Changes in water status during water stress at different stages of development in wheat

The dynamics of stomatal resistance and osmotic adjustment in response to plant water deficits and stage of physiological development was studied in the leaves of spring wheat ( Triticum aestivum L., GWO 1809). Plants were germinated and grown in pots in a growth chamber at the Duke University Phytotron to four physiological stages of development (4th leaf, 7th leaf, anthesis, and soft dough), during which time stomatal resistance, total water potential and osmotic potential were measured on the last fully developed leaf of water stressed and non-stressed plants. Pressure potential was obtained by difference. Stomatal closure of the abaxial and adaxial surfaces were independent of each other, each having a different critical total water potential. The total water potential required to close the stomata on the last fully developed leaf were different at different stages of physiological development, decreasing as the plants grew older. The development of osmoregulation in wheat allows the closure of stomata during the vegetative stage at a high total water potential, but insures that stomata remain open from anthesis through the ear filling period to a lower total water potential.     

Fern and lycophyte guard cells do not respond to endogenous abscisic acid

Stomatal guard cells regulate plant photosynthesis and transpiration. Central to the control of seed plant stomatal movement is the phytohormone abscisic acid (ABA); however, differences in the sensitivity of guard cells to this ubiquitous chemical have been reported across land plant lineages. Using a phylogenetic approach to investigate guard cell control, we examined the diversity of stomatal responses to endogenous ABA and leaf water potential during water stress. We show that although all species respond similarly to leaf water deficit in terms of enhanced levels of ABA and closed stomata, the function of fern and lycophyte stomata diverged strongly from seed plant species upon rehydration. When instantaneously rehydrated from a water-stressed state, fern and lycophyte stomata rapidly reopened to predrought levels despite the high levels of endogenous ABA in the leaf. In seed plants under the same conditions, high levels of ABA in the leaf prevented rapid reopening of stomata. We conclude that endogenous ABA synthesized by ferns and lycophytes plays little role in the regulation of transpiration, with stomata passively responsive to leaf water potential. These results support a gradualistic model of stomatal control evolution, offering opportunities for molecular and guard cell biochemical studies to gain further insights into stomatal control.     

Field Studies of the Wheat Stomata Resistance Influenced by Soil Water Content

Concurrent observations of soil water potential and leaf stomata diffusion resistance were made on two, plots of wheat grown at Datun Agro-ecological Experimental Station in Beijing under two different soil water conditions. These data were further complemented by weather and physiological observation. In this paper, we mainly analysed the influence of soil water potential on the status of wheat leaf stomatal resistance. The results indicate that: (1) there is a obvious influence of soil water potential on the status of wheat leaf stomata under normal conditions and (2) there is a different upper and lower epidermis stomata of wheat leaf respond to the soil water potential. The lower epidermis stomata are more sensitive to soil water potential than upper epidermis one. (3) There is a linear relationship between the ratio of lower and upper epidermis stomata resistance and soil water potential in root layer, according to this we can diagnose the degree of wheat water deficit.     

Stomatal sensing of the environment

The effects of environmental factors on stomatal behaviour are reviewed and the questions of whether photosynthesis and transpiration eontrol stomata or whether stomata themselves control the rates of these processes is addressed. Light affects stomata directly and indirectly. Light can act directly as an energy source resulting in ATP formation within guard cells via photophosphorylation, or as a stimulus as in the case of the blue light effects which cause guard cell H⁺ extrusion. Light also acts indirectly on stomata by affecting photosynthesis which influences the intercellular leaf CO₂ concentration ( C _i). Carbon dioxide concentrations in contact with the plasma membrane of the guard cell or within the guard cell acts directly on cell processes responsible for stomatal movements. The mechanism by which CO₂ exerts its effect is not fully understood but, at least in part, it is concerned with changing the properties of guard cell plasma membranes which influence ion transport processes. The C _i may remain fairly constant for much of the day for many species which is the result of parallel responses of stomata and photosynthesis to light. Leaf water potential also influences stomatal behaviour. Since leaf water potential is a resultant of water uptake and storage by the plant and transpirational water loss, any factor which affects these processes, such as soil water availability, temperature, atmospheric humidity and air movement, may indirectly affect stomata. Some of these factors, such as temperature and possibly humidity, may affect stomata directly. These direct and indirect effects of environmental factors interact to give a net opening response upon which is superimposed a direct effect of stomatal circadian rhythmic activity.     

