The Effect of Root Temperatures on Water and Sulphate Absorption in Intact Sunflower Plants

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Abstract
The effect of 15, 25, and 35°C root temperature on water absorption, transpiration, and sulphate uptake by the roots and transport to the shoots of intact sunflower plants has been studied using 0.5, 5.0, and 50.0 mM sulphate concentrations at two rates of transpiration induced (1) by light and low relative humidity and (2) by darkness and high relative humidity. Root temperatures and sulphate concentrations did not significantly affect the water absorption and transpiration and both these processes were approximately similar at the different treatments. There was a nearly twofold increase in water absorption and transpiration in the light and low relative humidity as compared to the dark and high relative humidity irrespective of the root temperatures and sulphate concentrations. The A.F.S. uptake in the roots was found to be independent of the root temperatures, sulphate concentrations, and transpiration rates, and amounted to 15 to 21 per cent based on the root weight. Sulphate accumulation in the roots was not significantly influenced by the root temperatures at 0.5 and 5.0 mM sulphate concentrations, but nearly doubled with temperature at 50.0 mM sulphate concentration of the external solution. The slow nature of accumulation of sulphate, the high sulphate status of the experimental plants, and the short duration of the experiments are considered as likely reasons for the absence of a clear effect of temperatures on accumulation of sulphate at the two lower concentrations of the external solution. Effects of high concentration on permeability and metabolism of the cells are suggested as the reasons for the decreased accumulation with an increase in temperature at 50.0 mM sulphate concentration. Accumulation of sulphate in the roots was not significantly influenced by the transpiration rates. Unlike root accumulation, sulphate transport to the shoots increased with increasing transpiration. However, a major part of the sulphate transport (70 to 75 per cent at 0.5 and 5.0 mM sulphate concentrations and 80 to 85 per cent at 50.0 mM sulphate concentration) appeared to have occurred at the low transpiration. The similarity of this transport to the accumulation of sulphate in the roots indicates that it was due to an active transport process sensitive to root temperatures and sulphate concentrations. A low concentration of sulphate in the xylem and an increased permeability of the root cells to ion movement induced by an increased suction in the xylem are considered as reasons for a small increase in the sulphate transport at high transpiration rate. The evidence for the existence of a barrier—probably endodermis—preventing the passive diffusion of sulphate and sensitivity of the Transpiration Stream Concentration to root temperatures and sulphate concentrations favour that the increased transport with increased transpiration was due to an active process.