Abstract:

Irrigation and fertilization are important measures to improve alfalfa seed productivity. Understanding the mechanisms of seed yield formation will be helpful for optimizing water and fertilizer management. The relationships between leaf nutrient characteristics at the reproductive growth stage and the seed yield of alfalfa remain unclear. We investigated leaf nutrient contents and nutrient resorption efficiencies in alfalfa plants at the reproductive stage， and their relationships with seed yield， under different irrigation and nitrogen （N） fertilization conditions. Six irrigation treatments were set up comprising differing rates of irrigation at re-greening in spring， budding and podding growth stages. The irrigation treatments were： full irrigation （450， 150 mm at each irrigation， W₁）； mild water deficit （360， 120 mm at each irrigation， W₂） and moderate water deficit （270， 90 mm at each irrigation， W₃）； 300 mm irrigation allocated 150， 75 and 75 mm， respectively， at re-greening， budding and podding stages （W₄）； 300 mm irrigation allocated 75， 150 and 75 mm， respectively， at re-greening， budding and podding stages （W₅）； and 300 mm irrigation allocated 75， 75 and 150 mm， respectively， at re-greening， budding and podding stages （W₆）. The N fertilization treatments were 10 kg·ha^-1 （N₁）^{or 40 kg·ha-1 （N₂） applied at re-greening. N and P contents of green and senescent leaves were measured at budding， podding stages and at crop maturity to allow calculation of N and P resorption. It was found that： 1） With decreasing irrigation and advancing growth stages， there were decreases in the N and phosphorus （P） contents in green leaves at the reproductive growth stage. The nutrient contents of green leaves were not lower in W₄ and W₅ than in W₃ or even W₂. The nutrient contents in green leaves at the budding and podding stages were significantly higher in N₂ than in N₁. The N and P contents in senesced leaves at the reproductive growth stage increased as irrigation decreased and growth stages advanced. The N content of senesced leaves at the budding stage was lower in W₄ and W₅ than in W₁-W₃， but the N content of senesced leaves at the podding and maturity stages was higher in W₄ and W₅ than in W₁ and even W₂. The N content of senesced leaves at the maturity stage was higher in N₂ than in N₁， and the P content at the podding stage was higher. 2） The leaf nutrient resorption efficiency during the reproductive growth stage decreased with decreasing irrigation and advancing growth stages. Nutrient resorption efficiency at the budding stage was higher in W₄ and W₅ than in W₁-W₃， but that at the podding and maturity stages was similar in W₄， W₅， and W₃. The N resorption efficiency at the budding stage was higher in N₂ than in N₁， but higher in N₁ than in N₂ at the other stages. The P resorption efficiency was higher in N₂ than in N₁ during the whole reproductive growth period. 3） The seed yield was numerically highest in W₂ and_{numerically lowest in W3 withother treatments intermediate. Hence W2 separated statistically only from W3 and W4， while W3 separated statistically only from W1 and W2. The seed yield under deficit irrigation was not lower than that in W1 or even W2. The seed yield was higher in N2 than in N1. 4） Seed yield was positively correlated with the N and P contents of green leaves and with nutrient resorption efficiencies at the podding stage， and negatively correlated with the nutrient contents of senesced leaves. Therefore， irrigation and N fertilization significantly affected the leaf nutrient contents during the reproductive growth stage of alfalfa， and their effects varied depending the growth stage. Nutrient resorption at the podding stage increased the supply of nutrients for seed formation. In conclusion， the highest alfalfa seed yield was obtained when N fertilizer was applied at 40 kg·ha-1 under slight water deficit during the whole growth period， or under deficit irrigation at the re-greening and budding stages plus full irrigation at the podding stage.}}

Key words: Hexi Corridor, alfalfa, regulated deficit irrigation, seed production, nutrient resorption