Transpiration Resistance of Plants

Abstract

An energy budget method is described for using potom- eters to measure the minimum diffusion resistance to transpiration of excised leaves. The method is used to infer the transpiration rate of intact leaves in situ. Values of minimum resistance are given for several coniferous and deciduous trees and shrubs from wet to dry habitats. energy between the leaf and the environment by radiation, convection, and transpiration. The loss of water vapor by a leaf cools a leaf and often keeps the temperature from rising too high for plant proteins when the incident energy is large. During transpiration, water vapor diffuses from the mesophyll cell walls lining the substomatal cavity, through the stomatal channel, to the free air beyond the leaf. In still air there is a boundary layer of moist air adjacent to the leaf surface. The transpiration rate is proportional to the vapor pressure gradient between the mesophyll cell walls and the free air beyond the leaf, and is inversely proportional to the resistance to flow offered by the stomatal channel and boundary layer. It is the purpose of this paper to describe a method of measurement of the diffusion resistance of intact leaves or branches of plants. During the daytime, plants receive direct solar radiation and in- direct solar radiation scattered by the sky and clouds and reflected from the ground and nearby objects. They also receive thermal radia- tion emitted by the atmosphere, the ground and nearby objects. A certain fraction of the incident radiation is absorbed. Leaf tempera- ture adjusts to the energy exchange until the absorbed radiation absR, is balanced by the loss by reradiation, R, the loss or gain by convec- tion, C, and the loss by transpiration, E. The energy budget of a plant is discussed in detail by Gates (1962) and is written as follows: