Leaf Water Potential Components Research Articles

Canopy warming caused photosynthetic acclimation and reduced seed yield in maize grown at ambient and elevated [CO2

Rising atmospheric CO2 concentration ([CO2 ]) and attendant increases in growing season temperature are expected to be the most important global change factors impacting production agriculture. Although maize is the most highly produced crop worldwide, few studies have evaluated the interactive effects of elevated [CO2 ] and temperature on its photosynthetic physiology, agronomic traits or biomass, and seed yield under open field conditions. This study investigates the effects of rising [CO2 ] and warmer temperature, independently and in combination, on maize grown in the field throughout a full growing season. Free-air CO2 enrichment (FACE) technology was used to target atmospheric [CO2 ] to 200μmolmol(-1) above ambient [CO2 ] and infrared heaters to target a plant canopy increase of 3.5°C, with actual season mean heating of ~2.7°C, mimicking conditions predicted by the second half of this century. Photosynthetic gas-exchange parameters, leaf nitrogen and carbon content, leaf water potential components, and developmental measurements were collected throughout the season, and biomass and yield were measured at the end of the growing season. As predicted for a C4 plant, elevated [CO2 ] did not stimulate photosynthesis, biomass, or yield. Canopy warming caused a large shift in aboveground allocation by stimulating season-long vegetative biomass and decreasing reproductive biomass accumulation at both CO2 concentrations, resulting in decreased harvest index. Warming caused a reduction in photosynthesis due to down-regulation of photosynthetic biochemical parameters and the decrease in the electron transport rate. The reduction in seed yield with warming was driven by reduced photosynthetic capacity and by a shift in aboveground carbon allocation away from reproduction. This field study portends that future warming will reduce yield in maize, and this will not be mitigated by higher atmospheric [CO2 ] unless appropriate adaptation traits can be introduced into future cultivars.

The response of aphids to plant water stress – the case of Myzus persicae and Brassica oleracea var. capitata

AbstractThe possible repercussions of decreasing water availability in plant–insect interactions largely remain unclear. To study this, we designed two factorial studies applying two levels of water stress to Brassica oleraceae var. capitata L. (Brassicaceae) plants inoculated with the aphid Myzus persicae Sulzer (Hemiptera: Aphididae: Macrosiphini). One study investigated the effect of aphids and water stress on leaf water potential components, aphid population size, aphid intrinsic rate of increase (rm), and fecundity. The other investigated the effect of aphids and water stress on gas exchange, leaf fluorescence yield, estimation of leaf chlorophyll, and above‐ground dry weight. Drought treatment reduced the size of the M. persicae population. Aphid numbers were strongly and positively correlated with soil moisture and negatively correlated with leaf temperature. Aphids on droughted plants had lower rm and lower fecundity per aphid. Under aphid stress, plants showed reduced maximum photosynthetic rate (Amax), lower leaf chlorophyll as estimated by soil plant analysis development (SPAD) values, higher osmotic potential, and reduced final above‐ground dry weight compared with control plants. Aphid infestation also led to significantly higher leaf water potentials and reduced light‐adapted fluorescence yield. Compared with control plants, water‐stressed plants showed reduced Amax, lower SPAD values, reduced leaf water potential, lower turgor, and reduced final above‐ground dry weight. Osmotic potential was lower in droughted plants when aphids were not present. Plants under combined aphid and water stress showed significantly reduced SPAD and final above‐ground dry weight, with comparatively higher leaf water potentials. These results show that aphid infestation limits solute accumulation in the vacuole of drought stressed cabbage. Aphid infestation reduced the host’s ability to photosynthesise optimally and led to reduced growth. Plant stress hypotheses propose that a senescence feeder such as M. persicae would show higher rates of growth in response to the increasing ‘quality’ of the phloem sap under water shortage. In the case of M. persicae, our results do not support the hypothesis that drought leads to improved aphid population growth and survival.