Short-term and long-term effects of plant water deficits on stomatal response to humidity in Corylus avellana L.

Short-term (hours) changes in plant water status were induced in hazel (Corylus avellana L.) by changing the evaporative demand on a major portion of the shoot while maintaining a branch in a constant environment. Stomatal conductance of leaves on the branch was influenced little by these short-term changes in water status even with changes in leaf water potential as great as 8 bars. Long-term (days) changes in plant water status were imposed by soil drying cycles. Stomatal conductance progessively decreased with increases in long-term water stress. Stomata still responded to humidity with long-term water stress but the range of the conductance response decreased. Threshold responses of stomata to leaf water potential were not observed with either short-term or long-term changes in plant water status even when leaves wilted. It is suggested that concurrent measurements of plant water status may not be sufficient for explaining stomatal and other plant responses to drought.     

Stomatal behaviour and leaf water potential of chilled and water-stressed Solanum melongena, as influenced by growth history

Abstract Leaf diffusion resistance and leaf water potential of intact Solanum melongena plants were measured during a period of chilling at 6 °C. Two pretreatments, consisting of a period of water stress or a foliar spraying of abscisic acid (ABA), were imposed upon the plants prior to chilling. The control plants did not receive a pretreatment. In addition to intact plant studies, stomatal responses to water loss and exogenous abscisic acid were investigated using excised leaves, and the influence of the pretreatment observed. Chilled, control plants wilted slowly and maintained open stomata despite a decline in leaf water potential to –2.2 MPa after 2 d of chilling. In contrast plants that had been water stressed or had been sprayed with abscisic acid, prior to chilling, did not wilt and maintained a higher leaf water potential and a greater leaf diffusion resistance. In plants that had not received a pretreatment, abscisic acid caused stomatal closure at 35 °C, but at 6°C it did not influence stomatal aperture. The two pretreatments greatly increased stomatal sensitivity to both exogenous ABA and water stress, at both temperatures. Stomatal response to water loss from excised leaves was greatly reduced at 6°C. These results are discussed in relation to low temperature effects on stomata and the influence of preconditioning upon plant water relations.     

Unraveling the Effects of Plant Hydraulics on Stomatal Closure during Water Stress in Walnut

The objectives of the study were to identify the relevant hydraulic parameters associated with stomatal regulation during water stress and to test the hypothesis of a stomatal control of xylem embolism in walnut (Juglans regia x nigra) trees. The hydraulic characteristics of the sap pathway were experimentally altered with different methods to alter plant transpiration (Eplant) and stomatal conductance (gs). Potted trees were exposed to a soil water depletion to alter soil water potential (Psisoil), soil resistance (Rsoil), and root hydraulic resistances (Rroot). Soil temperature was changed to alter Rroot alone. Embolism was created in the trunk to increase shoot resistance (Rshoot). Stomata closed in response to these stresses with the effect of maintaining the water pressure in the leaf rachis xylem (P(rachis)) above -1.4 MPa and the leaf water potential (Psileaf) above -1.6 MPa. The same dependence of Eplant and gs on P(rachis) or Psileaf was always observed. This suggested that stomata were not responding to changes in Psisoil, Rsoil, Rroot, or Rshoot per se but rather to their impact on P(rachis) and/or Psileaf. Leaf rachis was the most vulnerable organ, with a threshold P(rachis) for embolism induction of -1.4 MPa. The minimum Psileaf values corresponded to leaf turgor loss point. This suggested that stomata are responding to leaf water status as determined by transpiration rate and plant hydraulics and that P(rachis) might be the physiological parameter regulated by stomatal closure during water stress, which would have the effect of preventing extensive developments of cavitation during water stress.     

