Dynamics of patchy stomatal movements, and their contribution to steady-state and oscillating stomatal conductance calculated using gas-exchange techniques

Patchy stomatal movements were induced in leaves of Helianthus annuus L. and Xanthium strumarium L. by increasing Δw and decreasing light in a gas-exchange cuvette. The dynamics of the patchy movements were recorded and analysed using images of chlorophyll fluorescence, and the influence of heterogeneous stomatal activity on gas-exchange measurements of whole-leaf stomatal conductance was explored. Image series and gas-exchange measurements from two contrasting 100 min experiments are presented. One series of images, taken using Helianthus annuus, was characterized by strongly oscillating stomatal conductance induced by a decrease in light at high Δw. Fluorescence analysis revealed that individual patches of the leaf displayed a variety of behaviours (from static to strongly oscillating fluorescence), which, when averaged, matched the time dependence of the oscillating stomatal conductance measured by gas-exchange techniques. During the second series of images, taken using Xanthium strumarium, stomatal conductance (measured with gas exchange) declined slightly after an increase in Δw, and then maintained a steady state. Again, some patches in this leaf showed highly dynamic q_NP, although on the whole q_NP varied without any obvious pattern or frequency. When all patch activity in this series was averaged, it paralleled the steady whole-leaf stomatal conductance determined by gas-exchange measurements. It is clear from this work that coordinated patchy stomatal movements can contribute significantly to the dynamics of whole-leaf stomatal conductance, and, in contrast, that dynamic but uncoordinated patchy movements can average to produce a steady gas-exchange trace.     

Simultaneous measurement of stomatal conductance, non-photochemical quenching, and photochemical yield of photosystem II in intact leaves by thermal and chlorophyll fluorescence imaging

A new imaging system capable of simultaneously measuring stomatal conductance and fluorescence parameters, non-photochemical quenching (NPQ) and photochemical yield of photosystem II (Phi(PSII)), in intact leaves under aerobic conditions by both thermal imaging and chlorophyll fluorescence imaging was developed. Changes in distributions of stomatal conductance and fluorescence parameters across Phaseolus vulgaris L. leaves induced by abscisic acid treatment were analyzed. A decrease in stomatal conductance expanded in all directions from the treatment site, then mainly spread along the lateral vein toward the leaf edge, depending on the ABA concentration gradient and the transpiration stream. The relationships between stomatal conductance and fluorescence parameters depended on the actinic light intensity, i.e. NPQ was greater and Phi(PSII) was lower at high light intensity. The fluorescence parameters did not change, regardless of stomatal closure levels at a photosynthetically active photon flux (PPF) of 270 micro mol m(-2) s(-1); however, they drastically changed at PPF values of 350 and 700 micro mol m(-2) s(-1), when the total stomatal conductance decreased to less than 80 and 200 mmol m(-2) s(-1), respectively. This study has, for the first time, quantitatively analyzed relationships between spatiotemporal variations in stomatal conductance and fluorescence parameters in intact leaves under aerobic conditions.     

Effects of patchy stomatal closure on gas exchange measurements following abscisic acid treatment

