Water potentials, osmotic potentials and stomatal resistances were measured daily in a growth room during a 16-day period, in growing leaves of a drought-sensitive and a drought-resistant winter wheat (Triticum aestivum L. em. Thell.), which had the roots maintained under one of three root-temperature treatments: (1) constant temperature of 24°C (about the optimum); (2) constant temperature of 34°C (10° above the optimum and near the high temperature limit for growth); and (3) alternating temperature of 24 h at 24°C followed by 24 h at 34°C. Air temperature was constant at 24°C.
Water and osmotic potentials, stomatal resistances, and height of plants maintained at the 34°C root temperature were intermediate between those grown at 24°C (highest potentials, lowest resistances, tallest) and those alternated daily between the two temperatures (lowest potentials, highest resistances, shortest). Previous work showed that potentials, stomatal resistances, and growth of wheats subjected to constant root temperatures of 25, 29 and 33°C were linearly related to root temperature with lowest potentials, highest resistances, and least growth at the high temperature. Even though alternated plants in this experiment were at an average root temperature of 29°C, they grew the poorest. The wheat plants, therefore, did not integrate a root temperature stress in the same manner over ‘time’ as they did over ‘space’.
The results showed that the stomatal resistance, water and osmotic potentials, and height of wheat seedlings, maintained at a constant root temperature 10°C higher than the optimum, were about halfway between those of plants grown at the optimum and those alternated daily between the optimum and the optimum-plus-10 °C root temperature.