Stomatal diffusion resistance of snap beans. I. Influence of leaf-water potential

Concurrent measurements of abaxial and adaxial stomatal resistance and leaf-water potentials of snap beans (Phaseolus vulgaris L.) in the field and growth chamber show that the stomata on the 2 surfaces of the leaflet react differently to water deficit. The stomata on the abaxial surface, which are about 7 times more numerous than on the adaxial surface, are not significantly affected at leaf-water potentials greater than —11 bars, but with further decrease in leaf-water potential, the resistance rapidly increases. On the other hand, the resistance of the adaxial stomata increases sharply at a leaf-water potential of about —8 bars and is constant at higher water potentials. The average stomatal resistance for both surfaces of the leaf, which is the major diffusive resistance to water vapor, to a first approximation acts as an on-off switch and helps prevent further decline in leaf-water potential. The relation between the leaf-water potential and the stomatal resistance links the soil-water potential to the transpiration stream as needed for soil-plant-atmosphere models.     

Stomatal regulation by microclimate and tree water relations: interpreting ecophysiological field data with a hydraulic plant model

Dynamics in microclimate and physiological plant traits were studied for Pubescent oak and Scots pine in a dry inner-alpine valley in Switzerland, at a 10 min resolution for three consecutive years (2001-2003). As expected, stomata tended to close with increasing drought in air and soil. However, stomatal aperture in oak was smaller than in pine under relatively wet conditions, but larger under dry conditions. To explore underlying mechanisms, a model was applied that (i) quantifies water relations within trees from physical principles (mechanistic part) and (ii) assumes that signals from light, stomatal aperture, crown water potential, and tree water deficit in storage pools control stomata (systemic part). The stomata of pine showed a more sensitive response to increasing drought because both factors, the slowly changing tree water deficit and the rapidly changing crown water potential, closed the stomata. By contrast, the stomata of oak became less drought-sensitive as the closing signal of crown water potential was opposed by the opening signal of tree water deficit. Moreover, parameter optimization suggests that oak withdrew more water from the storage pools and reduced leaf water potentials to lower levels, without risking serious damage by cavitation. The new model thus suggests how the hydraulic water flow and storage system determines the responses in stomatal aperture and transpiration to drought at time scales ranging from hours to multiple years, and why pine and oak might differ in such responses. These differences explain why oaks are more efficient competitors during drought periods, although this was not the case in the extremely dry year 2003, which provoked massive leaf loss and, from July onwards, physiological activity almost ceased.     

The Effect of 2-Chloroethyltrimethyl Ammonium Chloride (CCC) on Stomatal Diffusive Resistance in Tomato

CCC (2-chloroethyltrimethyl ammonium chloride) at a concentration of 6.3 mM was applied to tomato plants (cv. Grosse Lisse) grown in a controlled environment. There was an increase in adaxial leaf diffusive resistance but not in abaxial resistance, the effect being apparent before any growth retardation was measurable. The partial closure of adaxial stomata in response to CCC reduced transpiration from that leaf surface. In plants deprived of water, leaf water potential was higher when CCC was applied and both adaxial and abaxial stomatal closure was delayed. The data do not suggest that CCC influenced the relationship between leaf water potential and conductance for either abaxial or adaxial stomata.     

Hydraulic and chemical signalling in the control of stomatal conductance and transpiration.

Abscisic acid (ABA) transported in the xylem from root to shoot and perceived at the guard cell is now widely studied as an essential regulating factor in stomatal closure under drought stress. This provides the plant with a stomatal response mechanism in which water potential is perceived in the root as an indication of soil water status and available water resources. There is also ample evidence that stomata respond directly to some component of leaf water status. This provides additional information about water potential gradients developing between root and shoot as the result of water transport, allowing for a more stable regulation of shoot water status and better protection of the transport system itself. The precise location at which leaf water status is sensed, however, and the molecular events transducing this signal into a guard cell response are not yet known. Major questions therefore remain unanswered on how water stress signals perceived at root and leaf locations are integrated at the guard cell to control stomatal behaviour.     