Gas exchange data and images of leaf fluorescence were collected concurrently as stomata responded to abscisic acid (ABA) application. When 10^?5kmolm^?3 ABA was applied to the transpiration stream in a short pulse, stomatal conductance (g_s), photosynthesis (A) and intercellular CO2 concentration (C_i) decreased rapidly after a short lag period and became approximately constant after 2h. There was an apparent reduction in the A versus c1 relationship as stomata closed, but the data returned to the A versus C1 curve while stomatal conductance was constant or slowly rising during the second hour after ABA treatment. Larger amounts of ABA administered during the pulse caused larger deviations from the A versus c1 relationship. When 10^?7kmolm^?3 ABA was applied continuously through the transpiration stream, gs, A and Cⁱ decreased, but there was no substantial deviation from the A versus c{ curve. Fluorescence images were patchy as stomata closed for all experiments, but became slowly more uniform during the time that gas exchange was returning to the A versus Cj curve. The distribution of con-ductance among patches was not bimodal, and larger devi-ations from the A versus ct curve had greater ranges of pixel values and more pixel values representing low values of Cj during stomatal closure than did experiments show-ing small or no deviation. Estimates of A and gs from fluo-rescence images compared favourably with measured val-ues in most cases, suggesting that the patchy distributions of fluorescence were caused by patchy distributions of stomatal conductance and that apparent reductions in the A versus ct relationship were the result of these patchy stomatai distributions and not direct effects of ABA on mesophyll functioning. The data show that stomatal patches can be temporary and that patchiness may not be reflected in gas exchange data if the range of stomatal con-ductances is not large. These observations may explain some of the discrepancies among previous studies concerning the effect of ABA on the A versus Cⁱ relationship.     

Topography of photosynthetic activity of leaves obtained from video images of chlorophyll fluorescence

The distribution of photosynthetic activity over the area of a leaf and its change with time was determined (at low partial pressure of O₂) by recording images of chlorophyll fluorescence during saturating light flashes. Simultaneously, the gas exchange was being measured. Reductions of local fluorescence intensity quantitatively displayed the extent of nonphotochemical quenching; quench coefficients, q_N, were computed pixel by pixel. Because rates of photosynthetic electron transport are positively correlated with (1 − q_N), computed images of (1 − q_N) represented topographies of photosynthetic activity. Following application of abscisic acid to the heterobaric leaves of Xanthium strumarium L., clearly delineated regions varying in nonphotochemical quenching appeared that coincided with areoles formed by minor veins and indicated stomatal closure in groups.     

High yields in advanced lines of Pima cotton are associated with higher stomatal conductance, reduced leaf area and lower leaf temperature

Advanced lines of Pima cotton ( Gossypium barbadense L.) bred for higher yield potential and heat resistance have higher stomata conductance and smaller leaf areas than those of obsolete lines. In controlled experiments, five commercial lines of Pima cotton having increasing lint yield and heat resistance showed a gradient of increasing stomatal conductance and decreasing leaf size. In field experiments, heat-sensitive, low yield Pima lines showed a lower stomatal conductance than high yielding, advanced lines. This indicates that selection for high yield potential and heat resistance has imposed a selection pressure for higher stomatal conductance and smaller leaf areas. The higher stomatal conductance and smaller leaf area in the advanced lines resulted in a lower leaf temperature in both controlled environments and in the field. The largest leaf temperature differences between obsolete and advanced lines were observed in the afternoon. These differences coincided with the largest differences in stomatal conductance and the highest air temperatures. Measurements of stomatal conductance and leaf temperature in field-grown progeny from a cross between the advanced line, Pima S-6. and the obsolete line, Pima 32, showed that genetically determined differences in stomatal conductance resulted in corresponding differences in leaf temperature. None of the altered physiological traits were selected for in the breeding program, indicating that selection for the desired agronomic traits imposed selection pressures on the altered physiological traits. The increases in stomatal conductance and decreases in leaf area could represent an integrated response to selection pressures on enhanced evaporative cooling, ensuing from selection for heat resistance.     

Higher rates of leaf gas exchange are associated with higher leaf hydrodynamic pressure gradients

