Leaf Temperature and Internal Water Status of Wheat Grown at Different Root Temperatures1

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Abstract
Both root temperature and plant water status are important in controlling plant growth. Yet few studies report the influence of root temperature on the water relations of plants. Therefore, to determine the effect of root temperature on leaf temperature and internal water status, a cultivar of winter wheat (Triticum aestivum L. em Thell. ‘Ponca’), considered to be drought sensitive, was grown for 82 days, under growth room conditions, at six different root temperatures (15.6, 19.9, 24.7, 28.5, 32.1 and 37.3 C). The air temperature remained constant at 25 C. Leaf temperature and stomatal resistance were measured daily starting on day 18. Leaf water, osmotic, and turgor potentials were determined every other day starting on day 44. No seeds germinated at 37.3 C. During the experiment, leaf temperatures were lower than air temperature by 1.46, 1.16, 1.12, 0.98, and 0.71 C for the 15.6, 19.9, 24.7, 28.5 and 32.1 C root temperatures, respectively. Stomatal resistances averaged 21.5, 19.9, 19.8, 22.1, and 27.4 sec/cm for the 15.6, 19.9, 24.7, 28.5, and 32.1 C root temperatures, respectively. Leaf water, osmotic, and turgor potentials were highest at the 24.7 C root temperature. Potentials were lowest at the highest root temperature (32.1 C). The experiment was repeated using a wheat cultivar considered to be drought resistant (‘KanKing’). KanKing seeds germinated at 37.3 C. For KanKing, leaf temperatures were cooler than air temperature by 1.75, 1.51, 1.36, 1.32, and 1.02 C for the 15.6, 19.9, 24.7, 28.5, and 32.1 C root temperatures, respectively. Stomatal resistances averaged 44.6, 41.9, 33.5, 37.0, and 37.4 sec/cm for the 15.6, 19.9, 24.7, 28.5, and 32.1 C root temperatures, respectively. Potentials were not determined for KanKing. The drought sensitive cultivar grew taller than the drought resistant cultivar. But the heights of both cultivars, decreased in the following order with root temperature: 24.7, 28.5, 19.9, 15.6, and 32.1 C. The results showed that the drought resistant cultivar maintained a larger difference between leaf and air temperature at the different root temperatures than did the drought sensitive cultivar. This suggested that leaf temperature measurements could be used to screen plants for drought resistance.