The effect of natural shade on chloroplast thylakoid membrane activity and composition was examined for soybean (Glycine Max. cv. Young) grown under field conditions. Plots with high (10 plants m -1 row) or low (1 plant m -l row) plant density were established. Expanding leaves were tagged at 50, 58 and 65 days after planting (DAP). At 92 DAP, tagged leaves were used as reference points to characterize canopy light environments and isolate thylakoid membranes. Light environments ranged from a photosynthetic photon flux density (PPFD) of 87% of full sun to a PPFD of 10% of full sun. The decline in PPFD was accompanied by an increase in the far-red/red (735 nm/645 nm) ratio from 0.9 to approximately six. The major effects of shade on chloroplast thylakoid membranes were a reduction in chloroplast coupling factor and a shift in light-harvesting capacity from Photosystem I to Photosystem II. Photosynthetic electron transport capacity was not affected by differences in PPFD, but was 20 to 30% higher in the 1 plant m-l row treatment. The plant density effect on electron transport was associated with differences in plastocyanin concentration, suggesting that plastocyanin is a limiting factor in soybean. Shade did not have a significant effect on the concentration of Photosystem II, Cyt b6f, or Photosystem I complexes.
Abbreviations: CF1-chloroplast coupling factor; DAP-days after planting; DBMIB-2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone; DCIP-2,6-dichlorophenolindophenol; FR/R-far-red/red; PBS-10 mM sodium phosphate (pH 7.0), 150 mM NaC1; PPFD-photosynthetic photon flux density; PS I-Photosystem I; PS I I -Photosystem II; P700-reaction center of Photosystem I; Rubisco-ribulose-1,5-bisphosphate carboxylase/oxygenase; TBS-20 mM Tris-HC1 (pH 7.5), 500 mM NaCI; T r B S -2 0 mM Tris-HCl (pH 7.5), 500 mM NaCI, 0.05% (w/v) polyoxyethylenesorbitan monolaurate (Tween-20) * The US Government right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged.