Effects of soil water deficit at different growth stages on rice growth and yield under upland conditions .1. Growth during drought

Upland rice can be subject to drought at any time during growth, and responses of various growth processes to water stress may depend on the timing of drought in relation to crop growth stages. In two experiments, rice plants at three or four growth stages were subjected to drought periods of 23-35 days to examine the response of shoot dry matter (SDM) production and associated processes to water stress. Young plants, 33 days old at the commencement of stress, maintained high leaf water potential during the stress period because of the low demand for water. Radiation-use efficiency (i.e. SDM production per unit intercepted radiation) of the young plants in the drought treatment was similar to that in the irrigated control, but SDM production was reduced by 71% because of 68% reduction in radiation interception. The young plants extracted water more slowly than older plants immediately after stress was imposed, whereas the total water extraction during the stress period was not significantly different from that of the older plants. The young plants produced less SDM than the older plants (90 versus 300 g m(-2)), and hence had lower water-use efficiency (i.e. SDM production per unit water used 1.4 versus 4.2 g kg(-1)). During early growth stages, root length growth was very sensitive to soil water deficit. When stress developed in the older plants with the canopy intercepting greater than about 60% of full solar radiation, reduction in SDM production was mostly due to reduced radiation-use efficiency (1.2 versus 2.3 g MJ(-1)). The plants extracted water more rapidly and from deeper soils, and leaf water potential decreased rapidly with the large demand for transpiration water. The results therefore show that rice responds to drought in a manner depending on the stage of canopy development.