蒸散に関する研究(3)

かんしよの蒸散と蒸散抵抗

抄録

Internal and external resistances to gaseous diffusion have been used in the physical expressions of water vapor flow during transpiration from plant leaves.
External resistances can be estimated with an appropriate method, for instance, on the basis of evaporation from a wetted surface, of heat transfer coefficient of a leaf-shaped plate, or of the detailed profiles of meteorological elements. Takechi and Haseba (1971) deduced that the external resistance can be strictly determined by use of the measured value of evaporation from a leaf-shaped wet surface with a same variation of surface temperature as a leaf.
The total resistance to transpiration can be obtained as division of the transpiration rate by departure of water vapor concentration on the mesophyll cell walls from that in ambient air. Then, the internal resistance is obtained by subtraction of the external component from the total. Otherwise, the internal resistance may be directly measured by means of a special porometer, or calculated from the measurement of stomatal density and aperture. However, there are few examples showing the relationship between internal resistances and stomatal aperture.
In this paper, transpiration from leaves of potted sweet potato was dealt. The transpiration was measured by a weighing method and internal resistances were estimatecd using external resistances calculated with the equations based on the theory of mass transfer through the boundary layer of a leaf. In addition, daily march of the transpiration rate is discussed with environmental factors measured.
Results are as follows:
1) When well irrigated, a marked increase in transpiration rate occurred after sunrise and the rate reached about 1×10^-5g/cm² sec at midday on clear days. In the afternoon it gradually decreased with time. During night the rate was very small at an order of 10^-7g/cm² sec. The transpiration rate showed a linear increase with water vapor concentration difference between leaf interior and air, so far as wind speed and stomatal aperture had constant values. The internal resistance was about 20sec/cm during mighthours. After sunrise it decreased rapidly and maintained a nearly constant values of about 2sec/cm during daytime. After sunset increase in the internal resistance with closing of stomata occurred.
2) At low soil moisture, transpiration was very small even at midday when internal resistance showed a large value of about 10sec/cm.
3) When insolation was shielded, transpiration was reduced to about 40% of that under full insolation, and the internal resistance doubled as a result of stomatal closing.
4) Under normal growing conditions, the internal rsistance in sweet potato leaves depended inversely on the stomatal opening in agreement with an inference by the diffusion theory.