Height-related trends in stomatal sensitivity to leaf-to-air vapour pressure deficit in a tall conifer

Stomatal responses to leaf-to-air vapour pressure deficit (LVPD), leaf water potential components, and cuticular properties were characterized for Douglas-fir (Pseudotsuga menziesii) foliage collected from tree tops along a height gradient from 5 m to 58 m in order to explore height-related trends in stomatal sensitivity to LVPD and to investigate the role of bulk leaf turgor and leaf cuticle thickness in determining stomatal behaviour. There were three distinct phases in the response of stomatal conductance (g(s)) to changes in LVPD. At low LVPD, g(s) increased with increasing LVPD (phase one). During the second phase, g(s) was maximal at low to intermediate LVPD and during the third phase g(s) declined steadily as LVPD increased. The responsiveness of g(s) to LVPD exhibited a height-related pattern such that maximum g(s) (g(s-max)) occurred at progressively greater LVPD with increasing height (r2=0.55, P=0.006). Bulk leaf osmotic potential at full turgor decreased with height (r2=0.77, P=0.00016), and LVPD at g(s-max) and at maximum crown conductance (g(c-max)) in the field were significantly correlated with leaf turgor (r2=0.92, P=0.0093). Cuticle thickness increased by 0.044 microm for every metre increase in height (r2=0.78, P=0.00015). The observed trends in the response of g(s) to LVPD along a height gradient, and their consistency with height-related trends in foliar osmotic potential suggest that osmotic adjustment at the tops of tall trees influences the relationship between g(s) and LVPD.

Effects of soil moisture deficits on the water relations of bambara groundnut (Vigna subterraneaL. Verdc.)

The components of leaf water potential (Ψ I ) and relative water content (RWC) were measured for stands of bambara groundnut (Vigna subterranea) exposed to three soil moisture regimes in controlled-environment glasshouses at the Tropical Crops Research Unit, Sutton Bonington Campus. Treatments ranged from fully irrigated (wet) to no irrigation from 35 days after sowing (DAS) (dry). RWC values varied between 92-96% for the wet treatment, but declined from 93% to 83% in the dry treatment as the season progressed. Ψ I at midday decreased in both the wet and dry treatments, but the seasonal decline was more pronounced in the latter: seasonal minimum values were -1.19 and - 2.08 MPa, respectively. Plants in the wet treatment maintained turgor (Ψ p ) at about 0.5 MPa throughout the season, whereas values in the dry treatment approached zero towards the end of the season. There was a linear relationship between Ψ p and Ψ I , with Ψ p approaching zero at a Ψ I of -2.0 MPa. Mean daily leaf conductance was consistently higher in the wet treatment (0.46-0.79 cm s -1 ) than in the intermediate and dry treatments (0.13-0.48 cm s -1 ). Conductances in the intermediate and dry treatments were similar, and the lower evapotranspirational water losses in the latter were attributable to its consistently lower leaf area indices (L): L at final harvest was 3.3, 3.3 and 1.9 for the wet, intermediate and dry treatments. Bambara groundnut was apparently able to maintain turgor through a combination of osmotic adjustment, reductions in leaf area index and effective stomatal regulation of water loss.

PLANT WATER RELATIONS OF ARTEMISIA CANA AND AGROPYRON INTERMEDIUM DURING AND AFTER WATER STRESS PRECONDITIONING

Typically, dormant seedlings are transplanted when revegetating disturbed lands to prevent transplant shock triggered by water stress. It may be possible to transplant nondormant seedlings by inducing drought-tolerant acclimation responses such as solute accumulation. Artemisia cana and Agropyron intermedium seedlings were subjected to three different water stress preconditioning treatments. After conditioning, seedlings were dried down in their containers until leaf senescence, or were transplanted to disturbed land sites. Leaf water potential components and relative water content were measured. Following treatments, water relations parameters of preconditioned seedlings were not markedly different from controls in either species. At the end of the final dry-down, water stress preconditioning had not induced active or passive solute accumulation, prolonged leaf survival during lethal drought conditions, or differences in transplant survival under the experimental conditions of this study.

Seasonal changes in leaf water potential components in two almond cultivars

SUMMARYAlmond trees (Amygdalus communisL. cvs Garrigues and Ramillete) were grown in the field under non-irrigated conditions in Murcia, Spain. Seasonal variations in leaf water potential components were studied in 1989. Predawn leaf water potential showed high values in both cultivars, due to the absence of soil water stress. Pressure-volume curve analysis indicated that the leaf osmotic potential at full saturation (Ψo(sat)) for cv. Garrigues remained fairly constant throughout the season. Bulk modulus of elasticity (E) showed, in both cultivars, a tendency to decrease as the season progressed. E values were higher in Ramillete than in Garrigues. The relative water content at the turgor loss point (RWCtlp) seemed to be controlled by E values. The larger relative apoplastic water content (RWCafound in Ramillete might have allowed it to retain more water at low leaf water potentials than Garrigues. These facts would support the suggestion that Ramillete is a more drought-resistant cultivar than Garrigues.

