Stomatal Limitation to Carbon Gain in Paphiopedilum sp. (Orchidaceae) and Its Reversal by Blue Light

Leaves from Paphiopedilum sp. (Orchidaceae) having achlorophyllous stomata, show reduced levels of stomatal conductance when irradiated with red light, as compared with either the related, chlorophyllous genus Phragmipedium or with their response to blue light. These reduced levels of stomatal conductance, and the failure of isolated Paphiopedilum stomata to open under red irradiation indicates that the small stomatal response measured in the intact leaf under red light is indirect.

The overall low levels of stomatal conductance observed in Paphiopedilum leaves under most growing conditions and their capacity to increase stomatal conductance in response to blue light suggested that growth and carbon gain in Paphiopedilum could be enhanced in a blue light-enriched environment. To test that hypothesis, plants of Paphiopedilum acmodontum were grown in controlled growth chambers under daylight fluorescent light, with or without blue light supplementation. Total photosynthetic photon flux density was kept constant in both conditions. Blue light enrichment resulted in significantly higher growth rates—of up to 77%—over a 3 to 4 week growing period, with all evidence indicating that the blue light effect was a stomatal response. Manipulations of stomatal properties aimed at long-term carbon gains could have agronomic applications.

     

The effect of exogenous abscisic acid on stomatal development, stomatal mechanics, and leaf gas exchange in Tradescantia virginiana

Gas exchange parameters and stomatal physical properties were measured in Tradescantia virginiana plants grown under well-watered conditions and treated daily with either distilled water (control) or 3.0 mM abscisic acid (ABA). Photosynthetic capacity (CO(2) assimilation rate for any given leaf intercellular CO(2) concentration [c(i)]) and relative stomatal sensitivity to leaf-to-air vapor-pressure difference were unaffected by the ABA treatment. However, at an ambient CO(2) concentration (c(a)) of 350 micromol mol(-1), ABA-treated plants operated with significantly lower c(i). ABA-treated plants had significantly smaller stomata and higher stomatal density in their lower epidermis. Stomatal aperture versus guard cell pressure (P(g)) characteristics measured with a cell pressure probe showed that although the form of the relationship was similar in control and ABA-treated plants, stomata of ABA-treated plants exhibited more complete closure at P(g) = 0 MPa and less than half the aperture of stomata in control plants at any given P(g). Scaling from stomatal aperture versus P(g) to stomatal conductance versus P(g) showed that plants grown under ABA treatment would have had significantly lower maximum stomatal conductance and would have operated with lower stomatal conductance for any given guard cell turgor. This is consistent with the observation of lower c(i)/c(a) in ABA-treated plants with a c(a) of 350 micromol mol(-1). It is proposed that the ABA-induced changes in stomatal mechanics and stomatal conductance versus P(g) characteristics constitute an improvement in water-use efficiency that may be invoked under prolonged drought conditions.     

Stomatal kinetics and photosynthetic gas exchange along a continuum of isohydric to anisohydric regulation of plant water status

Species' differences in the stringency of stomatal control of plant water potential represent a continuum of isohydric to anisohydric behaviours. However, little is known about how quasi‐steady‐state stomatal regulation of water potential may relate to dynamic behaviour of stomata and photosynthetic gas exchange in species operating at different positions along this continuum. Here, we evaluated kinetics of light‐induced stomatal opening, activation of photosynthesis and features of quasi‐steady‐state photosynthetic gas exchange in 10 woody species selected to represent different degrees of anisohydry. Based on a previously developed proxy for the degree of anisohydry, species' leaf water potentials at turgor loss, we found consistent trends in photosynthetic gas exchange traits across a spectrum of isohydry to anisohydry. More anisohydric species had faster kinetics of stomatal opening and activation of photosynthesis, and these kinetics were closely coordinated within species. Quasi‐steady‐state stomatal conductance and measures of photosynthetic capacity and performance were also greater in more anisohydric species. Intrinsic water‐use efficiency estimated from leaf gas exchange and stable carbon isotope ratios was lowest in the most anisohydric species. In comparisons between gas exchange traits, species rankings were highly consistent, leading to species‐independent scaling relationships over the range of isohydry to anisohydry observed.     

