RATE OF ADJUSTMENT OF LEAF TEMPERATURE TO INCIDENT ENERGY

Abstract

The variability of leaf mass as a factor in the energy relations of leaves was discussed (4) in a previous paper. The problem was considered with reference to the thermal effects of absorbed radiant energy, especially in the absence of energy loss through transpiration. Data were tabulated to show the specific variability in the mass of one cm.2 of leaf tissue, and the corresponding variations in the calculated rates of leaf temperature increase. These calculations were based primarily upon certain fundamental measurements of solar energy receipt, energy absorption by leaves, thermal emissivity of leaf substance, and specific heat of leaves made by Brown and Escombe (1) and Brown and Wilson (2) more than thirty years ago. In these data there is the implication that the rate of adjustment of leaf temperature to inflowing radiant energy would also vary with the leaf mass, and that the rate of thermal adjustment would be very rapid. This particular phase of the problem was not emphasized in the former paper, and it seems appropriate to call attention to the rapidity of adjustment that should be expected if the leaf is confined to thermal emissivity as a method of energy dissipation, and if the values for incident energy and absorption previously suggested (4) are accepted as average for thin leaves. As was indicated in the discussion of the mass factor, and also in an earlier discussion of the reflection of light from leaf surfaces (3), there are important reasons for modifying the values for incident radiation and absorption obtained by Brown and his co workers. Their figure for the calorie value of sunlight is considerably above the average value shown by their own measurements; and their coefficient of absorption included the reflected energy. It is now certain that this reflected energy is much too large to be neglected in such calculations. If one employs the lower values suggested (4) for energy receipt (B = 0.55) and absorption (a = 0.58) , retains Brown's values for the specific heat (s = 0.879) and thermal emissivity of leaf substance (e = 2 x 0.0366 cal./ cm.2/min./deg.), and applies these values to the leaves of plants listed in table I, the calculated rates of leaf temperature change are much reduced from those published several years ago (4). In column 3 of table I are shown the calculations (Ra/ms) of the temperature increase per minute in leaves exposed to radiation of 0.55 cal./cm.2/min., with absorption of 58