Growth, Osmotic Adjustment, and Cell‐Wall Mechanics of Expanding Grape Leaves during Water Deficits

The responses of growth, components of leaf water potential, and mechanical properties of leaves to ontogeny and soil water deficits were investigated in wine grape Vitis vinifera L. Vines were cultured in soil in controlled environments, and water deficits were imposed by withholding water. Osmotic adjustment of several hundred kilo‐pascals maintained turgor at or above that of controls, but growth was completely inhibited. The bulk modulus of elasticity increased almost twofold, from 8.1 MPa for a rapidly expanding leaf to 14.7 MPa for a similar leaf in which growth had ceased after withholding water for several days, and continued to increase to 20.3 MPa 2 d after growth had ceased. Stress‐strain relationships of isolated leaf strips were determined with the Instron analyzer. Total extension (elastic and plastic components) under 20‐ or 30‐g loads was linearly related to the rate of leaf expansion determined prior to sampling. Total extension of well‐watered leaves exhibited a diurnal pattern, with a minimum extensibility near midnight and high extensibility in the light. Plastic extensibility was estimated as the difference in extension in successive load cycles. Plastic extension was linearly related to the rate of leaf expansion. The results indicate that the extensibility (as determined by Instron analysis) of grape leaf tissue decreases concomitant with the rate of expansion as leaves age. Similarly, plastic and elastic extensibility decrease when growing leaves are exposed to water deficits. The results are consistent with the hypothesis that decreased cell‐wall extensibility contributes to the inhibition of leaf expansion caused by water deficits.

Alteration of Components of Leaf Water Potential and Water Content in Velvetleaf under the Effects of Long-Term Humidity Difference

Velvetleaf (Abutilon theophrasti Medik.) was grown in growth chambers set at 45 or 85% relative humidity at 30 degrees C, CO(2) 350 microliters per liter and 1000 micromoles per square meter per second of photosynthetically active radiation. Soil water potential was maintained at -0.05 megapascal by subirrigation with half strength Hoagland solution. The third, fourth, and fifth leaves from the base of 21- and 25-day-old plants were used for pressure-volume measurements. Components of leaf water status including water potential (osmotic and potential associated with the apoplast), leaf water content (apoplasmic and symplasmic water), and elastic modulus of leaf tissue were determined. Results indicate: (a) persistent dry air generated leaves with lower water potential at a given relative water content than did humid air; (b) the higher total leaf water content in plants grown in dry air was related to an increase in apoplasmic water, whereas symplasmic water remained similar in both humidity treatments; (c) difference in leaf water potential between low and high humidity treatments was related to decreased potential associated with the apoplast but not to a change in cell wall elasticity.

Physiological responses of two soybean [ Glycine max (L.) Merr] cultivars to short-term flooding

Flooding of soybean plants [ Glycine max (L.) Merr] significantly reduces crop growth and yield, but the underlying physiological responses are poorly documented. The short-term physiological effects of flooding on soybean were studied in the field on a relatively impermeable soil classified as Typic Albaqualf. Flooding treatments were imposed on two soybean cultivars, Essex and Forrest, at a vegetative (V4) and a reproductive (R2) growth stage. Diurnal measurements of net photosynthesis ( P n), stomatal conductance ( g s), and components of leaf water potential were recorded on 4 consecutive days following flood application, and again at 14 days after the flood was removed. Photosynthesis of Essex was reduced significantly within 48 h of flooding by 33 and 32% for the V4 and R2 growth stages, respectively, while reductions of 16 and 22% in P n of Forrest were evident. Reductions in g s of 46 and 24% occurred within 48 h for Essex and Forrest in the V4 stage, although in the R2 growth stage, both cultivars experienced an approximate 48% reduction in g s. However, the decline in P n with flooding was only partially explained by changes in g s. Photosynthesis was correlated with stomatal closure at low g s (< 0.5 mol m −2 sec −1), but a higher values of g s an approximate 20% reduction in P n was observed presumably due to non-stomatal limitations. Flooding did not affect components of leaf water potential indicating that the decreases in P n and g s were not associated with plant water-deficit stress. Flooding of Essex and Forrest at either the V4 or R2 growth stage significantly reduced the dry matter accumulation during the flooding treatment and the subsequent growth. Final seed yields were reduced significantly by a mean of 52 and 40% for Essex and Forrest, respectively. Overall, Forrest appeared more tolerant to excess water than Essex.