Water supply and demand remain balanced during leaf acclimation of Nothofagus cunninghamii trees

Higher leaf vein density (D(vein) ) enables higher rates of photosynthesis because enhanced water transport allows higher leaf conductances to CO(2) and water. If the total cost of leaf venation rises in proportion to the density of minor veins, the most efficient investment in leaf xylem relative to photosynthetic gain should occur when the water transport capacity of the leaf (determined by D(vein) ) matches potential transpirational demand (determined by stomatal size and density). We tested whether environmental plasticity in stomatal density (D(stomata) ) and D(vein) were linked in the evergreen tree Nothofagus cunninghamii to achieve a balance between liquid and gas phase water conductances. Two sources of variation were examined; within-tree light acclimation, and differences in sun leaves among plants from ecologically diverse populations. Strong, linear correlations between D(vein) and D(stomata) were found at all levels of comparison. The correlations between liquid- and vapour-phase conductances implied by these patterns of leaf anatomy were confirmed by direct measurement of leaf conductance in sun and shade foliage of an individual tree. ? Our results provide strong evidence that the development of veins and stomata are coordinated so that photosynthetic yield is optimized relative to carbon investment in leaf venation.     

Evidence from Amazonian forests is consistent with isohydric control of leaf water potential

Climate modelling studies predict that the rain forests of the Eastern Amazon basin are likely to experience reductions in rainfall of up to 50% over the next 50-100 years. Efforts to predict the effects of changing climate, especially drought stress, on forest gas exchange are currently limited by uncertainty about the mechanism that controls stomatal closure in response to low soil moisture. At a through-fall exclusion experiment in Eastern Amazonia where water was experimentally excluded from the soil, we tested the hypothesis that plants are isohydric, that is, when water is scarce, the stomata act to prevent leaf water potential from dropping below a critical threshold level. We made diurnal measurements of leaf water potential (psi 1), stomatal conductance (g(s)), sap flow and stem water potential (psi stem) in the wet and dry seasons. We compared the data with the predictions of the soil-plant-atmosphere (SPA) model, which embeds the isohydric hypothesis within its stomatal conductance algorithm. The model inputs for meteorology, leaf area index (LAI), soil water potential and soil-to-leaf hydraulic resistance (R) were altered between seasons in accordance with measured values. No optimization parameters were used to adjust the model. This 'mechanistic' model of stomatal function was able to explain the individual tree-level seasonal changes in water relations (r2 = 0.85, 0.90 and 0.58 for psi 1, sap flow and g(s), respectively). The model indicated that the measured increase in R was the dominant cause of restricted water use during the dry season, resulting in a modelled restriction of sap flow four times greater than that caused by reduced soil water potential. Higher resistance during the dry season resulted from an increase in below-ground resistance (including root and soil-to-root resistance) to water flow.     

弱光下生长的葡萄叶片蒸腾速率和气孔结构的变化

 植物能够对生长环境产生生态适应性,这种适应性可从气孔导度、光合速率、水分利用效率等生态指标上反映出来。为了研究葡萄蒸腾特性对弱光环境的适应性变化,本试验以‘京玉’葡萄幼苗（Vitis vinefera cv. Jingyu）为试验材料,通过遮光处理（2个处理,分别遮光65%和85%）营造弱光环境,测定了在弱光环境下生长的葡萄叶片蒸腾速率、气孔导度、水分利用效率对光照强度的响应,同时用扫描电镜技术观察了气孔的发育。结果表明,弱光环境下生长的葡萄幼苗,叶片的水势较高,但水分利用效率较低,叶片蒸腾速率和气孔导度变化对光照强度的响应缓慢,而自然光下生长的葡萄叶片则反应较迅速。通过对气孔结构的研究发现,与自然光照环境下生长的植株相比,在弱光环境下生长的葡萄幼苗,叶片下表皮的气孔横轴变宽,大小气孔之间差异减少,气孔外突,表皮细胞变大甚至扭曲,角质层变薄。说明葡萄幼苗能够对弱光环境产生适应性变化,其蒸腾特性的变化与其气孔结构的变化相关,具有一致性。     

Physiological effects of kaolin applications in well-irrigated and water-stressed walnut and almond trees