Steady-state leaf gas-exchange parameters and leaf hydraulic conductance were measured on 10 vascular plant species, grown under high light and well-watered conditions, in order to test for evidence of a departure from hydraulic homeostasis within leaves as hydraulic conductance varied across species. The plants ranged from herbaceous crop plants to mature forest trees. Across species, under standardized environmental conditions (saturating light, well watered), mean steady-state stomatal conductance to water vapour (g(w)) was highly correlated with mean rate of CO2 assimilation (A) and mean leaf hydraulic conductance normalized to leaf area (k(leaf)). The relationship between A and g(w) was well described by a power function, while that between A and k(leaf) was highly linear. Non-linearity in the relationship between g(w) and k(leaf) contributed to an increase in the hydrodynamic (transpiration-induced) water potential drawdown across the leaf (delta psi(leaf)) as k(leaf) increased across species, although across the 10 species the total increase in delta psi(leaf) was slightly more than twofold for an almost 30-fold increase in g(w). Higher rates of leaf gas exchange were therefore associated with higher k(leaf) and higher leaf hydrodynamic pressure gradients. A mechanistic model incorporating the stomatal hydromechanical feedback loop is used to predict the relationship between delta psi(leaf) and k(leaf), and to explore the coordination of stomatal and leaf hydraulic properties in supporting higher rates of leaf gas exchange.     

Stomatal patchiness and task-performing networks

BACKGROUND: Patchy stomatal conductance is a poorly understood and little-studied phenomenon. It is relatively common, yet it appears to be detrimental to water-use efficiency under some conditions and has no immediately obvious physiological function of any kind. Much of the difficulty in studying patchy stomatal conductance is tied to its unpredictability, both in occurrence and in characteristics. SCOPE AND CONCLUSIONS: Statistical analyses of the variability of stomatal patchiness reveal remarkable similarities to structures and behaviours found in locally connected networks of dynamic units that perform tasks. Such systems solve problems that reside at the level of the entire network despite the absence of a central processor or a mechanism for directly sharing information over the entire system. Frequently, task performance is emergent, in the sense that no unit independently performs the task. Because each unit in the network can communicate with only its immediate neighbours, problem solving is accomplished by the states of the individual units self-organizing into synchronized, collective patterns. In some cases, patches of states form and move coherently over the network, thus providing a means for distantly separated parts of the network to communicate. Often, exactly what form these patches take and how they move as the units synchronize is highly unpredictable. In analogy with such networks, it is suggested that stomatal patchiness may be a signature that plants optimize gas exchange in a more sophisticated and adaptive manner than if performed by their individual stomata independently.     

Separation of Direct and Indirect Responses of Stomata to Light: Results from a Leaf Inversion Experiment at Constant Intercellular CO2 Molar Fraction

Previous work has shown that stomata respond directly to light,but it was not clear whether the only additional response isthrough CO₂, or whether some other metabolite is involved inthis response. Gas exchange experiments were done with normallypositioned and inverted leaves of Hedera helix to investigatethis problem. The macroscopic optical properties of the leavesand their anatomical structure were also studied. These experimentssnowed that there is no need to postulate the existence of amessenger other than CO₂ to explain the indirect response ofstomata to light. The experiments also showed that leaf inversionaffects both stomatal conductance and photosynthesis, and highlightthe difficulties involved in the interpretation of the effectof leaf inversion on stomata when stomatal conductance measurementsare not done concurrently with measurements of CO₂ flux densityand intercellular CO₂ molar fraction Key words: Hedera helix, ivy, gas exchange, leaf inversion, stomatal conductance, light, CO₂ flux density, photosynthesis     

Use of thermography for quantitative studies of spatial and temporal variation of stomatal conductance over leaf surfaces

This paper describes a new approach to the calibration of thermal infrared measurements of leaf temperature for the estimation of stomatal conductance and illustrates its application to thermal imaging of plant leaves. The approach is based on a simple reformulation of the leaf energy balance equation that makes use of temperature measurements on reference surfaces of known conductance to water vapour. The use of reference surfaces is an alternative to the accurate measurement of all components of the leaf energy balance and is of potentially wide application in studies of stomatal behaviour. The resolution of the technique when applied to thermal images is evaluated and some results of using the approach in the laboratory for the study of stomatal behaviour in leaves of Phaseolus vulgaris L. are presented. Conductances calculated from infrared measurements were well correlated with estimates obtained using a diffusion porometer.     

