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CROP PHYSIOLOGY & METABOLISM

Influence of Phosphorus and Potassium on Alfalfa Yield, Taproot C and N Pools, and Transcript Levels of Key Genes after Defoliation

Dep. of Agronomy, Purdue Univ., 915 West State St., West Lafayette, IN 47907-2054

^* Corresponding author (jvolenec{at}purdue.edu).

Fertilization with K and P impacts alfalfa (Medicago sativa L.) yield, but how these nutrients influence taproot reserves and gene expression is unknown. Our objectives were to determine how P and K impact (i) alfalfa yield and yield components, (ii) accumulation and use of taproot carbon (C) and nitrogen (N) pools, and (iii) transcript levels for β-amylase, sucrose synthase, and the high molecular weight vegetative storage protein in alfalfa taproots. Yield and yield components were determined at 30-d intervals beginning in late May. Roots were sampled after the late June harvest (Day 0) and 1, 3, 6, 10, 14, 21, and 30 d thereafter. Addition of P and K increased forage yield by enhancing mass per shoot. High P resulted in rapid starch use, while taproots of plants fertilized with K had low sugar concentrations. Transcripts decline by Day 6 and by Day 10 were below detection limits. Transcripts for β-amylase and sucrose synthase accumulated rapidly after Day 10 in taproots of plants fertilized with both P and K. Balanced P and K nutrition resulted in the accumulation and effective utilization of C and N reserves and in improved alfalfa adaptation to defoliation.

Abbreviations: SDS-PAGE, sodium dodecyl sulfate–polyacrylamide gel electrophoresis • SDW, structural dry weight • VSP, vegetative storage protein