 ]. Based on analysis of inbred line B73, the reduced CO2 assimilation at elevated [O3 ] was associated with accelerated senescence decreasing photosynthetic capacity and not altered stomatal limitation. These findings across diverse maize genotypes could advance the development of more O3 tolerant maize and provide experimental data for parameterization and validation of studies modeling how O3 impacts crop performance.

In situ temperature relationships of biochemical and stomatal controls of photosynthesis in four lowland tropical tree species.

Net photosynthetic carbon uptake of Panamanian lowland tropical forest species is typically optimal at 30-32 °C. The processes responsible for the decrease in photosynthesis at higher temperatures are not fully understood for tropical trees. We determined temperature responses of maximum rates of RuBP-carboxylation (VCMax ) and RuBP-regeneration (JMax ), stomatal conductance (Gs ), and respiration in the light (RLight ) in situ for 4 lowland tropical tree species in Panama. Gs had the lowest temperature optimum (TOpt ), similar to that of net photosynthesis, and photosynthesis became increasingly limited by stomatal conductance as temperature increased. JMax peaked at 34-37 °C and VCMax ~2 °C above that, except in the late-successional species Calophyllum longifolium, in which both peaked at ~33 °C. RLight significantly increased with increasing temperature, but simulations with a photosynthesis model indicated that this had only a small effect on net photosynthesis. We found no evidence for Rubisco-activase limitation of photosynthesis. TOpt of VCMax and JMax fell within the observed in situ leaf temperature range, but our study nonetheless suggests that net photosynthesis of tropical trees is more strongly influenced by the indirect effects of high temperature-for example, through elevated vapour pressure deficit and resulting decreases in stomatal conductance-than by direct temperature effects on photosynthetic biochemistry and respiration.

Blue light differentially represses mesophyll conductance in high vs low latitude genotypes of Populus trichocarpa Torr. & Gray.

To explore what role chloroplast positioning might have in relation to latitudinal variation in mesophyll conductance (gm) of Populus trichocarpa Torr. & Gray (black cottonwood), we examined photosynthetic response to different blue light treatments in six representative genotypes (three northern and three southern). The proportion of blue (B) to red light was varied from 0:100, 10:90, 20:80, 40:60, and 60:40 while keeping the total photosynthetic photon flux density constant. Mesophyll conductance was estimated by monitoring chlorophyll fluorescence in combination with gas exchange. Compared to the control (10% B), gm was significantly lower with increasing blue light. Consistent with a change in chloroplast positioning, there was a simultaneous but reversible decrease in chlorophyll content index (CCI), as measured by foliar greenness, while the extracted, actual chlorophyll content (ACC) remained unchanged. Blue-light-induced decreases in gm and CCI were greater in northern genotypes than in southern genotypes, both absolutely and proportionally, consistent with their inherently higher photosynthetic rate. Treatment of leaves with cytochalasin D, an inhibitor of actin-based chloroplast motility, reduced both CCI and ACC but had no effect on the CCI/ACC ratio and fully blocked any effect of blue light on CCI. Cytochalasin D reduced gm by ∼56% under 10% B, but did not block the effect of 60% B on gm, which was reduced a further 20%. These results suggest that the effect of high blue light on gm is at least partially independent of chloroplast repositioning. High blue light reduced carbonic anhydrase activity by 20% (P<0.05), consistent with a possible reduction in protein-mediated facilitation of CO2 diffusion.

The Sites of Evaporation within Leaves.

The sites of evaporation within leaves are unknown, but they have drawn attention for decades due to their perceived implications for many factors, including patterns of leaf isotopic enrichment, the maintenance of mesophyll water status, stomatal regulation, and the interpretation of measured stomatal and leaf hydraulic conductances. We used a spatially explicit model of coupled water and heat transport outside the xylem, MOFLO 2.0, to map the distribution of net evaporation across leaf tissues in relation to anatomy and environmental parameters. Our results corroborate earlier predictions that most evaporation occurs from the epidermis at low light and moderate humidity but that the mesophyll contributes substantially when the leaf center is warmed by light absorption, and more so under high humidity. We also found that the bundle sheath provides a significant minority of evaporation (15% in darkness and 18% in high light), that the vertical center of amphistomatous leaves supports net condensation, and that vertical temperature gradients caused by light absorption vary over 10-fold across species, reaching 0.3°C. We show that several hypotheses that depend on the evaporating sites require revision in light of our findings, including that experimental measurements of stomatal and hydraulic conductances should be affected directly by changes in the location of the evaporating sites. We propose a new conceptual model that accounts for mixed-phase water transport outside the xylem. These conclusions have far-reaching implications for inferences in leaf hydraulics, gas exchange, water use, and isotope physiology.

Enhanced photosynthesis endows seedling growth vigour contributing to the competitive dominance of weedy rice over cultivated rice.

BACKGROUND: Weedy rice, as one of the worst paddy field weeds worldwide, bears vigorous seedlings and dominantly competes with cultivated rice causing serious crop yield losses. To elucidate the causes of its stronger seedling vigour endowing its dominant competition with cultivated rice, comparative studies on seedling growth characteristics were conducted among six weedy rice biotypes and the two indica and japonica cultivars Shanyou-63 (SY-63) and Zhendao-8 (ZD-8), respectively, in the greenhouse. RESULTS: Weedy rice emerged 2 to 3 days earlier, rapidly grew 1.3-1.7 cm taller daily, produced more secondary adventitious roots and greater aboveground fresh biomass than cultivated rice. Moreover, weedy rice exhibited greater photosynthetic pigment content, net photosynthetic rate, stomatal conductance, intercellular CO2 concentration, transpiration rate, and chlorophyll fluorescence kinetic parameters. An enhanced overall photosynthetic activity in weedy rices was attributed to the combined action of a larger antenna, more active reaction centres and higher quantum yield for electron transfer beyond QA . CONCLUSIONS: Enhanced photosynthesis of weedy rice at the seedling stage should be the main factor for leading to strong competitive dominance over cultivated rice. © 2016 Society of Chemical Industry.