A method for quantitative analysis of spatially variable physiological processes across leaf surfaces

Many physiological processes are spatially variable across leaf surfaces. While maps of photosynthesis, stomatal conductance, gene expression, water transport, and the production of reactive oxygen species (ROS) for individual leaves are readily obtained, analytical methods for quantifying spatial heterogeneity and combining information gathered from the same leaf but with different instruments are not widely used. We present a novel application of tools from the field of geographical imaging to the multivariate analysis of physiological images. Procedures for registration and resampling, cluster analysis, and classification provide a general framework for the analysis of spatially resolved physiological data. Two experiments were conducted to illustrate the utility of this approach. Quantitative analysis of images of chlorophyll fluorescence and the production of ROS following simultaneous exposure of soybean leaves to atmospheric O₃ and soybean mosaic virus revealed that areas of the leaf where the operating quantum efficiency of PSII was depressed also experienced an accumulation of ROS. This correlation suggests a causal relationship between oxidative stress and inhibition of photosynthesis. Overlaying maps of leaf surface temperature and chlorophyll fluorescence following a photoinhibition treatment indicated that areas with low operating quantum efficiency of PSII also experienced reduced stomatal conductance (high temperature). While each of these experiments explored the covariance of two processes by overlaying independent images gathered with different instruments, the same procedures can be used to analyze the covariance of information from multiple images. The application of tools from geographic image analysis to physiological processes occurring over small spatial scales will help reveal the mechanisms generating spatial variation across leaves.     

Stomatal responses to non-local changes in PFD: evidence for long-distance hydraulic interactions

Interactions among stomata within a single areole have recently been reported, and evidence suggests that hydraulic mechanisms may be responsible for these interactions. Such interactions may play a role in patchy stomatal behaviour by coordinating stomatal behaviour within areoles. However, models suggest that longer‐distance interactions may be required to produce the large‐scale discoordination that is characteristic of stomatal patchiness. This study was undertaken to characterize long‐distance interactions between ‘artificial patches’ of stomata under varying conditions of evaporative demand and soil water stress. Gas‐exchange was monitored in two adjacent regions (‘patches’) of a wheat leaf by two independent gas mixing and analysis systems. When photon flux density (PFD) was changed in only one of these patches, stomatal conductance responded in both patches in a manner consistent with hydraulic interactions propagated by changes in xylem water potential. These data are discussed in the context of mechanisms for patchy stomatal conductance and implications for the design and analysis of gas‐exchange experiments.     

Combining thermal and visible imagery for estimating canopy temperature and identifying plant stress

Thermal imaging is a potential tool for estimating plant temperature, which can be used as an indicator of stomatal closure and water deficit stress. In this study, a new method for processing and analysing thermal images was developed. By using remote sensing software, the information from thermal and visible images was combined, the images were classified to identify leaf area and sunlit and shaded parts of the canopy, and the temperature statistics for specific canopy components were calculated. The method was applied to data from a greenhouse water-stress experiment of Vicia faba L. and to field data for Vitis vinifera L. Vaseline-covered and water-sprayed plants were used as dry and wet references, respectively, and two thermal indices, based on temperature differences between the canopy and reference surfaces, were calculated for single Vicia faba plants. The thermal indices were compared with measured stomatal conductance. The temperature distributions of sunlit and shaded leaf area of Vitis vinifera canopies from natural rainfall and irrigation treatments were compared. The present method provides two major improvements compared with earlier methods for calculating thermal indices. First, it allows more accurate estimation of the indices, which are consequently more closely related to stomatal conductance. Second, it gives more accurate estimates of the temperature distribution of the shaded and sunlit parts of canopy, and, unlike the earlier methods, makes it possible to quantify the relationship between temperature variation and stomatal conductance.     

