| Nutrient cycling and recycling is important for maintaining plant growth and enhancing nutrient use efficiency. In the present study, two winter wheat cultivars (Triticum aestivum L.), CA9325 and JM2, were selected to investigate their growth, P uptake and recycling in xylem and phloem, translocation of P within plant to grains, and their relationships with sink strength as affected by P withdrawal at different developmental stages. The differences of P use ability by grains between two cultivars were compared. The plants were grown either in nutrient solution in lab under the controlled condition or in quartz sand in greenhouse. The main results were as followed:1. The appropriate P supply for wheat plant growth under the present condition was from 0.25 to 1.0 mmol L-1. Kinetics and mathematics simulation of H2PO4 uptake were studied using seedlings of the two cultivars grown in nutrient solution with different P levels for 23 days and depleted 48 h. The results showed that under P-deficient condition, CA9325 had greater P uptake rate, higher affinity to P and stronger endurance to low-P. The process of H2PO4 uptake by roots of wheat seedlings could be simulated using the transfer function model.2. After vernalization of both cultivars, all tillers were removed. P was withdrawn during the flag leaf expansion. After the treatment, the study period left was divided into two phases of one month each. In the first study period, net dry weight increments of whole plant in both cultivars were not influenced by P deficiency, while dry weight and P content accumulated into spike were accelerated. P transported in the xylem of the control plants came mainly from the roots' current uptake in both cultivars; while in the second study period, phloem retranslocation of P was increased, which accounted for 86% of P transported in xylem. In the P-deficient plants, however, almost all of the P deposition in grains was remobilized and exported from vegetative organs. These P were recycled to roots through phloem and then transported to spike via xylem, not directely transported to grains via phloem. The results suggested that dry matter and P allocation into grains were not synchronous, indicating independent regulatory processes. Although the finial dry weight of whole plant in both cultivars was markedly reduced by withdrawing P from the medium, the final P content of grains was not influenced.3. The studies were also carried out in greenhouse using both cultivars without moving tillers. P omitting from growth medium was performed either in jointing or in flagging stage. The results showed that in P-sufficient plants, total amount of P taken up by CA9325 was equal to JM2. However, CA9325 took up less P than JM2 between sowing and flagging stage, but more P than JM2 from flagging stage to grain filling stage. With the prolonged plant growth, the increase of P contents in main shoot, valid tillers and whole plants grown under P-sufficient conditions showed gradually increase and the peak of the P contents appeared at the grain filling stage. In the P-deficient plants of both cultivars with P omitting from medium either in jointing or flagging stage, P transferred into main shoot were not onlyfrom invalid tillers and roots, but also from valid tillers at the later growth stage. P contents in grains of main shoot in the P-sufficient CA9325 were higher than those in valid tillers; by contrast, that in the P-sufficient JM2 was reversed, since there were more tillers in JM2 than in CA9325. P contents in all vegetative organs and glumes of both cultivars with P omitting in jointing or flagging stage were lower than those in the P-sufficient plants, and gradually decreased with the prolonged plant growth.P contents of grains in the P-deficient plants were lower than those in the P-sufficient plants, but the percentage of P in grains to main shoot and valid tillers was higher in the P-deficient plant. At mature stage, P deficiency decreased the grain yields of both cultivars, especially in the plant with P withdrawi...
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