Alternate wetting and drying (AWD) irrigation is one of the most widely promoted water-saving irrigation strategy for rice. Repeated soil drying and re-wetting cycles in AWD irrigation not only can save water, but also increase mineral nutrient use efficiency and reduce fertilizer application. In this research, we compared the differences in grain yield, grain quality, irrigation water use efficiency (WUE), phosphorus use efficiency (PUE), growth states of roots and shoots of lowland and upland rice cultivars when they were under different irrigation and P fertilizer application treatments in the greenhouse study and also in the field. The greenhouse experiment included four rice varieties (2 lowland rice and 2 upland rice) under 3 irrigation regimes, including continuously flooded (CF), alternate wetting and moderate soil drying (AWMD) and alternate wetting and severe soil drying (AWSD). AWMD and AWSD enabled the plants to receive fewer irrigation frequencies and less irrigation water so that both water resources and labor force were saved. AWMD achieved the goal of increasing grain yield, WUE and PUE of upland rice; maintaining grain yield and increasing WUE and PUE of lowland rice. AWMD reduced redundant vegetative growth, enhanced root growth and increased root-shoot ratio and harvest index, which contributed to higher grain yield and higher resource use efficiency. More developed root systems under AWD helped upland rice to maintain a higher water status than lowland rice when plants were subjected to soil-drying, which resulted in a better performance of grain yield. As a comparison, AWSD regime also saved irrigation water, but it reduced grain yield.

Under the field condition, we combined irrigation regime and reduction of phosphate fertilizer application. Irrigation treatments consisted of two irrigation regimes including continuously flooded (CF), alternate wetting and moderate soil drying (AWMD). Phosphorus application treatments consisted of three P rates including 0, 45, and 90 kg ha-1 (0P, 45P, and 90P). Results showed that AWMD led to an increase of grain yield, improved WUE and PUE of upland rice at P45, whereas AWMD of lowland rice maintained grain yield, improved WUE and PUE at P45. Roots were longer and deeper under AWMD irrigation, which contributed to higher grain yield and higher resource use efficiency. When at lower P rates, the two rice types all translocated more P from vegetative tissues to grains, which led to a better P use efficiency. Our results also suggest that some plant hormones and members of OsPht1 family regulated the P homeostasis under the water-saving irrigation.

In summary, we found that AWD irrigation on rice not only can increase rice yield and water use efficiency and it also enhances P use efficiency, which potentially can reduce the use of P fertilizers. The results shall provide some theoretical and technical support for rice cultivation.