Prediction of the effects of soil water stress on grain yield and radiation use efficiency of rice (Oryza Sativa L.) using a simulation model

Abstract

Drought is the major abiotic stress factor which causes yield losses of rice in Sri Lanka. The present study quantified the effects on rice yield of two effects of drought, namely reduction of leaf area and modification of photosynthesis, respiration and assimilate partitioning characteristics. A process-based model which simulated radiation interception, canopy photosynthesis, respiration and assimilate partitioning was used to independently analyse the above effects of drought. A soil water sub-model which computed the daily soil water content (SWC) through a water balance was used to quantify the onset and magnitude of drought. The model was run for irrigated and water-stressed rice crops for a 12-year period (1985-1996) using daily meteorological data from Maha Illuppallama in the Low Country Dry Zone (DL1). Simulation of SWC showed that in DL1, during the maha season, water stress (≤ 60% available water) did not occur until the beginning of grain filling stage, whereas during the yala season it occurred at or before the end of the vegetative stage. When both leaf area index (LAI) and photosynthetic characteristics were reduced by water stress, the simulated yield reductions were 3 - 64% in maha and 23 - 81% in yala. Reduction of LAI accounted for 36 - 100% of the above yield reductions while modification of photosynthetic characteristics contributed 0 - 64%. Contribution of reduced LAI was greater in a majority (19 out of 24) of the seasons. Meanwhile, water stress reduced the radiation use efficiency (i.e. biomass produced per unit of radiation intercepted ) of rice by 1 - 73% to which the greater contribution (27 - 100%) came from the reduction of photosynthetic characteristics. It is concluded that to achieve higher rice yields under the type of drought encountered in DL1, mechanisms of maintaining a higher leaf area even under water stress (i.e. osmotic adjustment, lower cell wall extensibility and threshold turgor) should be incorporated through breeding. In addition, greater stomatal sensitivity to water stress should be incorporated as a reversible mechanism of surviving possible periods of severe water deficits.