Abstract: To clarify the physiological mechanism underlying grain yield of different stems and tillers of wheat in response to phosphorus application under water-saving supplementary irrigation and to determine the suitable phosphorus fertilization rate, we set water-saving supplementary irrigation (irrigation to the soil water content of 0-40 cm soil layer shall be supplemented to 70% of the field saturation capacity during the jointing stage and flowering stage, W₇₀) and no irrigation (W₀) on wheat variety ‘Jimai 22', and three phosphorous application rates, low (90 kg P₂O₅·hm^-2, P₁), medium (135 kg P₂O₅·hm^-2, P₂), and high (180 kg P₂O₅·hm^-2, P₃), with no phosphorus application as the control (P₀). We examined the photosynthetic and senescence characteristics, the grain yield of the different stems and tillers, as well as water and phosphorus use efficiency. The results showed that under both water-saving supplementary irrigation and no irrigation, the relative content of chlorophyll, net photosynthetic rate, sucrose content, sucrose phosphate synthase activity, superoxide dismutase activity, and soluble protein content of flag leaf of the main stem and tillers Ⅰ and Ⅱ (first degree tillers arising from axils of the 1st and 2nd true leaf of the main stem) under P₂ were significantly higher than those under P₀ and P₁, which contributed to the higher grain weight per spike of main stem and tillersⅠ and Ⅱ, but did not differ from P₃. Under water-saving supplementary irrigation, P₂ increased grain yield of main stem and tillers Ⅰ, Ⅱ and Ⅲ (first degree tillers arising from axils of the 3rd true leaf of the main stem) compared with that under P₀ and P₁, and increased grain yield of tillers Ⅱ and Ⅲ compared with P₃. Grain yield per hectare under P₂ was 49.1%, 30.5% and 8.9% higher than that under P₀, P₁ and P₃, respectively. Similarly, water use efficiency and phosphorus fertilizer agronomic efficiency under P₂ were the highest among the phosphorous treatments under water-saving supplementary irrigation. Under no irrigation condition, P₂ increased grain yield of main stem and tillers Ⅰ and Ⅱ compared with P₀ and P₁, yet grain yield of tiller Ⅱ was higher than that of P₃. Furthermore, under P₂, the grain yield per hectare, water use effi-ciency, and phosphorus fertilizer agronomic efficiency were higher than those under P₀, P₁ and P₃ under no irrigation. Under each phosphorous application rate, grain yield per hectare, phosphorus fertilizer agronomic efficiency, and water use efficiency of water-saving supplementary irrigation treatment were higher than those of no irrigation treatment. In conclusion, medium phosphorus application rate (135 kg·hm^-2) under water-saving supplementary irrigation would be the optimal treatment for both high grain yield and efficiency under the experimental condition.

Key words: wheat, water saving supplementary irrigation, phosphorus application rate, yield of stems and tillers, physiological mechanism