Less freeze injury was observed in unprotected large citrus trees during the freezes of 1962 and 1963 than in small. Earlier observations in the Coachella Valley of southern California prompted this study of the nocturnal heat exchange of a small (3—year—old) and a large (40—year—old) grapefruit tree in two orchards of their respective sized trees. Micro—climate studies were made in these orchards during several winter seasons. Other data for heat transfer calculations have been developed from numerous sources. The heat transfer budget for calm nights showed that leaves were the principal heat exchangers and that shading by neighboring trees reduced their radiation rates to about half in large trees. Stored heat of the ground beneath, and in the woody framework inside the large trees was reflected and reradiated within the leafy crown which had highly insulating properties. The top of the crown of large trees was warmed by the inversion layer.

Sunmiary. Thermal conductivity coefficients have been determined for longitudinal and transverse flow in 4 varieties of fresh Citrus wood using steady state-method's.Equations were developed from which thermal conductivity could be rapidly estimated from moisture content or electrical conductivity. The heat balance of large and small tree trunks on a freezing night has been calculated on the basis of the coefficients. Thermal Conductivity of Fuinctional Citruts TreeWood. Heat transfer of the woody stems of trees is chiefly by radiation, convection and conduction. Radiation loss is minimized by the insulating effect of the leafy crown, and convection assumes the dominant role only during the day because the transpiration stream flows most actively then (4); at night thermal conduction, though relatively small, is the principal mode of heat transfer. Although thermal conduction was correctly considered bv Raschke (18) to plav a minor role in the heat transfer of the plant, under the stress of nocturnal advection freezes and radiation frosts, this is not the case in subtropical plants which bear leaves and fruit during winter. The magnitude of thermal conduction in the wood, relative to its mass, is important in preventing frost damage to citrus trees (24). Therefore, we have determined longitudinal and transverse thermal coniductivity coefficients for 4 species of citrus and used these coefficients to analy)se the effects of freezing temperatures on 2 cm and 20 cm diameter trees. For this analysis we have used the weather parameters which existed in Weslaco, Texas, fronm Januarv 9 to 12, 1962 (32).Thermal conductivity coefficients (K) have been determined on about /70 species of dry woods (10,13,14,20). Across grain K for nearly dry woods, having about 12 % moisture, ranges from 0.1 to 0.S

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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.