Focus on Water: Systems Analysis of Guard Cell Membrane Transport for Enhanced Stomatal Dynamics and Water Use Efficiency

Stomatal transpiration is at the center of a crisis in water availability and crop production that is expected to unfold over the next 20 to 30 years. Global water usage has increased 6-fold in the past 100 years, twice as fast as the human population, and is expected to double again before 2030, driven mainly by irrigation and agriculture. Guard cell membrane transport is integral to controlling stomatal aperture and offers important targets for genetic manipulation to improve crop performance. However, its complexity presents a formidable barrier to exploring such possibilities. With few exceptions, mutations that increase water use efficiency commonly have been found to do so with substantial costs to the rate of carbon assimilation, reflecting the trade-off in CO₂ availability with suppressed stomatal transpiration. One approach yet to be explored in detail relies on quantitative systems analysis of the guard cell. Our deep knowledge of transport and homeostasis in these cells gives real substance to the prospect for reverse engineering of stomatal responses, using in silico design in directing genetic manipulation for improved water use and crop yields. Here we address this problem with a focus on stomatal kinetics, taking advantage of the OnGuard software and models of the stomatal guard cell recently developed for exploring stomatal physiology. Our analysis suggests that manipulations of single transporter populations are likely to have unforeseen consequences. Channel gating, especially of the dominant K⁺ channels, appears the most favorable target for experimental manipulation.Stomata are pores that provide the major route for gaseous exchange across the impermeable cuticle of leaves and stems (Hetherington and Woodward, 2003). They open and close in response to exogenous and endogenous signals and thereby control the exchange of gases, most importantly water vapor and CO₂, between the interior of the leaf and the atmosphere. Stomata exert major controls on the water and carbon cycles of the world (Schimel et al., 2001) and can limit photosynthetic rates by 50% or more when demand exceeds water supply (Ni and Pallardy, 1992). Stomatal transpiration is at the center of a crisis in water availability and crop production that is expected to unfold over the next 20 to 30 years; indeed, global water usage has increased 6-fold in the past 100 years, twice as fast as the human population, and is expected to double again before 2030, driven mainly by irrigation and agriculture (United Nations Educational, Scientific and Cultural Organization, 2009).Guard cell transport is integral to controlling stomatal aperture. Guard cells surround the stomatal pore and respond in a well-defined manner to an array of extracellular signals, including light, to regulate its aperture. Guard cells coordinate membrane transport within a complex network of intracellular signals (Willmer and Fricker, 1996; Blatt, 2000a, 2000b; Hetherington and Woodward, 2003; Shimazaki et al., 2007) to regulate fluxes, mainly of K⁺, Cl⁻, and malate, driving cell turgor and stomatal aperture. Our deep knowledge of these processes has made the guard cell the best known of plant cell models for membrane transport, signaling, and homeostasis (Willmer and Fricker, 1996; Blatt, 2000b; Roelfsema and Hedrich, 2010; Hills et al., 2012). This knowledge gives real substance to the prospect for reverse engineering of stomatal responses, using in silico design in directing genetic manipulation for improved crop yields, especially under water-limited conditions.Water use efficiency (WUE; defined as the amount of dry matter produced per unit of water transpired) is directly related to stomatal function. Thus, at the practical level, stomata represent an important target for breeders interested in manipulating crop performance. A large body of data relates stomata, transpiration, and carbon assimilation (Willmer and Fricker, 1996; Farquhar et al., 2001; Hetherington and Woodward, 2003; Lawson et al., 2011). Several examples illustrate how manipulating of stomatal characteristics can affect WUE (Fischer et al., 1998; Rebetzke et al., 2002; Masle et al., 2005; Eisenach et al., 2012). With few exceptions, however, mutations that increase WUE commonly do so at the expense of carbon assimilation, reflecting the trade-off in CO₂ availability with suppressed stomatal transpiration.Stomatal movements generally lag behind short-term changes in available light associated with sunflecks and shadeflecks (Pearcy, 1990; Lawson et al., 2012; Lawson and Blatt, 2014). This hysteresis in response, between stomatal aperture and gas exchange on one hand and photosynthetic capacity on the other, can lead alternately to periods of assimilation limited by stomatal conductance, and of high transpiration without corresponding rates of assimilation (Lawson et al., 2011). It has been argued that such hysteresis in stomatal responsiveness with the demand for CO₂ erodes assimilation and WUE, with substantial consequences for long-term yield (Vico et al., 2011; Eisenach et al., 2012; Lawson et al., 2012; Lawson and Blatt, 2014). If so, then improving WUE with gains in assimilation should be possible if the speed of stomatal responsiveness can be enhanced. However, the complexity of guard cell transport presents a formidable barrier to exploring such possibilities. Here we address this problem, taking advantage of OnGuard models of the stomatal guard cell. We explore in silico the potential for enhancing stomatal kinetics through single transporter (single gene product) manipulations. Our results identify the gating of the dominant K⁺ channels as the most promising target for experimental manipulation.     