BACKGROUND AND AIMS: Kaolin applications have been used to mitigate the negative effects of water and heat stress on plant physiology and productivity with variable results, ranging from increased to decreased yields and photosynthetic rates. The mechanisms of action of kaolin applications are not clear: although the increased albedo reduces leaf temperature and the consequent heat stress, it also reduces the light available for photosynthesis, possibly offsetting benefits of lower temperature. The objective of this study was to investigate which of these effects are prevalent and under which conditions. METHODS: A 6% kaolin suspension was applied on well-irrigated and water-stressed walnut (Juglans regia) and almond (Prunus dulcis) trees. Water status (i.e. stem water potential, psi(s)), gas exchange (i.e. light-saturated CO2 assimilation rate, Amax; stomatal conductance, g(s)), leaf temperature (T(l)) and physiological relationships in treated and control trees were then measured and compared. KEY RESULTS: In both species, kaolin did not affect the daily course of psi(s) whereas it reduced Amax by 1-4 micromol CO2 m(-2) s(-1) throughout the day in all combinations of species and irrigation treatments. Kaolin did not reduce g(s) in any situation. Consequently, intercellular CO2 concentration (C(i)) was always greater in treated trees than in controls, suggesting that the reduction of Amax with kaolin was not due to stomatal limitations. Kaolin reduced leaf temperature (T(l)) by about 1-3 degrees C and leaf-to-air vapour pressure difference (VPD(l)) by about 0.1-0.7 kPa. Amax was lower at all values of g(s), T(l) and VPD(l) in kaolin-treated trees. Kaolin affected the photosynthetic response to the photosynthetically active radiation (PAR) in almond leaves: kaolin-coated leaves had similar dark respiration rates and light-saturated photosynthesis, but a higher light compensation point and lower apparent quantum yield, while the photosynthetic light-response curve saturated at higher PAR. When these parameters were used to model the photosynthetic response curve to PAR, it was estimated that the kaolin film allowed 63% of the incident PAR to reach the leaf. CONCLUSIONS: The main effect of kaolin application was the reduction, albeit minor, of photosynthesis, which appeared to be related to the shading of the leaves. The reduction in T(l) and VPD(l) with kaolin did not suffice to mitigate the adverse effects of heat and water stress on Amax.     

Gas-exchange response and stomatal and non-stomatal limitations to carbon assimilation of sunflower under salinity

Sunflower (Helianthus annuus) was grown in both open-field and outdoor potted conditions in Southern Italy, and irrigated with water having electrical conductivity ranging between 0.9 and 15.6 dS m(-1) obtained by different NaCl concentrations. The aim of the work was to study the leaf area and photosynthetic responses of sunflower to mild salt stress. The response curve (A/c(i)) of assimilation (A) to leaf internal CO(2) concentration (c(i)) was used to determine leaf gas-exchange parameters, in order to evaluate stomatal and non-stomatal limitations to photosynthesis in relation to salt stress. In the field, a reduction of 19% in leaf area expansion occurred, while no correlation was observed between Psi(l) and stomatal conductance to water vapour (g(sw)) ranging between 0.76 and 1.35 mol m(-2) s(-1). This result was also evident at a higher salinity level reached in the pot experiment where leaf osmotic potential (psi(s)) varied from -1.35 to -2.67 MPa as compared with the field experiment, where psi(s) ranged from -1.15 to -1.42 MPa. Considering the two experiments as a unique data set, the assimilation rate, the stomatal conductance to CO(2) (g(sc)) and the sensitivity of A to c(i) variation (g*) were not significantly influenced by salinity in the whole range of psi(s). As a consequence, the stomatal and non-stomatal limitations to photosynthesis were not affected by salt treatment, averaging around 20 and 80%, respectively. The variation in A (from 44 to 29 μmol m(-2) s(-1)) was paralleled by the variation in g(sc) (from 0.47 to 0.84 mol m(-2) s(-1)), with a remarkable constancy of both c(i) (200+/-12.5 μmol mol(-1)) and normalized water-use efficiency (5+/-0.7 μmol mmol(-1) kPa), showing the optimal behaviour of the plant processes. These findings indicate that, under mild salt stress, the same as observed under water deficit, sunflower controls assimilation mainly by modulating leaf area rather than by stomatal closure, and that non-stomatal limitation of photosynthesis was not affected at all by the level of salinity reached in this study.     

