Rainfed lowland rice often grows in unsubmerged soil conditions during dry spells. Even if the drought effect is negligible, nutrient uptake and rice yield may decrease because of chronic unsubmerged soil stress on coarse-textured soil. The objective of this study was to evaluate the effect of unsubmerged soil conditions on N and P uptake, biomass accumulation and grain yield of 20 diverse rice genotypes. Unsubmerged and submerged field trials were conducted at the Ubon Ratchathani Rice Research Center in northeast Thailand in the wet seasons of 2010 and 2011. In the unsubmerged trial, rice was grown aerobically by draining the perched water; soil moisture at 20-cm depth fluctuated between -10 and -30 kPa. On average, the yield decline in unsubmerged soil conditions was 47% compared with submerged soil conditions (3.75 vs. 1.99 t ha-1), which was attributable to reduced biomass accumulation and N and P uptake. Unsubmerged soil stress had minimal effect on harvest index, days to heading and N concentration. In unsubmerged soil conditions, N and P uptake and N-use efficiency (biomass/N uptake) positively correlated with biomass accumulation at heading stage. Rice plants showed severe P deficiency because of unsubmerged soil stress, which limited the contribution of P-use efficiency to biomass accumulation. Our results showed that the physiological traits improving P uptake and plant P nutrition under unsubmerged soil stress would be important targets for future research. We suggest that genotype screening under unsubmerged soil stress on coarse-textured soil may further improve rainfed lowland rice for the drought-prone plains.
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