THE RELATIVE CONTRIBUTION OF REMOBILIZATION AND ROOT UPTAKE IN SUPPLYING NITROGEN AFTER DEFOLIATION FOR REGROWTH OF LAMINAE IN 4 GRASS SPECIES

Miniswards of Lolium perenne, Festuca rubra, Agrostis castellana and P oa trivialis were grown in a sand-limestone mixture and fed with a com plete nutrient solution containing 3 mol m-3 NH4NO3. The miniswards we re cut weekly to a height of 4 cm until a steady state of growth, base d on weight of clippings removed, was achieved. After clipping (day 0) the nutrient solution was replaced by one in which all the nitrogen ( N) was enriched with N-15 to 5 atom per cent, but was otherwise identi cal. This allowed discrimination of remobilization from current root u ptake when considering the supply of N for regrowth of laminae. Destru ctive harvests were made over the following 28-d period, while unharve sted plants continued to receive a weekly clip. The proportion of the total uptake of labelled N which appeared in the clipped material was of the order L. perenne > P. trivialis > A. castellana > F. rubra. The change in unlabelled N in the roots plus leaf bases over time was mod elled as a proportion of the amount present on day 0. A simple exponen tial model with a non-zero asymptote was found to model the data; the asymptote was assumed to represent the proportion of unlabelled N not readily available for remobilization. Species differences in the asymp totes were found, with F. rubra having a significantly larger asymptot e than A. castellana and P. trivialis, which in turn both had signific antly larger asymptotes than L. perenne. All species used both root up take and remobilization to supply N for post-defoliation regrowth of l aminae. However, the relative contribution of each source was species- dependent. The proportion of N in the clipped material derived from re mobilization was significantly greater for F. rubra than A. castellana , which in turn had a significantly greater proportion derived from re mobilization than P. trivialis and L. perenne.