Studies of Transpiration in Relation to the Environment (2)

Abstract

The rate of transpiration from plant leaves and leaf temperature in the case of changing transpiration with a meteorological factor have been calculated from the equation of stationary energy-balance for a single leaf. The results obtained are represented in the forms of the relations between the transpiration rate and the difference of leaf and air temperatures, under some climatological conditions. Further, some observations are compared with the calculated results.i) Variation with solar radiation.When assuming that the stomatal aperture in terms of the coefficient of internal vapor transfer increases with solar radiation and the other meteorological factors are fixed, an increase in solar radiation under the conditions of lower air-temperature, higher humidity and smaller internal-transfer-coefficient increases both transpiration rate and leaf temperature. Under the opposite conditions, the increase in solar radiation increases the transpiration rate and decreases the leaf temperature, until the increase of the radiation increases both transpiration rate and leaf temperature when the radiation is above a certain amount.When the internal-transfer-coefficient remains constant as solar radiation varies, the transpiration rate and leaf temperature show a nearly proportional relationship, i.e. the transpiration rate increases and the leaf temperature rises with increasing solar radiation.ii) Variation with air temperature.In both cases when the internal-transfer-coefficient increases with air temperature and when it remains constant as air temperature varies, an elevation of air temperature produces an increase in transpiration rate and a decrease in temperature difference between leaf and air; the leaf temperature rises with air temperature.iii) Variation with relative humidity.In all cases when the internal-transfer-coefficient decreases or increases with decreasing humidity and when the coefficient remains constant, the transpiration rate increases as humidity varies and the leaf temperature is decreased. The relationships between them are almost linear.iv) Variation with wind speed.Simulation was done on the assumption that the internal-transfer-coefficient remains constant as wind speed varies.When the transpiration is suppressed under the conditions of lower air temperature, higher humidity and smaller internal-transfer-coefficient and the leaf-air temperature difference becomes larger under the conditions of higher solar radiation, an increase in wind speed diminishes both transpiration rate and leaf temperature.Under the opposite conditions, the increase in wind speed increases both transpiration rate and leaf temperature.Under the medial conditions of air temperature, relative humidity, internal-transfer-coefficient and solar radiation, the tendency for the transpiration rate to increase with wind speed is general and then the leaf temperature falls.v) Observations.The relations between transpiration rate and leaf temperature obtained from some experiments using citrus, sweet potato and paddy rice plant in controlled environments were in good agreement with the simulation of the leaf-heat-balance.