The stomatal response to evaporative demand persists at night in Ricinus communis plants with high nocturnal conductance

Evidence is building that stomatal conductance to water vapour (g(s)) can be quite high in the dark in some species. However, it is unclear whether nocturnal opening reflects a mechanistic limitation (i.e. an inability to close at night) or an adaptive response (i.e. promoting water loss for reasons unrelated to carbon gain). Further, it is unclear if stomatal responses to leaf-air vapour pressure difference (D) persist in the dark. We investigated nocturnal stomatal behaviour in castor bean (Ricinus communis L.) by measuring gas exchange and stomatal responses to D in the light and in the dark. Results were compared among eight growth environments [two levels for each of three treatment variables: air saturation deficit (D(a)), light and water availability]. In most plants, stomata remained open and sensitive to D at night. g(s) was typically lower at night than in the day, whereas leaf osmotic pressure (Pi) was higher at night. In well-watered plants grown at low D(a), stomata were less sensitive to D in the dark than in the light, but the reverse was found for plants grown at high D(a). Stomata of droughted plants were less sensitive to D in the dark than in the light regardless of growth D(a). Drought also reduced g(s) and elevated Pi in both the light and the dark, but had variable effects on stomatal sensitivity to D. These results are interpreted with the aid of models of stomatal conductance.     

Form,development and function of grass stomata

Stomata are cellular breathing pores on leaves that open and close to absorb photosynthetic carbon dioxide and to restrict water loss through transpiration, respectively. Grasses (Poaceae) form morphologically innovative stomata, which consist of two dumbbell‐shaped guard cells flanked by two lateral subsidiary cells (SCs). This ‘graminoid’ morphology is associated with faster stomatal movements leading to more water‐efficient gas exchange in changing environments. Here, we offer a genetic and mechanistic perspective on the unique graminoid form of grass stomata and the developmental innovations during stomatal cell lineage initiation, recruitment of SCs and stomatal morphogenesis. Furthermore, the functional consequences of the four‐celled, graminoid stomatal morphology are summarized. We compile the identified players relevant for stomatal opening and closing in grasses, and discuss possible mechanisms leading to cell‐type‐specific regulation of osmotic potential and turgor. In conclusion, we propose that the investigation of functionally superior grass stomata might reveal routes to improve water‐stress resilience of agriculturally relevant plants in a changing climate.     

Simulation of the stomatal conductance of winter wheat in response to light, temperature and CO2 changes

BACKGROUND AND AIMS: The stomata are a key channel of the water cycle in ecosystems, and are constrained by both physiological and environmental elements. The aim of this study was to parameterize stomatal conductance by extending a previous empirical model and a revised Ball-Berry model. METHODS: Light and CO(2) responses of stomatal conductance and photosynthesis of winter wheat in the North China Plain were investigated under ambient and free-air CO(2) enrichment conditions. The photosynthetic photon flux density and CO(2) concentration ranged from 0 to 2000 micro mol m(-2) s(-1) and from 0 to 1400 micro mol mol(-1), respectively. The model was validated with data from a light, temperature and CO(2) response experiment. RESULTS: By using previously published hyperbolic equations of photosynthetic responses to light and CO(2), the number of parameters in the model was reduced. These response curves were observed diurnally with large variations of temperature and vapour pressure deficit. The model interpreted stomatal response under wide variations in environmental factors. CONCLUSIONS: Most of the model parameters, such as initial photon efficiency and maximum photosynthetic rate (P(max)), have physiological meanings. The model can be expanded to include influences of other physiological elements, such as leaf ageing and nutrient conditions, especially leaf nitrogen content.     