A Comparison of the Growth, Photosynthesis, Stomatal Conductance and Water Use Efficiency of Moricandia and Brassica Species

When grown in pots and well-watered, the relative growth ratesof the above ground parts of two species of Moricandia (M. arvensis,an intermediate C₃–C₄ species, and M. moricandioides,a C₃ species) were inferior to those of two cultivated Brassicaspecies (B. campestris and B. napus). The Moricandia specieshad thicker leaves (greater d.wt per unit leaf area) with morechlorophyll than the Brassica species and had slightly greaterrates of photosynthesis per unit leaf area at an irradiance(400–700 nm) of 2000 µmol quanta m^–2 s ^–1.Leaves of M. arvensis, known to have a CO₂ compensation pointbetween that of C₃ and C₄ species, had a lower ratio of theintercellular to atmospheric partial pressure of CO₂ (C₁/C_a)and a greater instantaneous water use efficiency (WUE) thanthose of M. moricandioides and the Brassica species. Carbon isotope discrimination (     

Diurnal and Seasonal Patterns of Water Potential,Photosynthesis, Evapotranspiration and Water Use Efficiency of Clusterbean

Diurnal patterns of leaf water potential (_W), canopy net photosynthetic rate (P _N), evapotranspiration rate (E), canopy temperature (T_c), and water use efficiency (WUE) of clusterbean [Cyamopsis tetragonoloba (L.) Taub., cv. Desi] were studied at six phenological stages of plant development under field conditions at CCS Haryana Agricultural University, Hisar. The highest P _N, E, and WUE were observed at pod initiation stage (61 DAS). Daily maxima of P _N were usually between 11:00 to 14:00 h while those of E and WUE between 12:30 and 16:00 h. P _N was mainly dependent on photosynthetically active radiation and E on air temperature (T_a) but the relationships varied at different growth stages. WUE declined with the increase in T _a. At mid-day, _W was highest during pod initiation.     

Response of Photosynthesis, Stomatal Conductance and Water Use Efficiency to Elevated CO2 and Nutrient Supply in Acclimated Seedlings of Phaseolus vulgaris L.

Plants of Phaseolus vulgaris L were grown from seed in open-topgrowth chambers at present day (350 µmol mol^–1)and double the present day (700 µmol mol^–1) atmosphericCO₂ concentration with either low (L, without additional nutrientsolution) or relatively high (H, with additional nutrient solution)nutrient supply Measurements of assimilation rate, stomatalconductance and water use efficiency were started 17 d aftersowing on each fully expanded, primary leaf of three plantsper treatment Measurements were made in external CO₂ concentrations(C₂) of 200, 350, 450, 550 and 700 µmol mol^–1 andrelated to both C_a and to C₁, the mean intercellular space CO₂concentration Fully adjusted, steady state measurements weremade after approx 2 h equilibration at each CO₂ concentration The rate of CO₂ assimilation by leaves increased and stomatalconductance decreased similarly over the range of C_a or C₁ inall four CO₂ and nutrient supply treatments but both assimilationrate and stomatal conductance were higher in the high nutrientsupply treatment than in the low nutrient treatment The relationbetween assimilation rate or stomatal conductance and C₁ wasnot significantly different amongst plants grown in present-dayor elevated CO₂ concentration in either nutrient supply treatment,i e there was no evidence of down regulation of photosynthesisor stomatal response Increase in CO₂ concentration from 350to 700 µmol mol^–1 doubled water use efficiency ofindividual leaves in the high nutrient supply treatment andtripled water use efficiency in the low nutrient supply treatment The results support the hypothesis that acclimation phenomenaresult from unbalanced growth that occurs after the seed reservesare exhausted, when the supply of resources becomes growth limiting CO₂ enrichment, Phaseolus vulgaris L., net CO₂ assimilation rate, stomatal conductance, water use efficiency     

Photosynthesis,Transpiration, and Water Use Efficiency of Two Puccinellia Species on the Songnen Grassland,Northeastern China

The intra- and inter-specific variations in net photosynthetic (P _N) and transpiration (E) rates and water use efficiency (WUE) of Puccinellia tenuiflora and Puccinellia chinampoensis leaves were compared. The two species experienced a similar habitat, but differed in leaf area, leaf colour, and nitrogen contents. Leaf P _N and E for both reproductive and vegetative shoots of the two species declined with leaf age. P _N for reproductive shoots was less than for vegetative shoots, but their E was greater than that of vegetative shoots in the dry season. The average P _N and E for reproductive shoots of P. tenuiflora were lower than those of P. chinampoensis, but higher for vegetative shoots.     

贺兰山10种典型植物光合及水分利用效率特征研究

为了探究贺兰山不同乔灌草的光合生理特性及其对自然环境的适应特性和机制,该研究采用Li-6400XT便携式光合仪测定了贺兰山10种乔灌草气体交换参数及自然环境因子并分析其相关性.结果表明:(1)净光合速率(Pn)日均值从大到小为披针叶黄华>灰榆>山杨>栒子>冰草>油松>小叶忍冬>小檗>青海云杉>苔草,从不同生活型来看表现...     

模拟酸雨对柚木幼苗生长、光合与水分利用的影响

模拟pH6.5(对照)、4.5和2.5三个酸雨梯度,研究其对1a生组培柚木(TectonagrandisL.f.)幼苗生长、光合与水分利用的影响。结果表明,尽管不同处理间的各项生理指标差异不明显,但模拟酸雨对柚木形态构件参数造成较严重的影响。pH4.5和pH2.5处理组柚木基径(D)和树高(H)增长明显下降,使得D2H下降更加显著;不同处理下柚木叶片净光合速率(Pn)和蒸腾速率(E)日变化趋于一致,气孔导度(gs)日变化与对应的叶片净光合速率日变化十分相似,同时,对照与两个处理的Pn与gs之间都表现正相关(p<0.01),且在pH4.5处理表现更为显著,但是对照和两个处理的E与gs的线性关系不显著;pH4.5和2.5处理的水分利用效率(WUE)日变化趋于一致;对照胞间CO2浓度与大气CO2浓度比(Ci/Ca)均值最低,表明对照柚木对CO2利用最有效。     

Photosynthesis,Transpiration, and Water Use Efficiency in Two Divergent Leymus Chinensis Populations from Northeast China

The net photosynthetic rate (P _N), transpiration rate (E), and water use efficiency (WUE) of two divergent Leymus chinensis populations from the grassland region of Northeast China were compared. The two populations experienced the similar habitats, but differed in leaf colour, stomata numbers, and chlorophyll contents. The leaf P _N for the grey-green (GG) population was greater than that for the yellow-green (YG) population, while the leaf E for GG population was lower than that for the YG population. The greater WUE for the GG population suggests that this type is more able to maintain higher P _N under drought and is more fit for the rangeland use in this climate region.     

Effect of Water Stress on Photosynthesis,Chlorophyll Fluorescence Parameters and Water Use Efficiency of Common Reed in the Hexi Corridor

Russian Journal of Plant Physiology - Water stress is the major environmental stress that affect agricultural production worldwide, especially in arid and semi-arid regions. This research...     

