Transpiration response of Atriplex nummularia Lindl. and upland cotton vegetation to soil-moisture stress

Abstract The effect of increasing soil-moisture stress upon the transpiration of a stand of Atriplex nummularia Lindl., a native Australian saltbush, and of the American Upland cotton, variety Empire, was investigated under open glasshouse conditions in the Australian semi-arid zone. By growing the plants at a high density and restricting their height to 18 cm, each species formed a low uniform stand of vegetation. The plants were grown in sandy clay loam soil contained in two large weighing lysimeters and in surrounding boxes that formed a guard area. In each experiment soil-moisture stress was imposed upon the vegetation growing in one lysimeter and the surrounding guard area, by withholding water until the plants became acutely wilted, while the soil in the second lysimeter and in its guard area was maintained close to field capacity. The effect of the water stress treatment upon transpiration was determined by recording the daily water loss from the two lysimeters. The experiment was carried out in late summer and repeated in winter with each species. No signs of water deficit were ever seen among the plants of either species in the watered area of the vegetation, even on the hottest days experienced when peak transpiration rates of 8 mm/day and 12 mm/day were recorded for the A: nummularia and cotton vegetation respectively. The transpiration responses of both the Atriplex and cotton vegetation to increasing soil-moisture stress were very similar, while in the summer and winter experiments the effective soil-moisture tension range over which transpiration was reduced was approximately the same also, i.e., from 10–60 bars. In each experiment the falling transpiration rate showed an approximately linear relationship to the logarithm of soil-moisture tension. Quantitative differences were found between the summer and winter transpiration rates, in the range limited by soil-moisture tension. Thus, for the same soil-moisture tension the transpiration rate of both species was initially about 3–4 times greater in the summer experiments than in the winter ones. As the soil-moisture tension increased, the reduction in the transpiration rate of each species in the summer experiment was much greater than in the respective winter one, and the reduction in the rate occurred about 5 times more rapidly. However, because of the lower level of transpiration in the winter, the summer transpiration rates at any given soil-moisture tension did not fall below the corresponding winter values. That the sensitivity of transpiration to increasing soil-moisture tension was relatively unaffected by the climatic change from summer to winter in both cotton and Atriplex may have been a reflection of the particular soil root system present in the experiments. Alternatively, this may have been caused by changes in the anatomy or physiology of each species in wintertime. The ability of A. nummularia to survive rainless periods under semi-arid climatic conditions is considered. It is suggested that since no special characteristic that would aid drought survival was found, this ability probably depends upon a feature that did not develop in the experiments. Such a feature could be the development of an extensive root system.