Effects of phosphorus nutrition on carbohydrate and protein metabolism in alfalfa roots

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Abstract
Alfalfa (Medicago sativa L.) root reserves are thought to provide nutrients to regrowing shoots, enhance stress tolerance, and improve plant persistence. Factors affecting carbohydrate and protein accumulation and metabolism in roots are important in alfalfa production. Our objectives were to determine 1) the influence of phosphorus (P) nutrition on alfalfa shoot growth and root carbohydrate and protein metabolism after defoliation and 2) how quickly growth and root carbohydrate and protein metabolism of P‐deficient alfalfa plants responds to supplemental P. In Experiment 1, nodulated alfalfa was grown in quartz sand with minus‐nitrogen (N) Hoagland's solution containing 0,1,2, or 6 mM P. Root P concentrations increased with increasing solution P levels. Phytate P in roots of plants grown with 6 mM P was greater than that of plants grown in 0, 1, or 2 mM P. Shoot mass and shoots per plant were reduced by 67 and 43%, respectively, in plants grown with 0 mM P as compared to plants grown with 6 mM P. Root starch and protein concentrations did not decline after defoliation in plants provided 0 mM P. Defoliation increased endoamylase activity in roots of plants that received P. Root total amylase activity declined as P supply increased, and was not associated with root starch degradation. In Experiment 2, P‐deficient alfalfa plants were provided with minus‐N Hoagland's solution containing 6 mM P either at defoliation (Day 0), or at Day 14, and these plants were compared to others provided 0 or 6 mM P throughout the study. Root P concentrations increased within 7 days of P application. Shoot regrowth of P‐deficient plants recovered to equal that of plants provided 6 mM P within 7 to 21 days after P application. Application of P at defoliation enhanced root starch use, whereas P application on Day 14 had no effect on root starch depletion. Normal root starch utilization occurred in plants provided 6 mM P when plants were defoliated a second time 35 days after the initial harvest. Normal root protein utilization was not restored by P application during regrowth. Roots of P‐deficient alfalfa contained high concentrations of organic reserves, but use of these reserves after defoliation was impaired.