Seasonal Changes in Nonstructural Carbohydrates, Protein, and Macronutrients in Roots of Alfalfa, Red Clover, Sweetclover, and Birdsfoot Trefoil

Abstract

Root total nonstructural carbohydrate (TNC) has been studied, but little is known about seasonal trends in proteins and other macronutrients in legume roots. Our objectives were to determine how winter hardening and resumption of growth in spring influence (i) concentrations of sugars and starch, and activities of amylases; and (ii) protein concentration and composition, and abundance of N, P, and K. Alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), birdsfoot trefoil (Lotus corniculatus L.), and biennial sweetclover (Melilotus officinalis L.) were seeded in rows in the field in May and roots were sampled at approximately monthly intervals in autumn and biweekly in spring. All species accumulated high root TNC and starch concentrations in autumn. Starch and TNC concentrations declined throughout winter, but increased in roots of alfalfa and red clover in May. Starch accumulation and degradation patterns in spring were correlated with root endoamylase activity. Root sugar concentrations increased threefold from October to November. Root N and soluble protein concentrations increased 50% between September and December, and declined between March and May. Specific proteins accumulated to high concentrations in alfalfa roots during autumn. Root P increased 50% between October and April and was associated with an increase in root phytate. Proteins, N, P, and phytate accumulated during winter hardening in autumn in roots of these forage legume species that, in conjuction with TNC, may supply nutrients to regrowing shoots in spring.