Plant Energy Budgets: Effects of Radiation and Temperature

Abstract

4A. The Plant's Energy Balance solar radiation is reflected, transmitted, or utilized for processes other than just heating. In bright sunlight the net absorption of solar radiation (SR net) is the main energy input to a leaf. If such a leaf had no means to dissipate this energy, then its temperature would reach 100°C in less than 1 minute (Jones 1985). Thus, processes that govern heat loss by a plant are critical for maintaining a suitable temperature for physiological functioning. Heat loss occurs by several processes (Fig. 1). A leaf emits long-wave infrared radiation (LR). At the same time, however, it absorbs LR emitted by surrounding objects and by the sky. The net effect of emission and absorption (L~et) may be negative or positive. When there is a temperature difference between leaf and air, convective heat transfer (C) takes place in the direction of the temperature gradient. Another major component of the energy balance is cooling caused by trans-piration (t..E; where t.. is the energy required for evaporation and E is transpiration). In addition, metabolic processes (M) generate heat, although this is typically small compared with the other components of the energy balance, and it is usually ignored. When the temperature rises in response to sunlight, most components of the energy balance that contribute to cooling increase in magnitude until energy gain and loss are in balance. At this point, the leaf has reached an equilibrium temperature (steady state), and the sum of all components of the energy balance must equal zero :