不同基质和生长调节剂对红桂木光合与水分利用效率的影响

利用不同的栽培基质和生长调节剂,探讨其对红桂木光合作用和水分利用效率的影响。结果表明,基质土经改良(S2)并配以适量的吲哚丁酸(IBA)和萘乙酸(NAA),能增强植株的光合速率、蒸腾速率和水分利用率。改良基质(S2)与传统基质(S1)相比,前者显著地促进植株同化物的产生,极显著提高净光合速率、气孔导度、蒸腾速率、水分利用率、叶绿素b含量;IBA+NAA处理与IBA处理、CK相比,前者显著提高净光合速率、水分利用率、总叶绿素、叶绿素a、叶绿素b含量。综合而言,S2-IBA-NAA栽培组合有利于增强红桂木光合作用,提高红桂木同化能力。     

Photosynthesis,Transpiration, and Water Use Efficiency of Caragana microphylla,C. intermedia,and C. korshinskii

In the order C. microphylla — C. intermedia — C. korshinskii, compensation irradiance, saturation irradiance, and optimum temperature for photosynthesis increased, net photosynthetic rate (P _N) at low irradiance and low temperature decreased, optimum air humidity decreased, and P _N at low air humidity increased. Daily cumulative value of P _N increased while daily cumulative value of transpiration (E) decreased, and hence water use efficiency (WUE =P _N/E) increased. Diurnal course of P _N of C. microphylla was a double-peak curve, but the second peak in the curves of C. intermedia and C. korshinskii was not visible. These physiological characteristics are biological basis for the geographical distribution of these three Caragana species, and are in relation to water conditions of their habitats and distinctiveness in leaf hair of plant.     

Photosynthesis,Transpiration, and Water Use Efficiency of Vegetative and Reproductive Shoots of Grassland Species from North-Eastern China

The differences in net photosynthetic rate (P _N), transpiration rate (E), and water use efficiency (WUE) between the vegetative and reproductive shoots of three native grass species from the grassland of northeastern China [grey-green and yellow green populations of Leymus chinensis (Trin.) Tzvel., Puccinellia tenuiflora (Griseb) Scrib & Merr, Puccinellia chinampoensis Ohwi] were compared. The two type shoots experienced similar habitats, but differed in leaf life-span and leaf area. The leaf P _N and WUE for the vegetative shoots were significantly higher than those for the reproductive shoots in the grasses, while their E were remarked lower in the dry season. Relative lower leaf P _N and WUE for the reproductive shoots of grassland grasses may explain the facts of lower seed production and the subordinate role of seed in the grassland renewal in north-eastern China.     

Photosynthesis, Transpiration, Leaf Temperature, and Stomatal Activity of Cotton Plants under Varying Water Potentials

Cotton plants, Gossypium hirsutum L. were grown in a growth room under incident radiation levels of 65, 35, and 17 Langleys per hour to determine the effects of vapor pressure deficits (VPD's) of 2, 9, and 17 mm Hg at high soil water potential, and the effects of decreasing soil water potential and reirrigation on transpiration, leaf temperature, stomatal activity, photosynthesis, and respiration at a VPD of 9 mm Hg.

Transpiration was positively correlated with radiation level, air VPD and soil water potential. Reirrigation following stress led to slow recovery, which may be related to root damage occurring during stress. Leaf water potential decreased with, but not as fast as, soil water potential.

Leaf temperature was usually positively correlated with light intensity and negatively correlated with transpiration, air VPD, and soil water. At high soil water, leaf temperatures ranged from a fraction of 1 to a few degrees above ambient, except at medium and low light and a VPD of 19 mm Hg when they were slightly below ambient, probably because of increased transpirational cooling. During low soil water leaf temperatures as high as 3.4° above ambient were recorded. Reirrigation reduced leaf temperature before appreciably increasing transpiration. The upper leaf surface tended to be warmer than the lower at the beginning of the day and when soil water was adequate; otherwise there was little difference or the lower surface was warmer. This pattern seemed to reflect transpiration cooling and leaf position effects.

Although stomata were more numerous in the lower than the upper epidermis, most of the time a greater percentage of the upper were open. With sufficient soil water present, stomata opened with light and closed with darkness. Fewer stomata opened under low than high light intensity and under even moderate, as compared with high soil water. It required several days following reirrigation for stomata to regain original activity levels.

