SummaryThree separate experiments were conducted in which tomato seedlings were grown at root zone temperatures of 21°, 26.5° or 32°C. Each experiment differed in the air temperature (11°, 16° or 21°C) of the growth chamber. Total plant dry weight accumulated more slowly at 11° than at 16° or 21°C. Total dry weight and root length were affected by the root temperature treatments at each air temperature. Optimum root temperature, as indicated by dry weight, was 26.5°C except at the lowest air temperature where it was 32°C. Optimum root temperature, as indicated by root length, was 26.5°C at all air temperatures. The rate of increase in root length was reduced at the lowest air temperature or when root temperature was 32°C. Relative growth rate (RGR) was decreased at the lower air temperatures and when root temperature was 32°C. Air temperature had a greater effect than root temperature on RGR except when the root temperature was above 26.5°C. Maintaining higher root temperatures is unlikely to offset the growth limitations at lower air temperatures except early in the season, and continuous high root temperatures were detrimental to growth.
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Maletta et al. (1987) studied this question.