Estimation of the leaf water potential in rice by the pressure chamber technique.

Abstract

The pressure chamber technique is now widely adopted for the measurement of leaf water potential. The technique is simple and rapid, but several precautions are necessary to ensure reliable results. It is especially important to prevent rapid water loss after leaf excision, as demonsterated by TURNER and LONG. This study was undertaken to examine the effects of the rate of pressure increase and of transpirational water loss on the estimate of the leaf water potential in rice by the pressure chamber technique. A lowland rice cultivar, Nipponbare, was used throughout the study. Gauge reading of a pressure chamber was affected by the rate of pressure increase under both high and low irradiances (Fig. 2). The reading first increased with pressurization rate up to 0.1 to 0.2 bar/s and thereafter declined slightly or remained almost constant. No difference was found in water loss from a leaf during measurement between rapid and slow rates of pressure increase (Table 1). The leaf to be sampled was covered with a vinyl-bag just before severing. The leaves from well watered plants in the naturally-lit glasshouse in which temperature, humidity and windspeed were controlled were used. From these results, together with the fact that acceptable agreement was obtained between the water potentials measured by the pressure chamber techique with pressurization rate of 0.3 bar/s and thermocouple psychrometry for rice leaves with stomata closed, we have decided that the rate of 0.2 bar/s is appropriate for rice and used in the following measurements. Similar rate, 0.36 bar/s, is used by O'TOOLE and MOYA although TURNER suggests the rate of 0.05 bar/s. We found large differences between fresh weights of a leaf covered with a vinyl-bag and a leaf left uncovered after excision (Fig. 3). Rapid water loss after excision was also evident in terms of RWC (relative water content) as well as water content per unit leaf area in leaves without a vinyl cover (Table 2). According to the moisture retention characteristics for the rice leaves we used (Fig. 4), decrease in RWC of the uncovered leaf in the first 30 seconds was equivalent to the lowering of the water potential by approximately 5 bar. On the other hand, corresponding decrease in the water potential of the covered leaves was only 1 bar. Recovery of the leaf water potential after rewatering was distinct when the leaf was covered with a vinyl-bag just before sampling (Fig. 5). Change in the leaf water potential in response to irrigation was vague and values were significantly lower when the measurements were taken in the leaf without a cover. These results suggest that the water potential in adequately sampled leaves only reflects dynamic response of plant water status to environment properly. Relation between the leaf water potential and RWC appeared to be relatively stable during the grain-filling period (Fig. 6). From above results we conclude that a large discrepancy between the leaf water potentials measured with a pressure chamber and a thermocouple psychrometer, as found in traspiring rice leaves, is primarily due to rapid water loss after excision as a result of an inadequate sampling techique. The thermocouple psychrometry, too, may be questioned for the following reasons. Since a small piece of leaf tissue is used in this technique, number of samples should be carefully determined so as to represent the leaf as a whole. Significant gradients in water potential may exist in transpiring leaves, as suggested by the large differences in RWC along leaf blade (Fig. 7). Small size of the sample creates another problem. Since the psychrometry measures the relative humidity in the chamber generated by the chemical potential of all water in the leaf sample, dilution effect by apoplastic water may lead to the overestimate of the leaf water petential. BARR and KRAMER found that the smaller the sample size was, the higher the leaf water potential as measured with a thermocouple psychrometer was. Thus, p