Mobilization of nitrogen reserves during regrowth of defoliatedTrifolium repensL. and identification of potential vegetative storage proteins

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Abstract
Although it is well established that carbon reserves contribute to shoot regrowth of leguminous forage species, little information is available on nitrogen reserves except in Medicaqo sativa L. and Trifolium subterraneum L. In this study, reserves were labelled with 15N to demonstrate the mobilization of endogenous nitrogen from roots and stolons to regrowing leaves and new stolons during 24 d of regrowth in white clover (Thfolium repens L.). About 55% and 70%, respectively, of the nitrogen contents of these organs were mobilized to support the regrowth of leaves. During the first 6 d, nitrogen in regrowing leaves came mainly from N reserves of organs remaining after defoliation. After these first 6 d of regrowth, most of the shoot nitrogen was derived from exogenous nitrogen taken up while the contribution of nitrogen reserves decreased. After defoliation, the buffer-soluble protein content of roots and stolons decreased by 32% during the first 6 d of regrowth. To identify putative vegetative storage proteins, soluble proteins were separated using SDS-PAGE or two-dimensional electrophoresis. One protein of 17.3 kDa in stolons and two proteins of 15 kDa in roots seemed to behave as vegetative storage proteins. These three polypeptides, initially found at high concentrations, decreased in relative abundance to a large extent during early regrowth and then were accumulated again in roots and stolons once normal growth was re-established.

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