Asymmetric patchy stomatal closure for the two surfaces of Xanthium strumarium L. leaves at low humidity

Images of chlorophyll fluorescence were used to demonstrate patchy stomatal closure at low humidities in leaves of well-watered Xanthium strumarium plants. The pattern and extent of patchy stomatal closure were shown to be different for the two surfaces of amphistomatous leaves by taking images of leaves with CO₂ available to only one leaf was exposed to low humidity, patchiness was more extensive on that surface. Gas-exchange experiments were also conducted to determine the apparent photosynthetic capacity of the mesophyll (photosynthesis rate at constant c_i when it was supplied with CO₂ through both surfaces or through each surface alone. These experiments showed declines in the apparent photosynthetic capacity of the mesophyll at low humidities that were consistent with patchy stomatal closure on one or both surfaces. The results suggest that patchy stomatal closure can be a factor in the steady-state reponses of stomata to humidity. In amphistomatous leaves this is further complicated by the fact that patches on one epidermis may not coincide with those of the other surface.     

The contribution of photosynthesis to the red light response of stomatal conductance

To determine the contribution of photosynthesis on stomatal conductance, we contrasted the stomatal red light response of wild-type tobacco (Nicotiana tabacum 'W38') with that of plants impaired in photosynthesis by antisense reductions in the content of either cytochrome b(6)f complex (anti-b/f plants) or Rubisco (anti-SSU plants). Both transgenic genotypes showed a lowered content of the antisense target proteins in guard cells as well as in the mesophyll. In the anti-b/f plants, CO(2) assimilation rates were proportional to leaf cytochrome b(6)f content, but there was little effect on stomatal conductance and the rate of stomatal opening. To compare the relationship between photosynthesis and stomatal conductance, wild-type plants and anti-SSU plants were grown at 30 and 300 micromol photon m(-2) s(-1) irradiance (low light and medium light [ML], respectively). Growth in ML increased CO(2) assimilation rates and stomatal conductance in both genotypes. Despite the significantly lower CO(2) assimilation rate in the anti-SSU plants, the differences in stomatal conductance between the genotypes were nonsignificant at either growth irradiance. Irrespective of plant genotype, stomatal density in the two leaf surfaces was 2-fold higher in ML-grown plants than in low-light-grown plants and conductance normalized to stomatal density was unaffected by growth irradiance. We conclude that the red light response of stomatal conductance is independent of the concurrent photosynthetic rate of the guard cells or of that of the underlying mesophyll. Furthermore, we suggest that the correlation of photosynthetic capacity and stomatal conductance observed under different light environments is caused by signals largely independent of photosynthesis.     

不同浇水量对毛乌素沙地沙柳幼苗气体交换过程及其光化学效率的影响

 利用毛乌素沙地优势灌木沙柳(Salix psammophila)幼苗,研究气体交换及其PSⅡ的光化学效率对4种浇水量响应。结果表明：不同浇水量明显影响沙柳幼苗净光合速率、蒸腾速率、气孔导度、胞间CO2浓度和叶片温度日变化动态;除幼苗叶片温度随着浇水量的增加而降低外,其它以上因素均随浇水量的增加而增大。浇水量少时,幼苗净光合速率明显有“午睡”现象,而充足浇水能有效消除幼苗净光合速率的“午睡”现象;浇水量增加降低了沙柳幼苗光补偿点,提高了饱和光强和表观量子效率;浇水量显著影响幼苗的PSⅡ光化学效率,随着浇水量减少,幼苗的最大荧光、可变荧光、最大荧光比和最大光化学效率逐渐减小。相关分析反映了在不同施水量处理下,环境因子光合有效辐射、大气温度、大气饱和水汽压差及其生理变量气孔导度、叶片温度、胞间CO2浓度均显著影响幼苗净光合速率。     

Photocontrol of the Functional Coupling between Photosynthesis and Stomatal Conductance in the Intact Leaf : Blue Light and Par-Dependent Photosystems in Guard Cells

The photocontrol of the functional coupling between photosynthesis and stomatal conductance in the leaf was investigated in gas exchange experiments using monochromatic light provided by lasers. Net photosynthesis and stomatal conductance were measured in attached leaves of Malva parviflora L. as a function of photon irradiance at 457.9 and 640.0 nanometers.

