The Effects of Nitrogen Supply and Defoliation on the Seasonal Internal Cycling of Nitrogen inMolinia caerulea

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Abstract
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 15N 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 15N 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.

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