Abstract:

This research explored the effect of different levels of phosphorus application on diurnal variation in photosynthesis and partitioning of photosynthetic products in alfalfa under drip irrigation. In particular， the relationship between photosynthetic product levels in the plant and leaf photosynthetic parameters of alfalfa was clarified. Four phosphorus （P₂O₅） application levels of 0 （CK）， 50 （P₁）， 100 （P₂） and 150 kg·ha^-1 （P₃） were used as treatments in a field experiment. Leaf photosynthetic parameters were measured with a Li-6400 portable photosynthesis instrument on sunny days from 10：00-20：00 during the early flowering stage， together with environment factors and sugar and starch content in the leaves， stems and roots. A feature of the results was a ‘photosynthetic noon-break’ phenomenon across all phosphorus application treatments， and stomatal limitation was the main factor responsible for the mid-day decline of net photosynthetic rate （P_n）. Photosynthetically active radiation had the greatest impact on the P_n of alfalfa leaves， while atmospheric CO₂ concentration， atmospheric temperature， and atmospheric environmental factors such as relative humidity and vapor pressure deficit based on leaf temp also affected P_n. Principal component analysis showed that alfalfa photosynthetic efficiency in the four P-treatments ranked P₂>P₁>P₃>CK. At the 100 kg·ha^-1 phosphorus level （compared to CK）， the hay yield of alfalfa was increased significantly， the soluble sugar contents of leaves， stems， and roots were increased by 11.6%， 5.0% and 4.6%， respectively， and the starch contents were increased by 15.2%， 9.6% and 5.3%， respectively. Higher overall levels of soluble sugar and starch typically manifested as greater concentration increases in leaves， and relatively smaller increases in roots and stems. The diurnal troughs in P_n， transpiration rate， leaf internal CO₂ concentration and water use efficiency （WUE） of alfalfa leaves were less marked in the P treatments， compared with the control treatment. Therefore， appropriate phosphorus application can increase the photosynthetic efficiency of alfalfa leaves， thereby significantly promoting crop growth and development. At the phosphorus （P₂O₅） application rate of 100 kg·ha^-1 （P₂）， the alfalfa photosynthetic enhancement effect was the most pronounced.

Key words: phosphorus application, alfalfa, diurnal photosynthesis variation, photosynthetic products