Studies of Transpiration in Relation to the Environment (2)

The transpiration from citrus leaves of potted Satsuma trees irrigated moderately was measured with the environmental elements from Octrber 1963 to October 1964. In addition, the evaporation from a horizontal circular wet plane surface was measured to be compared with the transpiration.An agrometeorological analysis of these data was made to clarify the annual change in transpiration and the efficiency of transpiration.In winter, the daily amount of the transpiration was about 0.015gr/cm2 day on clear days. From spring to summer transpiration became active and the daily amount reached about 0.14 gr/cm2 day on clear summer days.The transpiration rate showed the maximum in summer and the minimum in winter. The mean value of the transpiration rate in the daytime was about 2.8×10-6gr/cm2sec in summer and about 0.3×10-6gr/cm2sec in winter.The transpiration rate in the nighttime was very small for all seasons and its annual mean value was about 7×10-8gr/cm2sec.By a theoretical treatment, the transpiration rate from citrus leaves can be expressed by the following equation neglecting the terms of transfer by free convection, W≅(εs+2εc)D′fΔC, where W is the transpiration rate, εs and εc are, respectively, the efficiencies of the stomatal and of the cuticular transpirations, D′f is the coefficient of vapor transfer by forced convection from the leaf surface and ΔC is the mean difference in water vapor concentration between the leaf surface and the ambient air.The vapor transfer coefficient D′f under field conditions is confirmed to be proportional to the 4/5th power of wind velocity u, so that the following expression is obtained, W/u4/5ΔC∝εs+2εc.The calculated values of W/u4/5ΔC were small during the night and large in the daytime. On clear days except in midwinter, the transpiration efficiency showed an assymmetrical diurnal variation with respect to noon.Moreover, the diurnal variation of the ratio of the transpiration to the evaporation showed the similar assymmetrical course.The transpiration efficiency showed the minimumin in February and the maximum in July, and the efficiency in summer was about tenfold greater than that in winter.