NOCTURNAL THERMAL EXCHANGE OF CITRUS LEAVES

Turrell , F. M., S. W. Austin , and R. L. Perry . (U. California, Riverside & Los Angeles.) Nocturnal thermal exchange of citrus leaves. Amer. Jour. Bot. 49(2) : 97–109. Illus. 1962.—Cooling rates of leaves were measured with fine thermocouples inserted within the leaf laminae. From these rates, total thermal conductances were calculated for leaves of intact greenhouse-grown lemon cuttings, in the dark, in still air and moving air, and in open laboratory rooms of warm to freezing temperatures. Thermal conductances were also calculated for leaves of 4 commercial varieties of citrus picked from mature trees in the grove and measured in low light, in still air, in a microcosm at warm, constant temperatures. The total conductances were fractionated, first, by determining transpiration rates of detached leaves from both sources, in darkness and in still or moving air, through similar temperature ranges and humidities. From transpiration rates, transpiration conductances were calculated. Second fractions (radiation conductances) were calculated for lemon leaves from far-infrared reflectances; and the third fractions (free-convection conductances) were calculated by subtraction of the sum of the radiation and transpiration conductances from the total. A free-convection-conductance coefficient was calculated for lemon, and then applied to 3 other varieties of citrus for which infrared reflectances were unavailable, to obtain their free-convection conductances. These together with experimentally determined transpiration and total conductances permitted calculation of their radiation fractions. The conductances have been tested for 6 different measured microclimates in which the calculated leaf temperatures averaged ± 0.6 C of the measured temperatures, an error compatible with the precision of field temperature measurements. Total thermal conductances of lemon leaves were higher in both warm, still and warm moving air than in cold, whereas the radiation and free-convection fractions were about equal in still air. The transpiration fractions were very small in warm, still or warm moving air but negligible in cold. In cold still and cold moving air, all the conductances were larger for orange fruit than for lemon leaves. Leaves of plants native to tropical rain forests were more efficient in heat transfer than were leaves from the temperate zone.