Poster: Nutrient Biology

Abs # 407: Phosphorus and potassium effects on alfalfa (Medicago sativa L.) root physiology during post-defoliation shoot regrowth

Presenter:	Cunningham, Suzanne M., scunningham@purdue.edu
Authors	Cunningham, Suzanne M. (A)   Berg, William K (A)   Brouder, Sylvie M (A)   Joern, Brad C (A)   Johnson, Keith D (A)   Dalton, Kevin  (A)   Volenec, Jeffrey J (A)
Affiliations:	(A): Purdue University

Potassium (K) and phosphorus (P) are key nutrients that enhance defoliation tolerance and winter hardiness of alfalfa (Medicago sativa L.), but the impact of these nutrients on the physiology and biochemistry of this species remains a mystery. Our objectives were to determine how P and K nutrition impact alfalfa root biochemistry during shoot regrowth after defoliation and during cold acclimation in autumn. A factorial arrangement of two P (0 and 75 kg/ha) and two K (0 and 400 kg/ha) treatments was replicated in the field four times. Roots were sampled at defoliation in late June (Day 0), and 1, 3, 6, 10, 14, 21, and 30 days after shoot removal. Roots were sampled after defoliation in mid September and 7, 14, 21, 28, 42, 56, 70, and 80 days after shoot removal. Concentrations of sugar, starch, buffer-soluble protein, and amino-N in roots were determined. Expression of genes for P transporters, sucrose synthase, and vegetative storage proteins (VSPs), and polypeptide profiles using SDS-PAGE were analyzed. Enhanced shoot growth rates after defoliation resulted in greater forage yield in response to P and K application. Addition of P leads to greater utilization of starch reserves following defoliation. Roots supplied K contained decreased concentrations of sugar throughout regrowth. Defoliation in June resulted in a drastic decline in expression of all genes within 6 days, and expression levels of b-amylase, sucrose synthase, and the 32 kD VSP were below our detection limit on Day 10. Transcripts for b-amylase and sucrose synthase accumulated more rapidly in roots of plants supplied either P or K, and approached pre-defoliation levels in roots sampled on Day 21. Rapid recovery of expression of the 32 kD VSP was only enhanced when both P and K were supplied.