Apparent photosynthesis of cotton leaves occasionally oscillated with variable amplitude and frequency. When soil water was adequate, photosynthesis was nearly proportional to light intensity, with some indication of higher rates at higher VPD's. As soil water decreased, photosynthesis first increased and then markedly decreased. Following reirrigation, photosynthesis rapidly recovered.

Respiration was slowed moderately by decreasing soil water but increased before watering. Respiration slowed with increasing leaf age only on leaves that were previously under high light intensity.

     

Responses of Soybean Leaf Angle, Photosynthesis and Stomatal Conductance to Leaf and Soil Water Potential

The hypothesis that soil water potential (_s) is better correlatedto heliotropic leaf orientation, photosaturated photosyntheticCO₂ assimilation and stomatal conductance during periods oflimited water availability than is bulk leaf water potential(₁) was examined in greenhouse-grown soybean (Glycine max) plants,submitted to a progressive drought. Paired plants were exposedto either 1000 or 100 µmol m^–2 s^–1 photonflux densities (PFD) for 45–60 mins. The higher irradianceinduced short-term decreases in ₁, due to increased transpiration,while _l in the plant exposed to low PFD did not decrease. Thesechanges in ₁ occurred independently of changes in soil waterstatus. Concurrent to the light treatments, a single attachedleaf from each of the two plants was isolated from the restof the plant by shading, and the pulvinus of its terminal leafletwas exposed to a perpendicular PFD of 500 µmol m–²S^–1. Leaf movement of this leaflet was recorded in responseto this light, until a stable leaflet angle was achieved. Valuesof _s and _l (before and after light treatment), and photosaturatedrates of photosynthesis and stomatal conductance, were thenmeasured on these leaves. Leaflet angle and gas exchange werebetter correlated with _s (r² = 0.50, 0.50 and 0.57 for angle,photosynthesis and conductance, respectively) than with _l especiallywhen _l was the result of short-term, high-light induced changesin leaf water status (r² = 0.36, 0.32 and 0.49, for the sameparameters). Leaflet angle was also correlated with stomatalconductance (r² = 0.61) and photosynthetic rate (r² = 0.60),suggesting a close association between leaf orientation, leafmetabolism and soil water availability. Glycine max (L.) Merr. cv. Essex, soybean, heliotropism, water potential, photosynthesis, stomatal conductance, solar tracking     

Focus on Water: Separating Active and Passive Influences on Stomatal Control of Transpiration[OPEN]

