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Environmental and Stress Physiology

Effect of Photosynthetic Photon Flux Density on Carboxylation Efficiency ¹

Biological Station and Department of Biology, University of Michigan, Ann Arbor, Michigan 48109-1057, Systems Ecology Research Group, San Diego State University, San Diego, California 92182, Lehrstuhl für Botanik II, der Universität Würzburg, Mittlerer Dallenbergweg 64, D-8700 Würzburg, Federal Republic of Germany

The effect of photosynthetic photon flux density (PPFD) on photosynthetic response (A) to CO₂ partial pressures between 35 pascals and CO₂ compensation point () was investigated, especially below PPFD saturation. Spinacia oleracea cv `Atlanta,' Glycine max cv `Clark,' and Arbutus unedo were studied in detail. The initial slope of the photosynthetic response to CO₂ (A/C[]) was constant above a PPFD of about 500 to 600 micromoles per square meter per second for all three species; but declined rapidly with PPFD below this critical level. For there was also a critical PPFD (approximately 200 micromoles per square meter per second for S. oleracea and G. max; 100 for A. unedo) above which was essentially constant, but below which increased with decreasing PPFD. All three species showed a dependence of A/C() on PPFD at low PPFD. Simulated photosynthetic responses obtained with a biochemically based model of whole-leaf photosynthesis were similar to measured responses.