The stomata of the fern Adiantum capillus-veneris do not respond to CO2 in the dark and open by photosynthesis in guard cells

The stomata of the fern Adiantum capillus-veneris lack a blue light-specific opening response but open in response to red light. We investigated this light response of Adiantum stomata and found that the light wavelength dependence of stomatal opening matched that of photosynthesis. The simultaneous application of red (2 micromol m(-2) s(-1)) and far-red (50 micromol m(-2) s(-1)) light synergistically induced stomatal opening, but application of only one of these wavelengths was ineffective. Adiantum stomata did not respond to CO2 in the dark; the stomata neither opened under a low intercellular CO2 concentration nor closed under high intercellular CO2 concentration. Stomata in Arabidopsis (Arabidopsis thaliana), which were used as a control, showed clear sensitivity to CO2. In Adiantum, stomatal conductance showed much higher light sensitivity when the light was applied to the lower leaf surface, where stomata exist, than when it was applied to the upper surface. This suggests that guard cells likely sensed the light required for stomatal opening. In the epidermal fragments, red light induced both stomatal opening and K+ accumulation in guard cells, and both of these responses were inhibited by a photosynthetic inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The stomatal opening was completely inhibited by CsCl, a K+ channel blocker. In intact fern leaves, red light-induced stomatal opening was also suppressed by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. These results indicate that Adiantum stomata lack sensitivity to CO2 in the dark and that stomatal opening is driven by photosynthetic electron transport in guard cell chloroplasts, probably via K+ uptake.     

Potential impacts of global elevated CO(2) concentrations on plants

Early experiments investigating the effects of CO(2) enrichment on plants frequently showed photosynthetic stimulation and reduced stomatal aperture over short time periods. Work on the effects of elevated CO(2) has advanced in two major areas: by the extension of long-term and field experiments, and through investigations on the wide range of negative feedbacks affecting plant responses to CO(2). Downward photosynthetic acclimation in response to CO(2) enrichment is frequently observed over the short and long term, and indicates the activity of diverse feedback mechanisms. CO(2) is generally viewed as a limiting photosynthetic resource. However, recent work on stomatal development has shown that this view is simplistic: long- and short-distance signalling of CO(2) concentration are necessary components of normal plant development.     

Physiological framework for adaptation of stomata to CO2 from glacial to future concentrations

In response to short-term fluctuations in atmospheric CO(2) concentration, c(a), plants adjust leaf diffusive conductance to CO(2), g(c), via feedback regulation of stomatal aperture as part of a mechanism for optimizing CO(2) uptake with respect to water loss. The operational range of this elaborate control mechanism is determined by the maximum diffusive conductance to CO(2), g(c(max)), which is set by the size (S) and density (number per unit area, D) of stomata on the leaf surface. Here, we show that, in response to long-term exposure to elevated or subambient c(a), plants alter g(c(max)) in the direction of the short-term feedback response of g(c) to c(a) via adjustment of S and D. This adaptive feedback response to c(a), consistent with long-term optimization of leaf gas exchange, was observed in four species spanning a diverse taxonomic range (the lycophyte Selaginella uncinata, the fern Osmunda regalis and the angiosperms Commelina communis and Vicia faba). Furthermore, using direct observation as well as flow cytometry, we observed correlated increases in S, guard cell nucleus size and average apparent 1C DNA amount in epidermal cell nuclei with increasing c(a), suggesting that stomatal and leaf adaptation to c(a) is linked to genome scaling.     

Rapid and Specific Modulation of Stomatal Conductance by Blue Light in Ivy (Hedera helix) : An Approach to Assess the Stomatal Limitation of Carbon Assimilation

Low intensity (0.015 millimole per square meter per second) blue light applied to leaves of Hedera helix under a high intensity red light background (0.50 millimole per square meter per second red light) induced a specific stomatal opening response, with rapid kinetics comparable to those previously reported for stomata with `grass type' morphology. The response of stomatal conductance to blue light showed a transient `overshoot' behavior at high vapor pressure difference (2.25 ± 0.15 kiloPascals), but not at low vapor pressure difference (VPD) (0.90 ± 0.10 kilo-Pascal). The blue light-induced conductance increase was accompanied by an increase in net photosynthetic carbon assimilation, mediated by an increase in the intercellular concentration of carbon dioxide. Values of assimilation once the blue light-stimulated conductance increase reached steady state were less than those at the peak of the overshoot, but the ratios of assimilation to transpiration (A/E) and blue light-stimulated ΔA/ΔE were greater during the steady-state response than during the overshoot. These results indicate that significant stomatal limitation of assimilation can occur, but that this limitation may improve water use efficiency under high VPD conditions. Under high intensity red light, the decline in A/E associated with an increase in VPD was minimized when conductance was stimulated by additional low intensity blue light. This effect indicates that the blue light response of stomata may be important in H. helix for the optimization of water use efficiency under natural conditions of high irradiance and VPD.     

