THE EFFECTS OF NITROGEN SUPPLY AND DEFOLIATION ON THE SEASONAL INTERNAL CYCLING OF NITROGEN IN MOLINIA-CAERULEA

Plants of Molinia caerulea were grown in pots for two seasons at two levels of nitrogen (N) supply and two levels of defoliation. All N supplied was enriched with N-15 in the first season and was at natural abundance in the second season. This allowed the contribution of remobilization from overwintering stores to be discriminated from current root uptake in supplying N for new shoot growth in the second season. The effects of N supply and defoliation upon the internal cycling of N in M. caerulea were quantified. N was remobilized from both roots and basal internodes to support new shoot, especially leaf, growth in spring. Roots supplied more N than basal internodes. Since the remobilization mainly occurred before the onset of root N uptake, internal cycling was important for the earliest period of shoot growth. An increased N supply increased the amount of N remobilized to new shoot growth, however, the proportion of N remobilized from overwintering stores was independent of N supply. Defoliation increased the amount of N remobilized from the roots, and had no effect on the N-15 content of basal internodes of plants receiving a low supply of N. Remobilization of N from leaves of undefoliated plants occurred later in the season. Remobilization from leaves supplied flowers in plants receiving a low N supply and both flowers and new basal internodes in plants receiving a higher N supply.