STUDIES ON THE LIGHT MODULATION OF NITRATE REDUCTASE AND SUCROSE PHOSPHATE SYNTHASE ACTIVITIES IN WHEAT AND MAIZE LEAVES

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Date
1998-12-30
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MAHATMA PHULE KRISHI VIDYAPEETH, RAHURI- 413 722, DIST.- AHMEDNAGAR, MAHARASHTRA, INDIA.
Abstract
Nitrate reductasdc*EC 1.6.6.1) activity in the leaves of various crop plants declined slowly but gradually when the ligruVgrown seedlings were shifted to darkness. A minor temporary increase in the in vivo NR activity at the time of sudden darkening could probably be attributed to elimination of photosystem-I dependent nitrite reduction. The in vivo NR activity even on infiltration of the leaves with Mg2+ ions decreased slowly. The activity of the extracted enzyme also did not show any drastic reduction in presence of Mg2' ions upto 30 min of darkness and on transfer to light again a slow and gradual increase in the in vitro NR activity was noticed. The nitrate (NO.-T) content in the leaves of wheat and maize plants increased gradually on darkening, where as the nitrite (N02~) content decreased. Comparison of the rates of disappearance of N02" under light and dark aerobic conditions revealed that the reduction of accumulated nitrite in the dark was slow and commenced only after a time lag of 15 min. Infiltration and incubation of leaves with 2,4*DNP, an uncoupler of oxidative phosphorylation5showed a oncentration IV X dependent increase in N02" content under dark aerobic condition probably due to inhibition of nitrite reductase which requires ATP or suggesting activation (dephosphorylation) of NR itself. Under dark aerobic conditions nitrite is not expected to accumulate, however, exposure of leaf segments to CO:C>2 ratio of 10 caused significant nitrite accumulation even under dark aerobic onditions and prior infiltration of the leaves with SHAM further enhanced nitrite accumulation suggested that the competition for reducing equivalents from NADH between NR and O2 is a factor governing the regulation. Stimulation of nitrite accumulation in the leaves by EDTA under dark aerobic incubation in presence ofj(pNP and reversal of stimulatory effect by excess Ca2+ support the nvolvement of exogenous NADH-dehydrogenase located on the outer surface of the inner mitochondrial membrane in regulating the supply of reductants. The effect of light /dark transition on sucrose-phosphate synthase (SPS) activity was determined. The activity was drastically reduced both under non-selective (Vmax) and selective (V|jm) assay conditions on transfer to darkness. Comparatively more sensitivity of SPS under selective assay condition indicatesthat the enzyme activity is modulated by covalent modification (phosphorylation) during light / dark transition.
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