Initial ATP Formation, NADP Reduction, CO2 Fixation, and Chloroplast Flattening Upon Illuminating Pea Leaves

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Abstract
Chloroplasts in living cells of detached and sectioned leaves of Pisum sativum had a thickness of 2.68 +/- 0.04 mu in the dark as determined from photographs made using a phase contrast microscope. Upon illumination with 4000 lux for 10 min, the chloroplasts flattened to 2.15 +/- 0.04 mu. There was a short lag period of about 11 sec at 1000 lux and 2 sec at 4000 lux before appreciable light-induced flattening occurred. Both ATP and reduced nicotinamide adenine dinucleotide phosphate (NADPH) in detached pea leaves increased upon illumination and then fell during the initial 60 sec. The maximum ATP level was attained in 16 sec at 1000 lux and 10 sec at 4000 lux, while NADPH required about twice as long to reach a maximum. A sustained rate of carbon dioxide fixation occurred after a lag period coinciding in time with the drop in the NADPH level. ATP appeared to be involved not only with carbon dioxide fixation, but also with some reaction beginning sooner, perhaps the light-induced chloroplast flattening. Considering the initial photophosphorylation and the sustained CO(2) fixation rates, the ATP formation rate in vivo apparently increased after the leaves had been in the light for a few min.