The productivity of alfalfa is associated with a large amount of nitrogen (N) and phosphorus (P); the addition of exogenous N and P fertilizers can fully exploit the growth potential of alfalfa. However, there is uncertainty about the relationship between changes in alfalfa productivity and photosynthetic physiology and anatomy. We conducted field fertilization experiments on alfalfa in the second and third years under drip irrigation, as well as measurement of the photosynthetic physiology, anatomical structure and agronomic traits of alfalfa at different levels of N (0, 120 kg.ha(-1)) and different levels of P2O5 (0, 50, 100 and 150 kg.ha(-1)). The results showed that the dry matter yield (DMY), crude protein (CP), net photosynthetic rate (P-n) and specific leaf weight (SLW) were increased by 2.10 similar to 11.82%, 4.95 similar to 11.93%, 4.71-7.59% and 2.02 similar to 7.12% in the N application treatment compared with the non-N application treatment, while the DMY, CP, P-n and SLW were increased by 3.19-17.46%, 1.99 similar to 8.42%, 6.15 similar to 24.95% and 2.16 similar to 11.90% in the P application treatment compared with the non-P application treatment. N and P increase the thickness of the spongy tissue (ST) of alfalfa, which will facilitate the entry and exit of gas and water, and will further affect the photosynthetic indexes, such as stomatal conductance (G(s)) and transpiration rate (T-r), of alfalfa leaves. Increased palisade tissue (PT) thickness will also enhance the adaptability of plant leaves to strong sunlight, thereby increasing the maximum net photosynthetic rate (P-max) and light saturation point (LSP). Fertilization treatment showed the highest utilization efficiency for low light and better adaptation to strong light, but the Rd decreased. The comprehensive scores of principal component analysis for anatomical structure, photosynthetic performance and agronomic traits were N1P2 > N0P2 > N1P3 > N1P1 > N0P3 > N0P1 > N1P0 > N0P0. Therefore, the application of N and P fertilizers contributed to the adaptive changes in alfalfa leaf anatomy and the improvement of photosynthetic capacity, which were beneficial to the improvement of alfalfa dry matter yield, growth traits and nutritional quality, with the most obvious improvement effect obtained with the application of 120 kg.ha(-1) of N and 100 kg.ha(-1) of P2O5.