Light-emitting diodes as a light source for photosynthesis research

Light-emitting diodes (LED) can provide large fluxes of red photons and so could be used to make lightweight, efficient lighting systems for photosynthetic research. We compared photosynthesis, stomatal conductance and isoprene emission (a sensitive indicator of ATP status) from leaves of kudzu (Pueraria lobata (Willd) Ohwi.) enclosed in a leaf chamber illuminated by LEDs versus by a xenon arc lamp. Stomatal conductance was measured to determine if red LED light could sufficiently open stomata. The LEDs produced an even field of red light (peak emission 656±5 nm) over the range of 0–1500 mol m^-2 s^-1. Under ambient CO₂ the photosynthetic response to red light deviated slightly from the response measured in white light and stomatal conductance followed a similar pattern. Isoprene emission also increased with light similar to photosynthesis in white light and red light. The response of photosynthesis to CO₂ was similar under the LED and xenon arc lamps at equal photosynthetic irradiance of 1000 mol m^-2 s^-1. There was no statistical difference between the white light and red light measurements in high CO₂. Some leaves exhibited feedback inhibition of photosynthesis which was equally evident under irradiation of either lamp type. Photosynthesis research including electron transport, carbon metabolism and trace gas emission studies should benefit greatly from the increased reliability, repeatability and portability of a photosynthesis lamp based on light-emitting diodes.     

Stomatal sensitivities to changes in leaf water potential, air humidity, CO2 concentration and light intensity, and the effect of abscisic acid on the sensitivities in six temperate deciduous tree species

To characterise the stomata of six temperate deciduous tree species, sets of stomatal sensitivities to all the most important environmental factors were measured. To compare the importance of abscisic acid (ABA) in the different stomatal responses, the effect of exogenous ABA on all the stomatal sensitivities was determined.Almost all the stomatal sensitivities: the sensitivity to a decrease in leaf water potential, air humidity, CO₂ concentration ([CO₂]) and light intensity, and to an increase in [CO₂] and light intensity were the highest in the slow-growing species, and the lowest in the fast-growing species. Drought increased the sensitivity to the environmental changes that induce a decrease in the stomatal conductance, and decreased the sensitivity to the changes that induce an increase in this conductance. The sensitivities of the slow-growers were most strongly affected by drought and ABA. Therefore the success of the slow-growers in their ecological niches can be based on the highly sensitive and strictly regulated responses of their stomata. The fast-growers had the highest sensitivity to an increase in leaf water potential and this sensitivity was sharply reduced by drought and ABA. Thus, the dominance of the trees in riparian areas can be based on the ability of their stomata to quickly reach high conductance in well-watered conditions and to efficiently decrease this rate during drought.Stomatal sensitivities to the hydraulic environmental factors (water potentials in plant and air) had higher values in well-watered trees and a more pronounced response to drought than the sensitivities to the photosynthetic environmental factors ([CO₂] and light intensity). Thus, the hydraulic factors most likely prevail over the photosynthetic factors in determining stomatal conductance in these species.In response to exogenous ABA, the rates of stomatal closure, following a decrease in air humidity and light intensity, and an increase in [CO₂], were accelerated. Stomatal opening following an increase in air humidity and light intensity and a decrease in [CO₂] was replaced by slow closing. The rate of stomatal opening following an increase in leaf water potential was reduced. As the sensitivities to changes in light were modified less by the ABA than the other stomatal sensitivities, the prediction of stomatal responses on the basis of the sensitivity to light alone should be excluded in stomatal models.     