Photosynthetic rates and quantum yields of photosynthesis were higher under red light than under blue, on an absorbed or incident basis.

Stomatal conductance was higher under blue than under red light at all intensities. Based on a calculated apparent photon efficiency of conductance, blue and red light had similar effects on conductance at intensities higher than 0.02 millimoles per square meter per second, but blue light was several-fold more efficient at very low photon irradiances. Red light had no effect on conductance at photon irradiances below 0.02 millimoles per square meter per second. These observations support the hypothesis that stomatal conductance is modulated by two photosystems: a blue light-dependent one, driving stomatal opening at low light intensities and a photosynthetically active radiation (PAR)-dependent one operating at higher irradiances.

When low intensity blue light was used to illuminate a leaf already irradiated with high intensity, 640 nanometers light, the leaf exhibited substantial increases in stomatal conductance. Net photosynthesis changed only slightly. Additional far-red light increased net photosynthesis without affecting stomatal conductance. These observations indicate that under conditions where the PAR-dependent system is driven by high intensity red light, the blue light-dependent system has an additive effect on stomatal conductance.

The wavelength dependence of photosynthesis and stomatal conductance demonstrates that these processes are not obligatorily coupled and can be controlled by light, independent of prevailing levels of intercellular CO₂. The blue light-dependent system in the guard cells may function as a specific light sensor while the PAR-dependent system supplies a CO₂-modulated energy source providing functional coupling between the guard cells and the photosynthesizing mesophyll.

     

Genetic variation of stomatal conductance, blue light sensitivity and zeaxanthin content in guard cells of Pima cotton (Gossypium barbadense)

Pima S‐6 ( Gossypium barbadense L.) is a modern line with high stomatal conductance, while B368 is a primitive cotton with low conductance. The blue light sensitivity of adaxial guard cells, probed as the blue light‐dependent enhancement of the red light‐induced chlorophyll a fluorescence quenching, was investigated in these two cotton lines with contrasting stomatal conductance. Adaxial guard cells isolated from Pima S‐6 cotton plants had a significantly higher carotenoid content and a higher blue light sensitivity than those isolated from B368 plants. In a growth chamber‐grown F₂ population of a cross between these two lines, adaxial stomatal conductances of individual plants segregated over a range exceeding the average conductances of the parents. Carotenoid content and the blue light sensitivity of adaxial guard cells also segregated. The concentrations of xanthophylls and β‐carotene in the adaxial guard cells were poorly correlated with the blue light response, except for zeaxanthin. The co‐segregation of stomatal conductance and blue light sensitivity suggested that the stomatal response to blue light may play a role in the regulation of stomatal conductance in the intact leaf. Zeaxanthin content and blue light sensitivity also co‐segregated, suggesting that both parameters are under genetic control. The co‐segregation of zeaxanthin content, blue light sensitivity and stomatal conductance provides further evidence for a role of zeaxanthin in the blue light photoreception of guard cells.     

Sequential response of whole plant water relations to prolonged soil drying and the involvement of xylem sap ABA in the regulation of stomatal behaviour of sunflower plants

