Some Effects of the Environment on the Temperature of Tobacco Leaves in Field Plots in Rhodesia

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Abstract
Temperatures of field-tobacco leaves, at different heights and orientated in different directions, were continuously recorded during three periods with resistance thermometers in the proximal and distal parts of the upper tissue of the main leaf veins. The temperature of leaves, relative to that of the air at the same height (RLT), was positive during daylight and negative at night; sheltered leaves were sometimes warmer than the air all day. Maximum and minimum individual RLT values, between 09.00 and 16.00 hours, were +18° C and –4° C at air temperatures of 25–28° C. At night, maximum and minimum RLT values were between +0.2° and –3.0° C. Leaves facing NE were warmer in the morning thatn those facing NW or SW, but were cooler in the afternoon. Distal parts of leaves were cooler than proximal parts and upper leaves were cooler than lower ones at night: Mean daily maximum RLT was greatest in lower leaves and mean daily minimum RLT was smallest in upper ones. During daylight, exposure to solar radiation had the greatest effect on RLT: proximal parts of leaves were affected most by air temperature and humidity, and distal parts by cloud cover. At night, and often in the evenings, RLT depended largely on cloud, humidity, and leaf cover, all of which affected re-radiation: correlations between RLT and these environmental factors were found more often with distal than with proximal parts of leaves.