© 2004 Heron Publishing—Victoria, Canada
Photosynthetic responses and N allocation in Douglas-fir needles following a brief pulse of nutrients
C. R. Warren (1, 2), N. J. Livingston (1) and D. H. Turpin (3)
1. Centre for Forest Biology, Department of Biology, University of Victoria, P.O. Box 3020 Stn CSC, Victoria, BC V8N 3N5, Canada / 2. Forest Science Centre, The University of Melbourne, Water Street, Creswick, Victoria 3363, Australia (crwarren@unimelb.edu.au) / 3. Department of Biology, University of Victoria, P.O. Box 3020 Stn CSC, Victoria, BC V8N 3N5, Canada / Received July 14, 2003; accepted November 21, 2003; published online April 1, 2004
Summary
The temporal distribution of soil nutrients is heterogeneous, and thus the uptake, storage and later remobilization of brief
nutrient pulses may be critical for growth in nutrient-limited habitats. We investigated the response of photosynthesis and
the major nitrogen (N) fractions in needles of 2-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings to a 15-day nutrient pulse (containing 250 ppm N). The nutrient pulse (N pulse) was imposed in
late July, toward the end of the seedlings’ third growing season, and subsequent changes in photosynthesis and needle N fractions
were examined over the following 3 months. Needles are sites of photosynthesis and putative storage organs. Thus we tested
two hypotheses: (1) N from the N pulse is quickly synthesized from soluble non-protein N into soluble proteins, especially
Rubisco, and (2) the N pulse increases photosynthetic rates and thus growth. We also examined an alternative hypothesis that
Rubisco functions also as a storage protein, in which case we would predict increases in amount of Rubisco in response to
the N pulse without concomitant increases in photosynthesis. Soluble non-protein N was the most dynamic N pool and may have
constituted a temporary storage reservoir; however, the quantitative significance of soluble non-protein N is questionable
because this pool was at most only 7% of total N. Concentrations of Rubisco were unaffected by the N-pulse treatment and there
was little evidence that Rubisco served as a storage protein. Nutrient-pulse seedlings added twice as much dry mass as controls
during the 3 months post-treatment (Warren et al. 2003a). Over the same period, the maximum rate of light-saturated photosynthesis (Amax) declined to low rates in control seedlings, whereas Amax increased in N-pulse seedlings. Nevertheless, treatment and temporal trends in N and Rubisco content per unit area were poorly
related to Amax, and it seems likely that photosynthesis was limited by additional factors, perhaps thylakoid proteins or an inadequate supply
of other nutrients.
Keywords:
allocation, chlorophyll, growth, nitrogen, non-protein nitrogen, nutrient pulse, partitioning, Pseudotsuga menziesii, Rubisco, soluble protein.