Sunflower plants ( Helianihus animus cv. Tall Single Yellow} were grown in the greenhouse in drain pipes (100 mm inside diameter and 1 m long) rilled with John Innes No. 2 compost. When the fifth leaf had emerged, half of the plants were left unwatered for 6 days, rewatered for 2 days and then not watered for another 12 days. Measurements of water relations and abaxial stomatal conductance were made at each leaf position at regular intervals during the experimental period. Estimates were also made of soil water potentials along the soil profile and of ABA concentrations in xylem sap and leaves.
Soil drying led to some reduction in stomatal conductance alter only 3 days but leaf turgors were not reduced until day 13 (6 days after rewatering). When the water relations of leaves did change, older leases became substantially dehydrated while high turgors were recorded in younger leaves. Leaf ABA content measured on the third youngest leaf hardly changed over the first 13 days of the experiment, despite substantial soil drying, while xylem ABA concentrations changed very significantly and dynamically as soil water status varied, even when there was no effect of soil drying on leaf water relations. We argue that the highest ABA concentrations in the xylem, found as a result of substantial soil drying, arise from synthesis in both the roots and the older leaves, and act to delay the development of water deficit in younger leases.
In other experiments ABA solutions were watered on to the root systems of sunflower plants to increase ABA concentrations in xylem sap. The stomatal response to applied ABA was quantitatively very similar to that to ABA generated as a result of soil drying. There was a log-linear relationship between the reduction of leaf conductance and the increase of ABA concentration m xylem sap.     

Response of photosynthesis to high light and drought for Arabidopsis thaliana grown under a UV-B enhanced light regime

Arabidopsis thaliana grown in a light regime that included ultraviolet-B (UV-B) radiation (6 kJ m⁻² d⁻¹) had similar light-saturated photosynthetic rates but up to 50% lower stomatal conductance rates, as compared to plants grown without UV-B radiation. Growth responses of Arabidopsis to UV-B radiation included lower leaf area (25%) and biomass (10%) and higher UV-B absorbing compounds (30%) and chlorophyll content (52%). Lower stomatal conductance rates for plants grown with UV-B radiation were, in part, due to lower stomatal density on the adaxial surface. Plants grown with UV-B radiation had more capacity to down regulate photochemical efficiency of photosystem II (PSII) as shown by up to 25% lower φ_PSII and 30% higher non-photochemical quenching of chlorophyll fluorescence under saturating light. These contributed to a smaller reduction in the maximum photochemical efficiency of PSII (F _v/F _m), greater dark-recovery of F _v/F _m, and higher light-saturated carbon assimilation and stomatal conductance and transpiration rates after a four-hour high light treatment for plants grown with UV-B radiation. Plants grown with UV-B were more tolerant to a 12 day drought treatment than plants grown without UV-B as indicated by two times higher photosynthetic rates and 12% higher relative water content. UV-B-grown plants also had three times higher proline content. Higher tolerance to drought stress for Arabidopsis plants grown under UV-B radiation may be attributed to both increased proline content and decreased stomatal conductance. Growth of Arabidopsis in a UV-B-enhanced light regime increased tolerance to high light exposure and drought stress.     

Reversible, Water Stress-Induced Non-Uniform Chlorophyll Fluorescence Quenching in Wilting Leaves of Potentilla reptans may not be due to Patchy Stomatal Responses

Abstract: After exposure to full sunlight under natural conditions, attached leaves of the common meadow weed Potentilla reptans show non-uniform ("patchy") chlorophyll fluorescence quenching in the early stages of fluorescence transients. These areas of bright fluorescence can be readily reproduced in detached leaves that are allowed to wilt on the laboratory bench in weak light. The extent and duration of the patchiness increases with increasing water stress (higher relative saturation deficits). Images captured during saturating flashes show that the patches also display slow development of non-photochemical quenching, consistent with the possibility that photosynthetic metabolism is impaired in these areas. Wilted Potentilla leaves readily regain full turgor when petioles are placed in water, and uniform chlorophyll fluorescence is recovered with in 30mm. Epidermal impressions reveal closed stomata over areas of both low and high fluorescence in wilted leaves. Because highly fluorescent patches also persist when wilted tissues are exposed to high CO₂ (i.e., patchiness is unlikely to be due to local differences in CO₂ supply) the data suggest direct effects of water stress on metabolism in wilted leaves. Leaf transverse sections show that although major veins may isolate areas of the lamina, minor veins do not. Relationships to leaf anatomy are discussed.