MOBILIZATION OF NITROGEN IN SWARDS OF TRIFOLIUM-SUBTERRANEUM L DURING REGROWTH AFTER DEFOLIATION

The net mobilization of nitrogen compounds (total N, soluble protein, amino N, membrane-bound protein) from different plant parts after defoliation, and the extent to which mobilized nitrogen contributed to new leaf growth, were studied in small swards of Trifolium subterraneum L. cv. seaton Park (subterranean clover) either completely dependent on nitrogen fixation (-N treatment) or receiving abundant nitrate at defoliation (+N treatment). Net mobilization of nitrogen from nodulated roots (75 % of mobilized nitrogen) and branches (25 % of mobilized nitrogen) accounted for most of the nitrogen in new leaf growth for up to 5 d in -N swards and for at least 2 d in +N swards. In -N swards, nodules degenerating as a result of defoliation were a major source of mobilized nitrogen for 2 d (50 % of mobilized nitrogen), after which the fine roots became the largest net contributor of mobilized nitrogen. Overall, a net 17 % of the nitrogen present in residual organs was mobilized. Soluble protein was the major form of mobilized nitrogen for which assays were performed, accounting for 50 % of the decline in total N in the -N treatment. The relative loss of soluble protein from sward components declined in the order, nodules (47 % over 5 d), fine roots (44 % over 9 d), branches (38 % over 9 d) and taproots (37 % over 5 d). The amino N pool increased sharply in size for two days after defoliation, possibly indicating breakdown of soluble protein in excess of the nitrogen demand for regrowth or to provide a respiratory substrate. It was concluded that, in plants of adequate nitrogen status, leaf growth was unlikely to be nitrogen-limited during the first few days after defoliation.