EFFECTS OF NITROGEN SUPPLY ON THE SOURCE OF NITROGEN USED FOR REGROWTH OF LAMINAE AFTER DEFOLIATION OF 4 GRASS SPECIES

Plants of Agrostis castellana, Festuca rubra, Lolium perenne and Poa t rivialis were grown in sand culture and supplied with a complete nutri ent solution containing either 0.5 or 1.5 mol m(-3) NH4NO3. Plants wer e clipped weekly to a height of 4 cm. At the time of the ninth clippin g (day 0) the nutrient solution was replaced by one in which all nitro gen (N) was enriched with N-15 to 5 atom %, but otherwise identical. W eekly destructive harvests were taken from day 0 to day 35, whilst unh arvested plants continued to receive the weekly clipping. The N-15 lab elling allowed discrimination between remobilization within the plant from current root uptake in supplying N for laminae regrowth. For all four species a poor N supply reduced both N uptake by roots and the po tential N available for remobilization, as well as allocation of both these sources to laminae regrowth. Overall the N supply derived from r oot uptake was reduced by a greater extent, and the proportion of N in laminae regrowth derived from remobilization increased with low N sup ply. The rate of remobilization of the N which was accessible was slow er with a reduced supply of N. The species order for increasing propor tion of N for laminae regrowth being derived from remobilization was L . perenne < P. trivialis < A. castellana < F. rubra irrespective of N supply.