Leaf Conductance Response to Humidity and Water Transport in Plants1

Stcmatal response to humidity is a potentially important adaptive characteristic. The possibility that stomata may respond to humidity independently of changes in bulk leaf water status was tested. Also the basis for reported differences in leaf water potential response to transpiration was investigated. Simultaneous measurements of in situ leaf water potential, transpiration, and net photosynthesis were made in controlled environments using sunflower (Helianthus annuus L.) and pinto bean (Phaseolus vulgaris L.). Responses of leaf conductance and leaf water potential to changes in ambient humidity and root medium water potential were determined. Leaf conductance responded consistently to changes in the vapor pressure difference between leaf and air. Decreases in ambient humidity resulted in decreases in leaf conductance with constant, decreasing or increasing leaf water potentials. Leaf conductance responded to changes in leaf water potential only when manipulations of plant water supply resulted in rapid decreases in leaf water potential below a threshold level of -8 bars. These data demonstrate that stomata may respond to humidity independently of changes in leaf water potential. On occasions environmental perturbations resulted in changes in leaf water potential that were negatively COP related with changes in transpiration. On other occasions leaf water potential remained constant when transpiration rate or water potential of the root medium was changed. On one occasion an inverted water potential gradient between root medium and leaves was observed in a transpiring plant. Models of the simultaneous movement of water and solutes within plant roots provided qualitative explanations for these observations by predicting that changes in the uptake and translocation of solutes may influence leaf water potential response to transpiration. Additional index words: In situ leaf psychrometer, Photosynthesis, Stomata, Ion translocation, Leaf water potential, Sunflower, Pinto bean, Transpiration. TOMATAL response to humidity has been demonS strated with epidermal strips (Lange et al., 1971) and decreases in leaf conductance have been induced by decreasing ambient humidity with constant leaf water potentials (Camacho-B et al., 1971) and with increases (Schulze et al., 1972) or decreases (Aston, 1970) in relative water content of the leaves. These data indicate that stomata may respond to humidity independently of changes in bulk leaf water status. This hypothesis was tested more completely by making simultaneous measurements of leaf conductance to water vapor and leaf water potential, while changing ambient humidity and root medium water potential. These experiments also provided the opportunity for examining water transport in plants to try to explain the differences in leaf water potential response to transpiration that have been reported (Hailey lcontribution from the Dep. of Plant Sciences, Univ. of California, Riverside, CA 92502 and from the U. S. Salinity Lab., WR, ARS, USDA, P. 0. Box 672, Riverside, CA 92502. Received 21 Nov. 1975. Assistant plant physiologist and agricultural engineer. et al., 1973; Camacho-B et al., 1974). Effects on leaf water potential of changes in transpiration and root medium water potential were determined with intact plants and analyzed using recent models of the simultaneous uptake of water and solutes by plant roots (Dalton et al., 1975; Fiscus, 1975). MATERIALS AND METHODS Sunflower (Helianthus annuus L. cv. Mammoth Russian) and pinto bean (Phaseolus vulgaris L.) were grown in 2-liter pots containing Pachappa sandy loam, coarse: loamy, mixed, thermic Mollic Haploxeralf, soil or aerated, modified Hoagland's solution (Epstein, 1972). Plants were grown in a growth chamber with a 14-hour photoperiod at a daytime air temperature of 25 1 C and a relative humidity of 55 3%. Nighttime air temperature and relative humidity were 20 1 C and 78 5: 30/,. Light was supplied by metal halide vapor and color-improved mercury lamps having a ratio of input watts of 11 to 4. Irradiance at leaf level during plant growth was between 280 and 420 tVm -2. Young plants with four to six true leaves were selected for study. Older leaves comprising less than 50% of the total leaf area were removed and individual plants with 100 to 200 cma leaf area (one side only) were placed in a stirred, controlledenvironment, single-plant chamber (Hall and Kaufmann, l975). Leaf temperature was 25.0, variation with time was within f 0.1 C and the leaves were arranged to minimize shading and held horizontal by nylon filaments. Plants were subjected to low irradiances (25 Win-.) so that leaf water potentials could be tletermined in situ with silver-foil psychrometers attached to the lower leaf surface of unshaded leaves (Hoffman and Hall, 1976). Occasionally, these psychrometers did not function properly. Leaf necrosis was observed on some occasions with beans several hours after the psychronieters were attached. IHowever, damage was not observed with sunflower leaves. On other occasions, sunflower leaves expanded substantially within a few hours and broke the seal between leaf and psychrometer. Therefore, after each experiment water deficits were deliberately induced in the leaves to test psychrometer functioning. Data are presented only from experiments where these tests indicated that the psychrometers were functioning properly. Transpiration and net photosynthesis were determined simultaneously with leaf water potential using an open gas exchange system as dexribed by Hall and Kaufmann (1975). Total leaf conductance to water vapor (including boundary layer, stotr.ata1, and cuticular components) was calculated using the following relationship: T I gt (Pleat Pa~r)/Patm where T is transpiration/unit leaf area (one side), gt is total leaf conductance (with the same units as T), Pl,,r and Pa,, are the partial pressures of water vapor inside the leaf and in the ambient air, and Pa,, is atmospheric pressure. PI,,, ?as obtained from leaf temperature nieasurements by assuming water vapor saturation of the air at the evaporating surfaces inside the leaf. Pair was obtained from the air leaving the chamber, which was shown to have the same vapor pressure as the air in the chamber. The flux density units for conductaticc presented here are more valid than the velocity units that are commonly used, since the latter units imply that the s.bsolute humidity difference is the driving force, whereas i t is more appropriate to consider differences in relative partial presure as the driving force (Cowan, 1972). In some experiments the water potential of the root medium rvas changed by removing the solution from around the roots a i d replacing it within 3 sec by another solution. Tests denionstrated that exposing roots to air for periods much longer 1:han 3 sec did not appear to stress the plants. Presumably the water film around each root was sufficient to supply the plant with 876 Published November, 1976