Water Deficit Development in Old and New Soybean Cultivars

AbstractCrop productivity depends to some degree on a cultivar's capacity to maintain a high plant water status over a range of soil water conditions. We conducted this 2‐yr study to examine whether newer soybean [Glycine max (L.) Merr.] cultivars have more favorable leaf turgor potentials than older cultivars during the pod elongation and seed enlargement stages of crop development, and to determine whether the older and newer cultivars differ in leaf water potential components and stomatal resistances when grown under drought stress. Two older (‘Manchu’ and ‘Dunfield’) and two more recent (‘Clark 63’ and ‘Williams 82’) cultivars were field grown under irrigated (I) and nonirrigated (NI) conditions at Urbana, IL. Although large differences in soil water potential were found between the water treatments, afternoon leaf water potentials of the cultivars were decreased only an average of 0.14 MPa and afternoon leaf turgor potentials were not reduced as a result of the NI treatment. In both the NI and I plots, Manchu and Dunfield generally had lower and similar leaf turgor potentials, respectively, as did the modern cultivars during the day. Under NI conditions, greater stomatal closure occurred for the older cultivars than for the newer cultivars, indicating that the older cultivars are more efficient in conserving water under drought stress.

Water Relations of Field‐Grown Soybean under Drought1

Two strategies for decreasing plant vulnerability to drought are increased access to the stored soil water and increased amount of osmotic adjustment in tissue to preserve turgor. Two indeterminate soybean [Glycine max (L.) Merr.] cultivars that had been identified in preliminary studies as having differing responses to drought were studied to aid in the evaluation of these strategies. Drought stress was imposed on the cultivars during 2 yr at differing ontogenetic stages of growth. Soil water extraction was determined with Bouyoucos gypsum blocks. Stress during vegetative growth was associated with a failure of the drought‐susceptible cultivar to extract water from the deeper soil depths. Components of leaf water potential were determined from pressure‐volume curves obtained with a pressure chamber. These data showed that drought stress resulted in a decrease in leaf osmotic potential of 0.4 to 0.5 MPa in both cultivars. Since, as compared to the more drought‐tolerant cultivar, the total leaf water potential was as much as 0.9 MPa lower in the droughtsusceptible line, comparable osmotic adjustment did not prevent it from experiencing less turgor. Therefore, relatively higher drought tolerance was not associated with greater osmotic adjustment. It appears from the comparison of these two cultivars that maintaining the water supply to the plant tissue was of much greater benefit than tissue osmotic adjustment.

Interactive Effects of Nitrogen and Water Stresses on Water Relations of Field‐Grown Corn Leaves1

AbstractWater and N stresses often limit corn (Zea maysL.) grain yields. Although the effects of either water or N stress on crop growth and development, physiology, and yield have been widely studied, relatively little information is available on the interactive effects of these stresses when imposed in combination. The objective of this study was to define and evaluate the interactive effects of water and N stresses on leaf water potential components, transpiration rate, and stomatal resistance of field‐grown corn leaves. Corn grown on a Kendrick fine sand soil (loamy, siliceous, hyperthermic Arenic Paleudult) was subjected to two water management treatments (optimal irrigation, and a 10‐day water stress period, which immediately preceded 50% silking). Within each water management treatment, two N levels (low, 62 kg ha−1; high, 275 kg ha−1) were imposed. During the water stress period, both midday and diurnal measurements of leaf water potential (ΨL)leaf osmotic potential (Ψτ), leaf turgor potential (ΨP), leaf transpiration, and leaf diffusive resistance were determined on combinations of the water and N stress treatments. Although high‐N plants attained similar or slightly lower ΨLthan low N plants as a result of the water stress, high‐N plants maintained lower stomatal resistances at the lower ΨL. High‐N plants also extracted more water from depths below 0.3 m in the soil profile compared with the low‐N plants. Lower Ψτas ΨLdeclined resulted in higher ΨPin high‐N plants, which was probably responsible for the lower stomatal resistances and higher rates of transpiration that were observed. Lower Ψπof the water‐stressed, high‐N plants could have resulted in the more efficient extraction of water from the soil profile. We conclude that the high‐N plants were affected less by the water stress than low‐N plants as evidenced by the maintenance of ΨPopen stomata, and higher rates of individual leaf transpiration despite similar reductions in ΨLduring periods of low soil water availability.