The relations of stomatal closure and reopening to xylem ABA concentration and leaf water potential during soil drying and rewatering

Two tropical tree species, Acacia confusa and Leucaena leucocephala, were used to study the relationships among stomatal conductance, xylem ABA concentration and leaf water potential during a soil drying and rewatering cycle. Stomatal conductance of both A. confusa and L. leucocephala steadily decreased with the decreases in soil water content and pre-dawn leaf water potential. Upon rewatering, soil water content and pre-dawn leaf water potential rapidly returned to the control levels, whereas the reopening of stomata showed an obvious lag time. The length of this lag time was highly dependent not only upon the degree of water stress but also on plant species. The more severe the water stress, the longer the lag time. When A. confusa and L. leucocephala plants were exposed to the same degree of water stress (around –2.0 MPa in pre-dawn leaf water potential), the stomata of A. confusa reopened to the control level 6 days after rewatering. However, it took L. leucocephala about 14 days to reopen fully. A very similar response of leaf photosynthesis to soil water deficit was also observed for both species. Soil drying resulted in a significant increase in leaf and xylem ABA concentrations in both species. The more severe the water stress, the higher the leaf and xylem ABA concentrations. Both leaf ABA and xylem ABA returned to the control level following relief from water deficit and preceded the full recovery of stomata, suggesting that the lag phase of stomatal reopening was not controlled by leaf and/or xylem ABA. In contrast to drying the whole root system, drying half of the root system did not change the leaf water relations, but caused a significant increase in xylem ABA concentration, which could fully explain the decrease of stomatal conductance. After rewatering, the stomatal conductance of plants in which half of the roots were dried recovered more rapidly than those of whole-root dried plants, indicating that the leaf water deficit that occurred during the drying period was related to the post-stress stomatal inhibition. These results indicated that the decrease in stomatal conductance caused by water deficit was closely related to the increase in xylem ABA, but xylem ABA could not fully explain the reopening of stomata after relief of water stress, neither did the leaf ABA. Some unknown physiological and/or morphological processes in the guard cells may be related to the recovery process.     

Growth and Water Relations of Wilty Mutants of Tomato (Lycopersicon esculentum Mill.)

In contrast to some previous reports on the growth of the ABA-deficientwilty mutants of tomato, growth was at least as rapid in themutants as in the wild type, as long as an adequate plant waterstatus was maintained by growing the plants under mist. Moreover,shoot extension was greater and the rate of leaf productionmore rapid in the mutants. Stomatal changes in response to environmentand to time in the light-dark cycle were generally similar inboth wilty mutants and the wild type, though the wild-type weregenerally more closed. Grafting experiments confirmed that thegenotype of the shoot was dominant in determining stomatal aperture,though wild-type rootstocks could cause a slight reduction inthe stomatal conductance of mutant leaves. The effect on plantwater relations of draughting only part of the root system wasinvestigated in a ‘split-root’ experiment. Withholdingwater from only part of the root system was found to lower significantlythe mean leaf water potential, even though the potential evaporationrate was kept very small. Key words: Abscisic acid, stomata, tomato     

Xeromorphic traits help to maintain photosynthesis in the perhumid climate of a Taiwanese cloud forest

Previous flux measurements in the perhumid cloud forest of northeastern Taiwan have shown efficient photosynthesis of the endemic tree species Chamaecyparis obtusa var. formosana even under foggy conditions in which leaf surface moisture would be expected. We hypothesized this to be the result of ‘xeromorphic’ traits of the Chamaecyparis leaves (hydrophobicity, stomatal crypts, stomatal clustering), which could prevent coverage of stomata by precipitation, fog, and condensation, thereby maintaining CO₂ uptake. Here we studied the amount, distribution, and composition of moisture accumulated on Chamaecyparis leaf surfaces in situ in the cloud forest. We studied the effect of surface tension on gas penetration to stomata using optical O₂ microelectrodes in the laboratory. We captured the dynamics of condensation to the leaf surfaces with an environmental scanning electron microscope (ESEM). In spite of substantial surface hydrophobicity, the mean water film thickness on branchlets under foggy conditions was 80 µm (upper surface) and 40 µm (lower surface). This amount of water could cover stomata and prevent CO₂ uptake. This is avoided by the clustered arrangement of stomata within narrow clefts and the presence of Florin rings. These features keep stomatal pores free from water due to surface tension and provide efficient separation of plant and atmosphere in this perhumid environment. Air pollutants, particularly hygroscopic aerosol, may disturb this functionality by enhancing condensation and reducing the surface tension of leaf surface water.     

