Effects of Nitrogen and Phosphorus Limitation on the Growth and Phosphorus Allocation of Alfalfa (Medicago sativa L.)

As phosphorus (P) is one of the most limiting elements for crop growth and development, investigating the P allocation strategy of legume species with the nitrogen (N)-fixing functions is particularly important under N and P limitation. In this study, a pot experiment in a greenhouse was conducted in 2021 with three nutrient application treatments including N and P balanced situations, N limitation, and P limitation. The growth, photosynthetic properties, and P content of three organs (leaf, stem, and root P content) and four fractions within leaves (metabolic, nucleic, lipid, and residual P content) were measured for two cultivars of the legume forage crop alfalfa. In our results, plant growth and photosynthetic capacity all decreased under nutrient limitation (P < 0.05), but photosynthetic P-use efficiency was not significantly changed under P limitation. The further analysis showed that P allocation to leaves (ranged from 22% to 29%) significantly decreased but allocations to stems (ranged from 32% to 45%) or roots (ranged from 30% to 43%) increased under nutrient limitation, and the proportions of leaf metabolic P (ranged from 45% to 75%) and nucleic acid P (ranged from 15% to 20%) were increased under P and N limitations (P < 0.05), respectively. In addition, leaf acid phosphatase activity was strongly negatively correlated with leaf residual P (r = - 0.884, P < 0.01). Our results revealed that P allocation of alfalfa was changed by nutrient limitation to increase plant P-use efficiencies, and that the allocation strategy also differed between N and P limitation, in which leaf acid phosphatase played an important role in conversion among leaf P fractions.