Motivated by studies suggesting that the stomata of ferns and lycophytes do not conform to the standard active abscisic acid (ABA) -mediated stomatal control model, we examined stomatal behavior in a conifer species (Metasequoia glyptostroboides) that is phylogenetically midway between the fern and angiosperm clades. Similar to ferns, daytime stomatal closure in response to moderate water stress seemed to be a passive hydraulic process in M. glyptostroboides immediately alleviated by rehydrating excised shoots. Only after prolonged exposure to more extreme water stress did active ABA-mediated stomatal closure become important, because foliar ABA production was triggered after leaf turgor loss. The influence of foliar ABA on stomatal conductance and stomatal aperture was highly predictable and additive with the passive hydraulic influence. M. glyptostroboides thus occupies a stomatal behavior type intermediate between the passively controlled ferns and the characteristic ABA-dependent stomatal closure described in angiosperm herbs. These results highlight the importance of considering phylogeny as a major determinant of stomatal behavior.Stomata regulate parallel diffusive paths of water and carbon dioxide between leaves and the atmosphere, thus assuming a governing role over the processes of transpiration and photosynthesis. Guard cell movements that open and close stomata are increasingly characterized (similarly to animal cells) as being mediated by rapid changes in the polarization state of membranes (Schroeder et al., 2001; Hedrich, 2012; Hills et al., 2012). Despite this membrane-dominated view of stomatal function, the critical goal of modeling stomatal behavior to render predictions of transpiration and photosynthesis typically relies on a hydraulic framework built around the direct impact of leaf hydration on epidermal and guard cell turgor (Buckley, 2005; Damour et al., 2010). Although recent advances in modeling the ionic balance of guard cells (Chen et al., 2012; Hills et al., 2012) yield predictions of stomatal aperture, no macro-scale stomatal model has been able to predict stomatal conductance from the perspective of ion movements into and out of guard cells; however, the effects of key components, such as light, carbon dioxide, and abscisic acid (ABA), on membrane polarization have been studied in detail. Reconciling the dynamics of leaf-scale and canopy-scale transpiration with physical and chemical processes at the guard cell remains a major challenge.Among the obstacles preventing the formulation of a large-scale transpiration model based on membrane ion transport is the fact that much of the characterization of guard cell membrane processes has been confined to a handful of small, ruderal, herbaceous angiosperms. Although species like Arabidopsis (Arabidopsis thaliana) provide the ideal molecular system for identifying guard cell signal transduction pathways, most of these model species are of little agricultural relevance and being herbaceous, poor physical analogs for the tree species that dominate terrestrial gas exchange. Hence, there is a need to understand whether the same principles governing stomatal control in angiosperm herbs, like Arabidopsis, equally apply to plants that dominate forests and agricultural production. Recent studies suggest that there are important differences in the ion-transport machinery among vascular plants, and despite the presence of potential guard cell signaling pathways throughout the plant kingdom (Dreyer et al., 2012; Brodribb and McAdam, 2013b; Chater et al., 2013), there is evidence of a systematic shift in the behavior of stomata among vascular plants (Doi et al., 2006; Brodribb and McAdam, 2011; McAdam and Brodribb, 2012a). In particular, the critical closing tendency of stomata during leaf water deficit seems to have evolved from a passive process mediated directly by water potential (passive hydraulic) to an active process controlled by the extrusion of anions from guard cells (active closure; Brodribb and McAdam, 2011). The stomata of ferns and lycophytes predictably respond to plant water deficit as passive hydraulic valves, closing rapidly on dehydration and opening on rehydration (Brodribb and McAdam, 2011; McAdam and Brodribb, 2012a). Despite the stomata in these lineages only ever showing functionally passive responses to changes in leaf water status (Brodribb and McAdam, 2011; McAdam and Brodribb, 2012a, 2013), some have challenged the concept of a passive origin of stomatal control in vascular plants by showing a conserved activity of key genes involved in active stomatal responses (Ruszala et al., 2011). In seed plants, the closure of stomata in response to water deficit is mediated by augmented levels of ABA, which leads to a depolarization of guard cell membranes triggering osmotic ion efflux and a loss of guard cell turgor (Mittelheuser and Van Steveninck, 1969; Thiel et al., 1992; Geiger et al., 2009, 2011; Bauer et al., 2013). In light of this variation in stomatal control, it seems that a key step to finding a general model for stomatal behavior would be to understand the interactions between active and passive processes in the stomatal movements of major lineages of plants.Conifers contribute significantly to global transpiration and productivity and also seem to have a stomatal control system that is somewhat different from model angiosperm herbs. These distinctions include insensitivity to elevated carbon dioxide (Beadle et al., 1979; Morison and Jarvis, 1983; Brodribb et al., 2009); a lack of epidermal mechanical advantage, resulting in no Ivanov effect (the increase in transpiration from a leaf after excision or exposure to low humidity; Huber, 1923; Stålfelt, 1944; McAdam and Brodribb, 2012a), likely because of heavily lignified dorsal walls (the walls closest to the epidermal cells; Sack, 1987), and a very high length-to-width ratio of open stomatal pores (Copeland, 1902). Furthermore, recent research suggests that different conifer species depend more or less on ABA as an agent of stomatal closure during extended periods of water stress (Brodribb and McAdam, 2013a). The apparent lack of epidermal mechanical advantage in conifer stomata provides an unusual opportunity to examine the impacts of changing leaf water content and evaporation on stomatal conductance and guard cell turgor without the confusing Ivanov effect produced by changes in the ratio of epidermal and guard cell turgor pressure (Raschke, 1970). Manipulating the hydration status of the leaf, thus, allows quantification of the interacting influences of leaf water potential (Ψ_l) and ion transport on stomatal aperture (Brodribb and McAdam, 2013a).Our aim in this study was to determine under what conditions passive (hydraulic) and active (ABA mediated) closures of stomata were important in a representative conifer species. We assumed that, in the absence of a mechanical interaction from the epidermis, it would be possible to characterize both dynamic and steady-state stomatal behavior based on intrinsic leaf properties of ABA sensitivity, hydraulic conductance, capacitance, and hydraulic vulnerability. We chose the conifer Metasequoia glyptostroboides (Cupressaceae) as our subject, because it has leaf characteristics within the range of deciduous angiosperm trees; also, it is one of the few conifer species where stomata are sufficiently visible (unoccluded by waxes) to observe stomatal responses in the isolated epidermis.     

