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Diurnal changes in adenylates and nicotinamide nucleotides in sugar beet leaves
Authors:J Madhusudana Rao  A Raviraj Arulanantham  Norman Terry
Institution:(1) Department of Plant and Soil Biology, University of California, 94720 Berkeley, CA, USA
Abstract:Sugar beets (Beta vulgaris L. cv. F58-554H1) were cultured hydroponically in growth chambers at 25°C, with a photon flux density of 500 mgrmol m-2s-1. Measurements were made of net CO2 exchange, leaf adenylates (ATP, ADP and AMP), and leaf nicotinamide nucleotides (NAD+, NADP+, NADH, NADPH), over the diurnal period (16h light/8 h dark) and during photosynthetic induction. All the measurements were carried out on recently expanded leaves from 5-week-old plants. When the lights were switched on in the growth chamber, the rate of photosynthetic CO2 uptake, and the levels of leaf ATP and NADPH increased to a maximum in 30 min and remained there throughout the light period. The increase in ATP over the first few minutes of illumination was associated with the phosphorylation of ADP to ATP and the increase in NADPH with the reduction of NADP+; subsequently, the increase in ATP was associated with an increase in total adenylates while the increase in NADPH was associated with an accumulation of NADP+ and NADPH due to the light-driven phosphorylation of NAD+ to NADP+. On return to darkness, ATP and NADPH values decreased much more slowly, requiring 2 to 4 hours to reach minimum values. From these results we suggest that (i) the total adenylate and NADPH and NADP+ (but not NAD+ and NADH) pools increase following exposure to light; (ii) the increase in pool size is not accompanied by any large change in the energy or redox states of the system; and (iii) the measured ratios of ATP/ADP and NADPH/NADP+ for intact leaves are low and constant during steady-state illumination.Abbreviations AEC adenylate energy charge - DHAP dihydroxyacetone phosphate - MTT 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide - PES phenazine ethosulfate - PEP phosphoenolpyruvate - PGA 3-phosphoglycerate - PFD photon flux density - Ru5P ribulose-5-phosphate - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase
Keywords:adenylates  diurnal changes  nicotinamide nucleotides  photosynthetic induction  sugar beet
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