Aspects of tissue water relations and seasonal changes of leaf water potential components of evergreen and deciduous species coexisting in tropical dry forests.

This study compared the tissue water relations and seasonal changes in leaf water potential components of an evergreen tree,Morisonia americana, and two evergreen shrubs,Capparis verrucosa andC. aristiquetae, with two deciduous trees,Humboltiella arborea andLonchocarpus dipteroneurus, and the deciduous vineMansoa verrucifera. All these species coexist in a tropical dry forest in Venezuela. Leaves of the evergreen species are sclerophyllous, while those of the deciduous species are mesophytic. Leaf area to leaf weight ratios of fully mature leaves were about 75 and 170 cm2 g-1 in evergreen and deciduous species, respectively. Seasonal fluctuations of leaf water content per unit of dry weight, water potential, and turgor pressure were smaller in evergreen than in deciduous species. The analysis of tissue water relations using pressurevolume curves showed that evergreen species could develop a higher leaf turgor and lose turgor at lower leaf water potentials than deciduous species. This was related to a lower osmotic potential at full turgor in evergreen (≃-3.0 MPa)_than in deciduous (≃-2.0 MPa) species, rather than to the elastic properties of leaf tissue. The volumetric modulus of elasticity was 14 MPa in evergreen compared with 7-10 MPa in deciduous species. Thus, leaf characteristics are important in determining the drought resistance of evergreen species of this tropical dry forest.

Physiological and Growth Responses of Seashore Paspalum to Salinity

Abstract Two cultivars of Seashore Paspalum (Paspalum vaginatum Swartz.), ‘Adalayd’ (‘Excaliber’) and ‘FSP-1’, were grown in solution culture at 6 levels of salinity derived from synthetic sea water. Cultivars differed in changes of leaf water potential, leaf water potential components, and in growth responses to increased salinity. ‘Adalayd’ exhibited a linear decrease whereas ‘FSP-1’ exhibited a quadratic decrease in leaf water potential with increasing salinity. Leaf osmotic potentials decreased linearly for both cultivars, but there was a significant interaction. Leaf turgor potential decreased linearly for ‘Adalayd’ but quadratically for ‘FSP-1’. ‘FSP-1’ had greater tolerance to salinity in solution culture than Adalayd.

Physiological Response of St. Augustinegrass to Irrigation Scheduling

AbstractIrrigation is an integral part of a turfgrass management program and accounts for a large part of the energy inputs on a turf area. As potable water supplies decrease, use of limited water will impose stress on turfgrass areas which may result in reduced quality. Research on irrigation practices should help to minimize the effects of water stress resulting from reduced irrigation frequency. Response of St. Augustinegrass [Stenotaphrum secundatum(Walt.) Kuntze] to irrigation scheduling was studied during the summers of 1980 and 1981 on a loamy, siliceous, hyperthermic Grossarenic Palendult soil. An irrigation schedule of 2‐, 3‐, 4‐, and 6‐day intervals simulated conditions ranging from grass being under no water stress to severe water stress. Field plots were constructed to maintain an undisturbed soil profile that horizontally isolated adjacent plots to prevent lateral water movement. Irrigation treatments were implemented by irrigating with evapotranspiration estimates of 0.64 cm day−1in 1980 and 0.38 cm day−1in 1981 at either 2‐,3‐,4‐, or 6‐day intervals. Plots received equal volumes of water, only irrigation timing was varied. Turfgrass swards were evaluated on 12‐day cycles under water stress prior to irrigation and after 24 h of recovery following irrigation. Measured parameters included turfgrass quality, carbon exchange rates (CER), evapotranspiration rates (ET), plant water potential component changes, and leaf diffusive resistance, and transpiration. Irrigation scheduling affected many of the parameters studied. The 6‐day irrigation schedule caused a reduction in CER and ET and concurrent increase in stomatal resistance and a loss of turgor pressure prior to irrigation. Following irrigation, CER and ET increased, turgor pressure increased, and diffusive resistance decreased during each 12‐day cycle. When averaged over the entire study period preirrigation values of CER, ET, leaf water potential components, and transpiration were lowest and diffusive resistance highest for the 6‐day schedule, and these increased or decreased during recovery following irrigation. Even under the most severe conditions of 0.38 cm of water day−1applied every 6 days, the sward remained in positive carbon balance. Turfgrass quality and percent cover were not affected by irrigation treatments. Scheduling irrigation every 6 days with 2.3 cm of water will not affect St. Augustinegrass turf quality if adequate mowing height is maintained.

