Origin of Foliar Nitrogen and Changes in Free Amino-Acid Composition and Content of Leaves, Stubble, and Roots of Perennial Ryegrass During Re-growth after Defoliation

Nitrogen re-mobilization and changes in free amino acids were studied as a function of time in leaves, stubble, and roots during ryegrass (Lolium perenne L.) re-growth. Experiments with 15N labelling clearly showed that during the first days nearly all the nitrogen in new leaves came from organic nitrogen re-mobilized from roots and stubble. On the days of defoliation, stubble had the highest content of free amino acids with 23 mg per g dry weight against 15 mg and 14 mg in leaves and roots, respectively. The major amino acids in leaves were asparagine (23% of total content in free amino acids), aminobutyrate, serine, glutamine, and glutamate (between 7% and 15%) whereas in roots and stubble the contribution of amides was high, especially asparagine (about 50%). Re-growth after cutting was associated with a rapid increase of the free amino acid content in leaves, with a progressive decrease in roots while stubble content remained virtually unchanged. In leaves, asparagine increased from the first day of re-growth, while the aspartate level remained unchanged and glutamine increased strongly on the first day but decreased steadily during the next few days of re-growth. Asparagine in stubble and roots changed in opposite directions: in stubble it tended to increase whereas in roots it clearly decreased. In contrast, stubble and roots showed a similar decrease in glutamine. In these two plant parts, as in leaves, aspartate remained at a low level. Results concerning free amino acids are discussed with reference to nitrogen re-mobilization from source organs (stubble and roots) to the sink organ (regrowing leaves).