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 Poa trivialis were grown in a sand-limestone mixture and fed with a complete nutrient solution containing 3 mol m-3 NH4NO3. The miniswards were cut weekly to a height of 4 cm until a steady state of growth, based 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 identical. This allowed discrimination of remobilization from current root uptake when considering the supply of N for regrowth of laminae. Destructive harvests were made over the following 28-d period, while unharvested 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 modelled as a proportion of the amount present on day 0. A simple exponential 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 asymptotes were found, with F. rubra having a significantly larger asymptote than A. castellana and P. trivialis, which in turn both had significantly larger asymptotes than L. perenne. All species used both root uptake and remobilization to supply N for post-defoliation regrowth of laminae. However, the relative contribution of each source was species-dependent. The proportion of N in the clipped material derived from remobilization was significantly greater for F. rubra than A. castellana, which in turn had a significantly greater proportion derived from remobilization than P. trivialis and L. perenne.