Nitrogen allocation and the fate of absorbed light in 21-year-old Pinus radiata
Sabine Posch (1, 2, 3), Charles R. Warren (4), Jörg Kruse (1), Helmut Guttenberger (2) and Mark A. Adams (5)
1. School of Forest and Ecosystem Science, University of Melbourne, Water Street, Creswick VIC 3363, Australia / 2. Department for Plant Sciences, Karl-Franzens-University Graz, Schubertstraße 51, 8010 Graz, Austria / 3. Corresponding author () / 4. School of Biological Sciences, University of Sydney, Heydon-Laurence Building A08, Sydney NSW 2006, Australia / 5. School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney NSW 2052, Australia / Received December 18, 2006; accepted September 16, 2007; published online January 2, 2008
Summary
We investigated effects of nitrogen (N) fertilizer and canopy position on the allocation of N to Rubisco and chlorophyll as
well as the distribution of absorbed light among thermal energy dissipation, photochemistry, net CO2 assimilation and alternative electron sinks such as the Mehler reaction and photorespiration. The relative reduction state
of the primary quinone receptor of photosystem II (QA) was used as a surrogate for photosystem II (PSII) vulnerability to photoinactivation. Measurements were made on needles
from the lower, mid and upper canopy of 21-year-old Pinus radiata D. Don trees grown with (N+) and without (N0) added N fertilizer. Rubisco was 45 to 60% higher in needles of N+ trees than
in needles of N0 trees at all canopy positions. Chlorophyll was ~80% higher in lower- and mid-canopy needles of N+ trees than
of N0 trees, but only ~20% higher in upper-canopy needles. Physiological differences between N+ and N0 trees were found only
in the lower- and mid- canopy positions. Needles of N+ trees dissipated up to 30% less light energy as heat than needles of
N0 trees and had correspondingly more reduced QA. Net CO2 assimilation and the proportions of electrons used by alternative electron sinks such as the Mehler reaction and photorespiration
were unaffected by N treatment regardless of canopy position. We conclude that the application of N fertilizer mainly affected
the biochemistry and light-use physiology in lower- and mid-canopy needles by increasing the amount of chlorophyll and hence
the amount of light harvested. This, however, did not improve photochemistry or safe dissipation, but increased PSII vulnerability
to photoinactivation, an effect with likely significant consequences during sunflecks or sudden gap formation.
Keywords:
canopy, chlorophyll, N fertilizer, photoprotection, photosynthesis, Rubisco.
