Leaf Temperatures and the Cooling of Leaves by Radiation

Abstract

There has been marked disagreement among plant physiologists relative to the effectiveness and importance of transpiration in cooling the leaves of plants. SMITH (12), CLUM (4), CURTIS (5), and others have concluded that the cooling effect is slight and rarely of importance in preventing excess heating, whereas, SHULL (10), EATON and BELDEN (7), ARTHUR and STEWART (1), CLEMENTS (3), and others have concluded that the cooling effect may be great. The latter investigators especially have claimed that the cooling effect is much greater than is commonly recognized. ARTHUR and STEWART, and CLEMENTS have claimed that the excessively high temperature of leaves, when they are inclosed in cellophane envelopes, or in glass chambers and exposed to strong light, is due to the fact that the cellophane or the glass effectively stops transpiration; while the failure of wilted, or vaselined leaves to rise in temperature more than 2°to 5°C. above those not so treated is due to the ineffectiveness of these treatments in preventing transpiration. As the writer pointed out in a recent paper (6), however, these investigators overlooked the fact that glass and cellophane are heat traps permitting the passage of visible radiation, but preventing much of the loss of heat by interfering with cooling by air currents, as well as by reducing loss by radiation in the infra-red. It was found that dry black paper showed similar marked rises in temperature when inclosed in cellophane envelopes, and in this case evaporation could play no part.Although the effects of various factors upon leaf temperatures have been investigated by several workers, almost no attention has been given to the loss of heat by radiation in the infra-red. BROWN and ESCOMBE (2) speak of loss of heat by emission, and include loss by both conduction and radiation. In their preliminary discussion of a hypothetical case they state that the walls of the inclosure must be at the same temperature as the inclosed air, but, in the actual experiments cited, no mention is made of the wall temperature, and it is assumed that the air temperature is the same as that of the leaf 's surroundinas. It is also assumed that if the air temperature is equal to or exceeds that of the leaf, there will be no loss of heat from the leaf by radiation. They, WArATSON (13), and other investigators seem to have failed to realize, that the oxygen and nitrogen of the atmosphere are almost transparent to radiation in the infra-red, and that in such an atmosphere leaves may become cooler than the air about them, owing to radiation

Temperature of Potato and Tomato Leaves

“…Differences as large as 80 C were measured under very clear, calm conditions. This observation agrees also with the data of others (6,7,12,29). The observations, table V, that lower sheltered leaves on clear and cloudy days and upper exposed leaves on clear and cloudy nights are cooler than the air agrees with the negative value of (T'-T) predicted in table IV.…”

“…MILLER and SAUNDERS (21) and EATON and BELDEN (10) attempted to observe leaf temperatures by clamping a thermocouple against leaves by means of a cork. CURTIS (7) found that leaf temperatures taken by this method were 0.9 to 3.7°C cooler than the temperature of a couple threaded into the leaf because the cork shaded the leaf. Many workers (6,7,24,32) have measured leaf temperatures by threading a thermocouple into the leaf.…”

Management of the greenhouse microclimate in relation to disease control: a review

“…Radiation to and from leaves and fruit, and convective heat transfers, can lead to tissue temperatures significantly different from ambient air temperatures [29,126,129], and cooler leaves often are sites for dew deposition [72]. Dew and other forms of surface wetness invite bacterial and fungal pathogens (Sect.…”

Influence of Bordeaux Mixture and Its Component Parts on Transpiration and Apparent Photosynthesis of Apple Leaves

“…If, in the experiments reported here the temperature of Bordeaux-sprayed leaves was lowered from 1 to 20 C., as they state in case of sprayed potato leaflets, a significant change in the vapor pressure gradient between the leaf and the external air would have taken place. CUR.S (6) shows that a 1 to 20 C. reduction in leaf temperature, between a range of 10 and 400 C., is equivalent to raising the external humidity as much as 5 to 14 per cent. Since the rate of transpiration is governed by the steepness of the vapor pressure gradient existing between the intercellular spaces and the external atmosphere, any comparative decrease in vapor pressure within the leaf, other conditions remaining constant, would reduce this gradient and lower the rate of transpiration.…”