Relationships Among Water Potential Components, Relative Water Content, and Stomatal Resistance of Field-Grown Peanut Leaves1

Abstract Limited data exist describing the physiological responses of peanut (Arachis hypogaea L.) plants to tissue water deficits. Detailed field experiments which accurately define the water status of both the plant and soil are required to better understand the effects of water stress on a peanut crop. The objectives of the present study were 1) to describe the changes in leaf water potential components during a drying cycle, and 2) to define the relationships among soil water content, leaf water potential, leaf turgor potential, relative water content, leaf-air temperature differential, and leaf diffusive resistance as water stress was imposed on a peanut crop. During a 28-day drying period where both rainfall and irrigation were withheld from peanut plants, midday measurements of the physiological parameters and volumetric soil water contents were taken concurrently. As soil drying progressed, water extraction from the upper soil depths was limited as soil moisture approached 0.04 m3m-3. Leaf water potentials and leaf turgor potentials of nonirrigated plants decreased to approximately −2.0 and 0 MPa, respectively, by the end of the experimental period. Leaf water potentials declined only gradually as the average volumetric soil water content in the upper 90 cm of soil decreased from 0.12 to 0.04 m3m-3. Further reductions in soil water content caused large reductions in leaf water potential. As volumetric soil moisture content decreased slightly below 0.04 m3m-3 in the upper 90 cm, leaf relative water content dropped to 86%, leaf water potential approached −1.6 MPa and leaf turgor potential decreased to 0 MPa. Concurrently, stomatal closure resulted and leaf temperature increased above air temperature. Osmotic potentials measured at 100% relative water content were similar for irrigated and nonirrigated plants, suggesting little or no osmotic regulation.

Water balance in developing leaves of four tropical savanna woody species

Water balance was studied in developing leaves of Curatella americana, Byrsonima crassifolia, Bowdichia virgilioides and Casearia sylvestris, four widespread tropical savanna evergreen woody species that renew their leaves during the dry season. Water potential components of leaves of different ages were estimated in the laboratory by determination of pressure-volume (PV) curves. Data from PV curves were used to help interpret the significance of variations in leaf water potential (ΨL) and stomatal conductance observed in leaves of different ages in the field.Initial osmotic potential at full turgidity as well as osmotic potential at the turgor loss point changed considerably during leaf development. Values of these water potential components for mature leaves were similar to those reported for temperate mesophytic woody species. Passive osmotic adjustment defined as the change in osmotic potential between full turgidity and turgor loss averaged only 0.3 MPa, much smaller than values for temperate mesophytic and drought hardy woody species. Since environmental conditions, especially soil moisture in the rooting zone, were essentially constant during leaf development, changes in leaf water potential components were not seasonal, but rather ontogenetic responses to diurnal water stress.In all species except C. sylvestris there were differences in ΨL between adjacent leaves exposed to the same environmental conditions, with immature leaves generally maintaining higher ΨL than mature leaves. The probable causes for this were 1) lower liquid flow resistance and 2) lower transpiration rates in younger leaves. At low transpiration rates liquid flow resistance was flux-dependent in all species, but became nearly independent of transpiration flux density (E) as E increased. This and their lower flow resistance permit high E to be sustained in developing leaves without excessive ψ drop across the leaf.During two typical dry season days E was high and even though complete or nearly complete turgor loss was sustained, these water deficits were not severe enough to cause complete stomatal closure. The possible relationship between a high stomatal conductance and the maintenance of a favorable carbon balance in these species is discussed.All of the water relations features observed suggested that in spite of their scleromorphic appearance, leaves of the four species under the conditions studied were adapted only to short term diurnal water deficits.

