Phosphate Nutrition and Defoliation Effects on Growth and Root Physiology of Alfalfa

ABSTRACT

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.

Keywords:

• phosphate nutrition
• Medicago sativa L.
• phosphate transporter
• defoliation
• starch
• sugar
• protein
• amino acids