Photosynthesis, Transpiration, Leaf Stomatal and Mesophyll Resistance Measurements by the Use of a Ventilated Diffusion Porometer

A ventilated diffusion porometer was modified and adapted for simultaneous measurements of leaf resistance and photosynthesis (using ¹⁴C). The system enables measurements to be made under field and laboratory conditions with different concentrations of CO₂ and vapor pressure gradients between the evaporating surfaces inside the leaf and the external atmosphere. The leaf is subjected to the porometer's atmosphere only for short periods (up to 30 seconds) and it is assumed that stomata are not affected. Establishing the linear regression of the effect of CO₂ concentration on net photosynthesis makes it possible to extrapolate for CO₂ compensation point, to calculate the overall resistance to CO₂ and the mesophyll resistance to CO₂.     

Influence of Rate of Development of Leaf Water Deficits upon Photosynthesis, Leaf Conductance, Water Use Efficiency, and Osmotic Potential in Sorghum

This study reports the effect of rate of development of leaf water deficits in soil-grown sorghum (Sorghum bicolor) on the relationship of net photosynthesis, leaf conductance, and water use efficiency to leaf water potential, and on the degree of solute accumulation (osmotic adjustment). Recovery of these processes on rewatering, and responses during a second stress cycle were also studied. The most rapid rate of stress (1.2 MPa day^?1) resulted in no solute accumulation and the lowest rate of net photosynthesis and leaf conductance for any given leaf water potential during stress. Stress at 0.7 and 0.15 MPa day^?1 led to equal solute accumulations of approximately 0.6 MPa, but net photosynthesis, leaf conductance, and water use efficiency at a given leaf water potential were lower with the faster rate of stress (0.7 MPa day^?1). Additionally, leaf conductance at a given leaf turgor potential was lowest at the 1.2 MPa day^?1 stress rate, slightly higher at the intermediate rate of stress, and clearly highest at the slowest rate of stress. Recovery of both net photosynthesis and leaf conductance upon rewatering was rapid, taking less than 3 days, but full recovery of osmotic potential took between 6 and 11 days. One slow stress cycle had no influence on relationships during a second cycle. The concept of a threshold leaf water potential for stomatal closure is discussed and the conclusion reached that stomatal closure occurs slowly over a wide range of leaf water potential (> 1.0 MPa), the range being greater for slower rates of stress.     

Species- and Habitat-variability of Photosynthesis,Transpiration and Water Use Efficiency of Different Plant Species in Maowusu Sand Area

Photosynthesis ( Pn ), transpiration (E) and water use efficiency (WUE) of more than 66 add sand species from different environmental habitats, shifting sand dune, fixed sand dune, lowland and wetland in the Maowusu Sand Area were analyzed and the relation among these characteristics and the resource utilization efficiency, taxonomic categories and growth forms of the species were assessed. The results showed that species from Chenopodiaceae, Gramineae, keguminosae which possessed the C4 photosynthesis pathway, or C3 pathway and also with nitrogen-fixation capacities had higher or the highest Pa values, i.e., 20 ～30μmol CO2·m－2·s－l, while that of evergreen shrub of Pinaceae had the lowest Pa values, i.e., 0 ～ 5 μunol CO2·m－2·s－1. Those species from Compositae, Scrophuladaceae, and Gramineae with C3 pathway but no N- fixation capacity had the highest E rates, i.e., 20 ～30 mmol H2O· m－2·s－1 and again the evergreen shrub together with some species from Salicaceae and Compositae had the lowest E rates, i.e., 0 ~ 5 mmol H2O·m－2·s－1. Species from kegmninosae, Gramineae and Chenopodiaceae with C4 pathway or Cs pathway with N- fixation capacity, both shrubs and grasses, generally had higher WUE. However, even the physiological traits of the same species were habitat- and season-specific. The values of both Pa and E in late summer were much higher than those in early summer, with average increases of 26%, 40% respectively in the four habitats. WUE in late summer was, however, 12% lower. Generally, when the environments became drier as a result of habitats changed, i.e., in the order of wetland, lowland, fixed sand dune and shifting sand dune, Pn and E decreased but WUE increased.     