Field Use of Screen‐Caged Thermocouple Psychrometers in Sample Chambers1

These investigations attempt to answer questions still remaining concerning the use of thermocouple psychrometers for measurement of leaf water potential when laboratory facilities are located some distance from the field. Our results show that with the necessary precautions, field use of thermocouple psychrometers provides a practical and useful method for measurement of plant water potential components. Wheat [Triticum aestivum (L.) em. Thell.] leaf samples taken in the field were sealed in sample chambers containing screencaged thermocouple psychrometers, and transported 30 km to the laboratory. Thermal equilibrium was attained in 4 min, and vapor pressure equilibrium in 4 h, after placement of the sample chambers in a constant temperature waterbath. Osmotic potential measurements could be delayed for up to 24 h as the equilibrium value remained constant after 4 h in the waterbath. Equilibration times were the same for different wheat species and cultivars tested and were unaffected by tissue water content. The temperature of the sample chamber during transit from the field to a laboratory did not affect the vapor pressure equilibration time or the water potential measured. Water potentials were however affected if transit time exceeded 2 h. Significant water loss (233 kPa in 15 s) was found to occur immediately after leaf excision before the leaf sample was sealed in the chamber. Procedures which take account of all these affects enable reliable measurements of leaf water potential components to be obtained when laboratory facilities are located some distance from the field. Leaf water potential values compared favorably with those in the literature and gave a near linear (1:1) relationship with measurements made using a Scholander pressure chamber. The osmotic potential at incipient plasmolysis as measured by the pressure‐volume method compared favorably with the osmotic potential as measured with psychrometers.

Water relations of winter wheat: 3. Components of leaf water potential and the soil-plant water potential gradient

SUMMARYDiurnal and seasonal changes in the total, osmotic and turgor potentials of winter wheat leaves are compared in two seasons of mild and severe soil water stress. Gradients of total water potential in the soil-plant system are also presented. In both seasons the total water potential of the leaves decreased in parallel with the soil water potential, concurrently leaf osmotic potential also decreased sufficiently to maintain positive leaf turgor potential. Eventually, under severe water stress, soil water potential approached –1·5 MPa and leaf turgor potential tended to zero during the middle of the day.The potential drop across the soil-root system was twice that along the stem. Estimates of the water potential at the root surface varied diurnally and were often lower than the bulk soil water potential. In dry soil plants were unable to equilibrate with the soil water potential overnight. These results are consistent with the existence of significant resistance to water flow across the rhizosphere.

Water Relations and Carbon‐14 Assimilation of Cotton with Different Leaf Morphology1

The purpose of this research was to determine the usefulness of simply inherited genetic differences in cotton (Gossypium hirsutum L.) leaf morphology (normal, laciniate, and superokra) as it relates to drought tolerance or water use efficiency. This initial report describes the leaf water potential components, C02 fixation rates, and the rate of loss of assimilated 14C from various source leaves on plants subjected to soil water stress at select growth stages under glasshouse conditions.No significant genotypic differences were observed in leaf water potential components or in CO2 fixation rates when the soil water content was near field capacity. Trends did exist for the superokra to be slightly superior to the normaleaf type. Significant differences were observed in both the rate and extent of 14C loss from different source leaves in the various genotypes. The superokra genotype had the highest activity, whereas the normal leaf genotype was the lowest in activity.As soil water availability began to affect leaf water status, significant genotypic differences existed in all measured physiological parameters. The superokra genotype was able to maintain a more favorable leaf water status through a combination of higher leaf water potential and lower osmotic potentlal than the normal leaf genotypes. Photosynthetic rates were reduced by water stress in all genotypes; however, the magnitude of the reduction varied significantly among the genotypes reflecting specific differences in turgor pressure. The removal of assimilated 14C from various source leaves subjected to water stress also exhibited significant genotypic differences both in the rate of removal and in the total amount of assimilate removed during a 24‐hour period. As water stress intensified the photosynthetic assimilation of 14CO2 was found to be more sensitive than the removal of assimilate from the source leaf.The results of these experiments indicate a possible advantage of altered leaf morphology when cotton leaves are subjected to soil water stress. The benefits of improved water relations and CO2 assimilatlon to yield in the altered leaf types of cotton remain to be evaluated.