Physiological responses of wheat to drought stress and its mitigation approaches

Drought is a polygenically controlled stress and a major agricultural risk that reduces crop productivity and limits the successful insight of land potential throughout the world. This review article has been divided into two parts, i.e., effect of drought stress on physiology of wheat and potential drought mitigation approaches. In the first part, physiological responses of wheat to stress were discussed. Cell membrane stability, relative water content, early maturity, decreased leaf area, small plant size, increased dry weight and root–shoot ratio, and the whole-plant transpiration rate response to enhanced atmospheric vapor pressure deficit are physiological traits associated with drought tolerance in wheat. Reduction of relative water content closes stomata and thereby reduces stomatal conductance. Osmotic adjustment improves drought tolerance by allowing cell enlargement, plant growth, and stomata to stay partially open and by maintaining CO₂ fixation under severe water deficit. The wheat plant accumulates several organic and inorganic solutes in its cytosol to lessen its osmotic potential for maintenance of cell turgor. Drought affects photosynthesis negatively by changing the inner structure of chloroplasts, mitochondria, and chlorophyll content and minerals. Destruction of the photosystem II (PSII) oxygen releasing complex and reaction center can disturb production and use of electrons, causing lipid peroxidation of cell membrane through the production of reactive oxygen species. In the second part, drought mitigation approaches were discussed. Seed, drought, bacterial, and hormonal priming are common approaches used to lessen the effects of water deficit. Physiological trait-based breeding, molecular breeding, marker-assisted backcrossing, aerial phenotyping, water budgeting, and resource allocation are modern approaches used to develop drought tolerant wheat cultivars. Wheat genotypes produced as a result of a combination of all these methodologies will increase food security regarding the currently changing climate.     

Water Relations,CO2 Exchange,Water-use Efficiency and Growth of Welwitschia mirabilis Hook. fil. in three Contrasting Habitats of the Namib Desert1

CO₂ exchange, transpiration and leaf water potential of Welwitschia mirabilis were measured in three contrasting habitats of the Namib desert. From these measurements stomatal conductance, internal CO₂concentration and WUE were calculated. In two of the three habitats photosynthetic CO₂ uptake decreased and transpiration increased with increasing leaf age while in the third habitat CO₂ uptake increased and transpiration decreased with leaf age. Except for the stomata of young leaf sections in this habitat, stomata closed with increasing δw leading to a pronounced midday depression of CO₂ uptake. The high stomatal limitation of photosynthetic CO₂ uptake of glasshouse-grown plants was verified in the natural habitat. Photosynthetic CO₂ uptake saturated between 800 and 1300 μmol photons m^?2 s^?1depending on leaf age and habitat. CO₂ uptake had a broad temperature optimum declining significantly beyond 32 °C. Predawn leaf water potential reflected water availability and atmospheric conditions in the three habitats and ranged from ? 2.5 to ? 6.2 MPa. There was a pronounced diurnal course of leaf water potential in all habitats. During the day a gradient in water potential developed along the leaf axis with the lowest potential at the leaf's tip. With respect to whole plant balances of CO₂ exchange and transpiration, there were marked differences between Welwitschias in the three habitats. Despite a negative CO₂ balance over a period of five months, leaves in the driest habitat grew constantly at the expense of carbon reserves in the plant. Only at the wettest site did carbon gain exceed carbon demand for growth. The WUE of whole plants was insignificant in all habitats. The results were as contrasting as the habitats and plants and did not allow generalisations about adaptational features of Welwitschia mirabilis.     