In situ observation of stomatal movements and gas exchange of Aegopodium podagraria L. in the understorey

Observations of stomata in situ while simultaneously measuring CO(2) gas exchange and transpiration were made in field experiments with Aegopodium podagraria in a highly variable light climate in the understorey of trees. The low background photosynthetic photon flux density (PPFD) caused a slight opening of the stomata and no visible response to sporadic lightflecks. However, if lightflecks were frequent and brighter, slow opening movements were observed. Small apertures were sufficient to allow maximal photosynthetic rates. Therefore, the small apertures observed in low light usually only caused minor stomatal limitations of lightfleck photosynthesis. The response of stomata to step-wise changes in PPFD under different levels of leaf to air vapour pressure difference (Delta(W)) was observed under controlled conditions. High Delta(W) influenced the stomatal response only slightly by reducing stomatal aperture in low light and causing a slight reduction in the initial capacity to utilize high PPFD levels. Under continuous high PPFD, however, stomata opened to the same degree irrespective of Delta(W). Under high Delta(W), opening and closing responses to PPFD-changes were faster, which enabled a rapid removal of the small stomatal limitations of photosynthesis initially present in high Delta(W) after longer periods in low light. It is concluded that A. podagraria maintains a superoptimal aperture in low light which leads to a low instantaneous water use efficiency, but allows an efficient utilization of randomly occurring lightflecks.     

Kinetic characterization of reduced pyridine nucleotide dehydrogenases (duroquinone-dependent) in cucurbita microsomes

Stomatal conductances of normally oriented and inverted leaves were measured as light levels (photosynthetic photon flux densities) were increased to determine whether abaxial stomata of Vicia faba leaves were more sensitive to light than adaxial stomata. Light levels were increased over uniform populations of leaves of plants grown in an environmental chamber. Adaxial stomata of inverted leaves reached maximum water vapor conductances at a light level of 60 micromoles per square meter per second, the same light level at which abaxial stomata of normally oriented leaves reached maximum conductances. Abaxial stomata of inverted leaves reached maximum conductances at a light level of 500 micromoles per square meter per second, the same light level at which adaxial stomata of normally oriented leaves reached maximum conductances. Maximum conductances in both normally oriented and inverted leaves were about 200 millimoles per square meter per second for adaxial stomata and 330 millimoles per square meter per second for abaxial stomata. Regardless of whether leaves were normally oriented or inverted, when light levels were increased to values high enough that upper leaf surfaces reached maximum conductances (about 500 micromoles per square meter per second), light levels incident on lower, shaded leaf surfaces were just sufficient (about 60 micromoles per square meter per second) for stomata of those surfaces to reach maximum conductances. This `coordinated' stomatal opening on the separate epidermes resulted in total leaf conductances for normally oriented and inverted leaves that were the same at any given light level. We conclude that stomata in abaxial epidermes of intact Vicia leaves are not more sensitive to light than those in adaxial epidermes, and that stomata in leaves of this plant do not respond to light alone. Additional factors in bulk leaf tissue probably produce coordinated stomatal opening on upper and lower leaf epidermes to optimally meet photosynthetic requirements of the whole leaf for CO₂.     

Steady-state stomatal responses of C3 and C4 species to blue light fraction: Interactions with CO2 concentration

Blue light induced stomatal opening has been studied by applying a short pulse (~5 to 60 s) of blue light to a background of saturating photosynthetic red photons, but little is known about steady-state stomatal responses. Here we report stomatal responses to blue light at high and low CO₂ concentrations. Steady-state stomatal conductance (g_s) of C₃ plants increased asymptotically with increasing blue light to a maximum at 20% blue (120 μmol m⁻² s⁻¹). This response was consistent from 200 to 800 μmol mol⁻¹ atmospheric CO₂ (C_a). In contrast, blue light induced only a transient stomatal opening (~5 min) in C₄ species above a C_a of 400 μmol mol⁻¹. Steady-state g_s of C₄ plants generally decreased with increasing blue intensity. The net photosynthetic rate of all species decreased above 20% blue because blue photons have lower quantum yield (moles carbon fixed per mole photons absorbed) than red photons. Our findings indicate that photosynthesis, rather than a blue light signal, plays a dominant role in stomatal regulation in C₄ species. Additionally, we found that blue light affected only stomata on the illuminated side of the leaf. Contrary to widely held belief, the blue light-induced stomatal opening minimally enhanced photosynthesis and consistently decreased water use efficiency.