Phosphate nutrition and defoliation effects on growth and root physiology of alfalfa

Phosphorus (P) deficiency reduces forage yield and stand persistence of alfalfa ( Medicago sativa L.). The objective of this study was to determine the influence of P nutrition and defoliation on alfalfa shoot growth, root carbohydrate and protein metabolism, and steady-state mRNA levels for high-affinity P transporters. In a greenhouse study, P-deprived plants were provided with 0, 0.25, 2, and 6 mM P beginning 7 d before shoot removal. Plants were sampled immediately (day -7) on days -5, -2, 0 (day of shoot removal), and on days 1, 2, 6, and 9 post-shoot removal. Addition of P to P-deficient plants stimulated growth of shoots but not roots. Taproot bark sugar concentrations were reduced significantly in cut plants at any rate of P, whereas only the 6 mM P treatment reduced taproot wood sugar concentrations in uncut plants. There was a significant defoliation-induced decline in both wood and bark sugar and amino-acid concentration that was enhanced at high P rates. Low P reduced utilization of starch and protein reserves in taproots. Transcripts for a high-affinity P transporter were not detected in any root or shoot tissue assayed, irrespective of defoliation or P treatment. The uncertain relationship between P availability and P-transporter transcript abundance in our greenhouse-grown plants requires additional investigation.