Effects of Water-Deficit Stress on Photosynthesis, Its Components and Component Limitations, and on Water Use Efficiency in Wheat (Triticum aestivum L.)

It is of theoretical as well as practical interest to identify the components of the photosynthetic machinery that govern variability in photosynthesis rate (A) and water-use efficiency (WUE), and to define the extent by which the component processes limit A and WUE during developing water-deficit stress. For that purpose, leaf exchange of CO₂ and H₂O was determined in two growth-chamber-grown wheat cultivars (Triticum aestivum L. cv TAM W-101 and cv Sturdy), and the capacity of A was determined and broken down into carboxylation efficiency (c.e.), light- and CO₂-saturated A, and stomatal conductance (g_s) components. The limitations on A measured at ambient CO₂ concentration (A₃₅₀) were estimated. No cultivar difference was observed when A₃₅₀ was plotted versus leaf water potential (Ψ_w). Light- and CO₂-saturated A, c.e., and g_s decreased with decreasing leaf Ψ_w, but of the corresponding photosynthesis limitations only those caused by insufficient c.e. and g_s increased. Thus, reduced stomatal aperture and Calvin cycle activity, but not electron transport/photophosphorylation, appeared to be major reasons for drought stress-induced inhibition of A₃₅₀. WUE measured as A₃₅₀/g_s first increased with stomatal closure down to a g_s of about 0.25 mol H₂O m⁻² s⁻¹ (Ψ_w = −1.6 MPa). However, it was predicted that A₃₅₀/g_s would decrease with more severe stress due to inhibition of c.e.     

Changes in Stomatal Conductance,Transpiration and Water Use Efficiency of Ten Species Experienced in High CO2 Concentrations in Biosphere 2

The stomotal conductance, transpiration and water use efficiency (WUE) were measured using a LI-6400 portable photosynthesis system for 5 tropical rain forest species and 5 desert species in Biosphere 2, USA. All the species have experienced in very high CO2 ( > 2 200 μmol• mol- 1 ) for more than 4.5 years. The results showed that the stomatal conductance and transpiration of rain forest species decreased from ( 127.4 ± 65.6) and (2.04 ± 0.61 ) mmol• m- 2•s- 1 to (61.3 + 30.5) and ( 1.54 ± 0.65 ) mmol• m-2• s -1 respectively, while WUE increased from (2.90 ± 0.55) to (8.45 ± 2.71) μmol CO2 •mmo1-1 H2O, with CO2 increasing from 350 – 400 to 700 – 820 μmol• mol-l. For the desert species, stomatal conductance and transpiration decreased from respectively (142.8±94.6) and (2.09±0.71) mmol•m-2•s-1 to (57.7±35.8) and (1.36±0.52) mmolm-2•s-l, but WUE increased from (4.69 ± 1.39) to (9.68 ± 1.61) μmol CO2•mmo1-1 H2O, with the CO2 increase from 320 - 400 to 820 – 850 μtmol• mol- 1. The stomatal conductance, transpiration and WUE were less influenced by light intensity under high CO2 than low CO2 concentrations. Most rain forest species reached their light saturation points at light intensity of 500 μmol• m-2•s-1, while desert species at 1 000 μmol•m-2•s-1. Among different species, the desert C3 tree, Nicotiana glauca Grah., had the highest decrease in stomatal conductance and transpiration and the highest increase in WUE, by 78%, 69% and 310% respectively. The enhancement of increasing CO2 to the stomatal, transpiration and WUE of species with different photosynthesis pathway and life forms in Biosphere 2 could be concluded as: C3 species > C4 species, and desert C3 species > rain forest C3 species.