Physiological responses to various water saving systems in rice

The conventional system for irrigating rice uses a large amount of water. Recently a few water saving technologies (saturated soil culture, well-watered aerated conditions such as aerobic rice) have been trialled in the field to reduce water use and to improve water use efficiency. However rice appears to be sensitive to small water deficits; yields are often lower in the water saving technologies. Pot experiments were conducted to simulate rice grown in saturated soil conditions with varying depths to the free water and to measure the physiological responses of rice seedlings and mature plants to them compared to the conventional flooded system of growing rice and to rice grown in aerated conditions. A field study compared the performance of one saturated soil system with flooded rice. A total of 10 cultivars were used in the study. In the glasshouse experiments the different water systems reduced water use by 28-16% relative to the continuously flooded treatment (control) and improved water use efficiency by up to 20% and thus appeared to simulate the effects on water use and water use efficiency reported in field trials. Leaf water potential ranged from 97 to 88% of the control and in most treatments (except in the fully aerated treatment) the roots had access to water at depth in the pot. Thus the plants were exposed to minimum water deficits (except is some aerated rice experiments). However there were significant effects of the water systems on some of the physiological parameters measured. Stomatal conductance was most sensitive with reductions of 27-13% of that of the control. Leaf elongation rate was reduced particularly in the aerated system where water was added to the pots to bring the soil to field capacity every day. Even when minimum available water was kept above 83% there was a significant reduction in stomatal conductance and in leaf elongation rate. On the other hand the effect of the water treatments on many other physiological parameters was very small particularly when the water table was maintained at around 10 cm below the soil surface. Thus while rice appears to be sensitive to small reductions in available water, both in the SSC and aerated systems, water use can be reduced using the SSC system with minimum effects on physiological processes provided free water is maintained around 10 cm below the soil surface. Field experiments using saturated soil culture in the rice growing area of southern New South Wales, Australia, tended to confirm the sensitivity of stomatal conductance and leaf elongation rate although there was some early un-planned drought stress in this treatment