Some environmental effects on the water status of wheat

The influence of soil moisture status and various meteorological parameters on the water relations of wheat were studies both in the field and under controlled environment conditions. Water potentials of the leaf and ear were measured with a pressured bomb, and leaf diffusion resistance with a diffusion porometer. In the field there was a marked diurnal variation in leaf and ear water potential which closely followed the daily pattern of air temperature and humidity changes, ear water potentials being higher than leaf water potential. Differences in evaporative conditions between days had little effect on leaf water potential when soils were dry. Irrigation only partially alleviated the fall in leaf and ear water potential during the day. Leaf diffusion resistance was closely related to solar radiation, but was unaffected by leaf water potential under relatively moist soil conditions. Diffusion resistance of the lower leaf surface was about double that of the upper surface. Under controlled environment conditions, leaf water potential and diffusion resistance were related to soil moisture status and transpiration under three different temperature and humidity conditions. Despite large differences in transpiration, leaf water potential was not affected by the differences in evaporative demand, but was closely related to soil moisture status. Diffusion resistance changed little until a critical leaf water potential was reached thereafter increasing rapidly, particularly on the lower surface of the leaf. This increase was accompanied by a fall in the transpiration rate, which in turn, was closely linked to an increase in the resistance to water flow from the soil to the leaf. When wilted plants were rewatered, leaf water potential recovered more rapidly than did diffusion resistance