WATER RELATIONS OF WATER-STRESSED,SPLIT-ROOT C4 (SORGHUM BICOLOR; POACEAE) AND C3 (HELIANTHUS ANNUUS; ASTERACEAE) PLANTS

Sorghum [Sorghum bicolor (L.) Moench] and sunflower (Helianthus annuus L.) were grown in a greenhouse with roots divided between sand irrigated with nutrient solution (–0.097 MPa) or nutrient solution containing polyethylene glycol (PEG) (–0.570 MPa) to compare the effect of unequal root zone stress on plant water relations of a C₄ (sorghum) and a C₃ (sunflower) plant. Roots also were divided between two pots of sand irrigated only with nutrient solution (controls) or only with PEG in nutrient solution. In addition to plant water-status measurements, photosynthetic rate, growth (height, root, and shoot dry weights), and evolution of ethylene (a gaseous hormone indicative of stress) were measured. Under all three split-root treatments, sunflower had a lower leaf water potential and produced more ethylene than sorghum. Sunflower was able to survive the PEG stress if half of its root system was under nonstressed conditions. Sunflower with half its root system irrigated with PEG usually had values of leaf water potential, osmotic potential, stomatal resistance, transpiration rate, photosynthetic rate, ethylene evolution, height, and dry weights that were close to those of the control plants. Sunflower with all roots exposed to PEG was wilted severely. Sorghum was little affected by PEG stress applied either to half or all the root system. Growth of sorghum was the same under all treatments. Apparently because stomata of sorghum were more closed in the partial stress test than those of sunflower, sorghum conserved water and had a higher leaf water potential, which might have permitted growth with stress.     

Stomatal and nonstomatal regulation of water use in cotton, corn, and sorghum

Stomata of corn (Zea mays L.) and sorghum (Sorghum bicolor L.) responded to changes in leaf water potential during the vegetative growth phase. During reproductive growth, leaf resistances were minimal and stomata were no longer sensitive to bulk leaf water status even when leaf water potentials approached −27 bars. Stomata of corn, cotton (Gossypium hirsutum L.), and sorghum appear to respond to changes in the humidity deficit between the leaf and air and in this manner, regulated transpirational flux to some degree. Distinct differences in water transport efficiency were observed in the three species. Under nonlimiting soil water conditions, sorghum exhibited the greatest efficiency of water transport while under limiting soil moisture conditions, cotton appeared most efficient. Corn was the least efficient with respect to nonstomatal regulation of water use. Differences in drought tolerance among the three species are partially dependent on stomatal regulation of water loss, but efficiency of the water transport system may be more related to drought adaptation. This is particularly important since stomata of all three species did not respond to bulk leaf water status during a large portion of the growing season.     

Behavior of Corn and Sorghum under Water Stress and during Recovery

Corn (Zea mays L.) and sorghum (Sorghum vulgare, Pers.) plants were grown in a vermiculite-gravel mixture in controlled environment chambers until they were 40 days old. Water was withheld until they were severely wilted, and they were then rewatered. During drying and after rewatering stomatal resistance was measured with a diffusion porometer each morning, and water saturation deficit and water potential were measured on leaf samples. The average resistance of the lower epidermis of well watered plants was lower for corn than for sorghum. When water stress developed, the stomata began to close at a higher water potential in corn than in sorghum. The stomata of both species began to reopen normally soon after the wilted plants were rewatered, and on the 2nd day the leaf resistances were nearly as low as those of the controls. The average leaf water potential of well watered corn was −4.5 bars; that of sorghum, −6.4 bars. The lowest leaf water potential in stressed corn was −12.8 bars at a water saturation deficit of 45%. The lowest leaf water potential in stressed sorghum was −15.7 bars, but the water saturation deficit was only 29%. At these values the leaves of both species were tightly rolled or folded and some injury was apparent. Thus, although the average leaf resistance of corn is little lower than that of sorghum, corn loses much more of its water before the stomata are fully closed than does sorghum. The smaller reduction in water content of sorghum for a given reduction in leaf water potential is characteristic of drought-resistant species.     

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.