Adequate phosphorus (P) nutrition during early stages is critical for maize growth. Our objective was to evaluate the relative contribution of seed P reserves and exogenous P to maize nutrition during early growth stages. Seedlings were grown with labeled nutrient solution ( 32 P). Seedlings were harvested periodically over the course of the three-week study. Initially, 87% and 77% of the total C and N in seeds were located in the endosperm, whereas 86% of seed P was located in the scutellum as phytate. Up to the 7th day after sowing, 96% of phytate was hydrolyzed. Hydrolyzed forms of P were temporarily stored in the seed before being translocated to growing organs, suggesting that the hydrolysis of phytate was not a limiting step for P supply to seedlings. Significant P uptake by roots was observed from the 5th day after sowing on. Both sources of P supplied roots and leaves, with a slightly higher proportion of P from seed reserves going to leaves rather than to roots. Of total seed P, 60% and 92% was exported towards newly growing seedlings till 7th and 17th days after sowing and ceased to be a significant source of P for growth thereafter. We conclude that although both P supply processes overlap in time, seed P was the main P source during early growth stages.
The aim of this study was to determine the mechanisms underlying the uptake of Cd by durum wheat (Triticum turgidum L. ssp. durum cv. “acalou”) in the presence of citrate under hydroponic conditions. Wheat seedlings were exposed for 3 h to simplified nutrient solutions initially containing 35 nM of free Cd with or without citrate. Uptake experiments with citrate alone were also performed. Solutions were radio labelled with 109Cd or citrate-14C. The depletion of Cd and citrate in the exposure solution was followed, and at the end of the exposure Cd and citrate were quantified in all compartments of the experimental system (root surface, inner roots and aerial parts). The apparent rates of internalization of Cd (r Cd) and citrate (r cit) were estimated from the ratio of the uptake flux to the contents adsorbed at the root surface. About two times more Cd was taken up in the presence of citrate. Assuming that citrate and Cd-citrate complexes were taken up at the same rate, a direct uptake of intact complexes could not account for the higher uptake of Cd in presence of citrate. In addition, Cd was internalized at the same rate in the absence or presence of citrate (r Cd = 3.62 h−1), whereas the internalization rate constants of Cd and citrate were different (r Cd ≈ 8 r cit). This strengthens the idea that a direct uptake of non-dissociated Cd-citrate complexes probably did not account for the higher Cd uptake in presence of citrate. A dissociation of Cd-citrate complexes within the diffusion layer or at the root surface and the subsequent additional supply of free Cd were more likely to explain the higher Cd uptake in presence of citrate
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