Measurement of plant water status

Physiological response of plants and plant parts under various water regimes has been under study for several years. Modern physiologists recognise that the state of water in any biological system is paramount in determination of ultimate biological condition. Recent reviews of plant responses to environmental extremes (Parker, 1969; Levitt, 1969; Alden and Herman, 1971) show water deficits directly affect plant growth by enzyme based changes in metabolism and alteration of physical relationships. The present study was undertaken to examine two techniques recently adapted to plant physiology: first, beta-gauging of leaf water content and, second, determination of water vapour diffusive resistance with a sensor-element diffusion porometer. Determination of film thickness by monitoring beta particle absorption has been used in physical sciences and industry for many years (Putman, 1964) but has only recently found use in plant physiology (Yamada et al., 1959, cited in Nakayama and Ehrler, 1964). Recent research particularly that examining short term autonomic cycling of leaf water content, has used the technique increasingly (see Barrs, 1971). Sensor-element diffusion porometers have found wide application in field determination of stomatal resistance particularly since Kanemasu et al., (1969) and Stiles, (1970) improved their portability. In the experiments reported here both techniques are examined in relation to relative water content determined by floating leaf discs (Barrs and Weatherley, 1962) since this has been a standard method used by many investigators for several years. Beta gauging was examined as a suitable method for continuously monitoring leaf relative water content without the destructive sampling required by the disc flotation method. Experiments with the porometer were done to examine at which level of leaf water content stomata close to a degree where gas or vapour exchange would be affected.