Abstract
Heat pulse method is an effective method for measuring sap-flow in stem of trees and vegitables under field condition. It is generally considered hard to use this method for the grasses having short and slender stems because stems are injured. The experiment for examining the application of the heat pulse method to some grasses was carried to clarify the plant-water relationship in Japanese millet (Echinochloa utilis Ohwi et Yabuno), sorghum (Sorghum bicolor Moench) and maize (Zea mays L.). 1) The velocity of sap-flow in stem, evaluated by the heat pulse method, was closely related to the water loss due to the transpiration expressed by means of weighing. Linear relationships and positive correlations were observed in all species between the velocity of sap-flow and transpiration rate. And the deviation of observed points from the regression line were less than about 10 percent for all range (Fig.3). 2) All species had similar patterns in the diurnal courses of the sap-flow velocity. The sap-flow velocity increased rapidly with increasing solar radiation in the morning, and reached at the highest level in around midday, then declined gradually with decreasing solar radiation (Fig.2). 3) The velocity of sap-flow was closely related to the factors affecting transpiration, such as stomatal aperture, stomatal conductance, leaf temperature and light intensity. There were positive correlations among these transpiration factors with each other (Table 1, Fig.4). 4) These results indicate that the heat pulse method is suitable for the study of water consumption in individual plant of Japanese millet, sorghum and maiz grown under field condition.
