Nitrogen Mobilization from Stubble and Roots during Re-growth of Defoliated Perennial Ryegrass

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Abstract
Nitrogen re-mobilization during ryegrass (Lolium perenne L.) re-growth was studied as a function of time. Roots and stubble compounds were labelled with 15N before clipping. Experimental results clearly showed that ryegrass re-growth involves two periods of differing physiological significance. The first occurs during the first 6 d when nearly all the nitrogen of the new leaves comes from organic nitrogen re-mobilized from roots and stubble. At the same time, the uptake of nitrogen from the medium is very low, no reduction of endogenous nitrate occurs and proteolysis and amino amido nitrogen re-mobilization in stubble are not fully compensated for by synthesis.Thus, the change in protein-N content, expressed per plant, in stubble decreases from 998 μg (uncut plant) to 866 μg and 661 μg for 2 d and 4 d respectively after cutting. The second period (after the sixth day) may be more representative of stable behaviour of ryegrass, with assimilation of mineral nitrogen from the medium providing the predominant source of nitrogen for re-growth.