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Existing unsaturated zone soil water samplers have several deficiencies that jeopardize their utility for field sampling. Suction cups function only when a vacuum is applied, and sample from an unknown volume of soil. Pan samplers sample only saturated flow. A capillary‐wick sampler was developed to overcome these problems and was tested in both the laboratory and field to evaluate its performance. Breakthrough curves for selected inorganic ions and organic chemicals were established in the laboratory. No adsorption/desorption of these chemicals was found for the capillary‐wick sampler made entirely of glass. Banks of eight capillary‐wick samplers were installed in field plots of Padina sand (loamy, siliceous, thermic, Grossarenic Paleustalf), Weswood silt loam (fine‐silty, mixed, thermic Fluventic Ustochrept), and Lufkin clay (fine, montmorillonitic, thermic Vertic Albaqualf) soils. Saturated soil Br breakthrough curves were determined at each location and used to estimate the number of samplers required to characterize the flow of contaminants. To achieve 95% confidence in the chemical data from the three soils and plot sizes tested, 31 samplers were required for the sandy soil, six for the silt loam soil, and only two for the clay soil. The wick‐activated sampler collected soil solution samples from soils having soil water potentials ranging from 0 to −6.0 kPa. The capillary‐wick sampler collected samples continuously across this range of water potentials without the need for a continuous vacuum and is thus an improvement over existing samplers. While the capillary‐wick samplers usually collected sample volumes representative of the flux at potentials of −5 kPa, samples collected at greater and lesser potentials were not representative of the flux.

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Young growth‐chamber‐grown cotton plants were subjected to a series of eight periods of soil water stress, which served as a preconditioning treatment. After preconditioning, water was withheld and changes in the stomatal resistance and leaf water potential were determined and compared with similar well watered control plants. The stomatal response of stress preconditioned plants adjusted such that the diffusion resistance of the lower surface of the leaf did not reach a value greater than 20 s cm−1 until the leaf water potential dropped 14 bars below that required to reach the same resistance on previously unstressed plants. The resistance—leaf water potential relation for the adaxial surface was unaltered by the preconditioning treatment. Adjustment of the osmotic potential of the guard cells on the abaxial surface provides at least a partial explanation of this change in response. The lack of adjustment of stomatal response on the adaxial surface of the leaves was correlated with a lack of adjustment in osmotic potential of guard cells on that surface.

The stomatal resistance of individual leaves of young cotton plants (Gossypium hirsutum L. var. Stoneville 213) was measured during a period of soil moisture stress under conditions of constant evaporative demand. When plants were subjected to increasing soil water stress, increases in stomatal resistance occurred first on the lower leaves and the stomata on the upper surfaces were the most sensitive to decreasing leaf-water potential. Stomatal closure proceeded from the oldest leaves to the youngest as the stress became more severe. This apparent effect of leaf age was not due to radiation differences during the stress period. Radiation adjustments on individual leaves during their development altered the stomatal closure potential for all leaves, but did not change the within-plant pattern. Our data indicate that no single value of leaf water potential will adequately represent a threshold for stomatal closure in cotton. Rather, the stomatal resistance of each leaf is uniquely related to its own water potential as modified by age and radiation regime during development. The effect of age on stress-induced stomatal closure was not associated with a loss of potassium from older leaves. Increases in both the free and bound forms of abscisic acid were observed in water-stressed plants, but the largest accumulations occurred in the youngest leaves. Thus, the pattern of abscisic acid accumulation in response to water stress did not parallel the pattern of stomatal closure induced by water stress.It is generally recognized that light intensity and leaf water potential are two of the most important factors controlling stomatal resistance of whole plants or individual leaves. The possible interaction between leaf age and stomatal response to reduced light intensity or low soil or plant water potential is frequently alluded to, but has seldom been the primary topic of research reports.Studies by Turner (25,26), Turner and Incoll (28) were reached by Gee and Federer for beech and birch leaves (7).Brown and Rosenberg (3) also demonstrated that the Rs2 of similarly illuminated sugar beet leaves was independent of age except for the most immature leaves.In contrast with the above reports, data by Slatyer and Bierhuizen (20) and Holmgren et al. (9) indicated that R8 of leaves of cotton grown under controlled conditions increased with age. The authors (20) suggested that additional leaf growth during the aging period may have been associated with solute translocation and consequent senescence of the test leaves. This same general result was obtained by Solarova (22) with primary leaves of bean. The R8 of both surfaces increased gradually with aging, but the increases were retarded when shading was eliminated or the plants were decapitated. In a recent study, Davis (5) found increases in leaf resistance of beans occurred at a nearly constant "developmental age" (80% of time from leaf maturity to death) regardless of leaf position on the plant.The above cited reports support the conclusion that reduced illumination...