NICOTINAMIDE ADENINE-DINUCLEOTIDE PHOSPHATE PHOSPHATASE FACILITATES DARK REDUCTION OF NITRATE - REGULATION BY NITRATE AND AMMONIA

Leaves of 15 - 30-d-old plants of sunflower and jute were harvested at 10.00 or 23.00 (local time) and measured immediately, or those harves ted at 10.00 were incubated for one hour in sunlight either in water o r 5 mM methionine sulfoximine (MSX) solution and then for three hours in dark either in water or 15 mM KNO3 solution. Nitrate feeding during dark incubation, in general, increased nitrate reductase (NR) and nit rite reductase (NiR) activities, and NADH and soluble sugar contents. Increase in tissue nitrate concentration in MSX fed but not in control samples suggested reduction of nitrate in dark. NADPH-dependent NR ac tivity increased considerably upon feeding with nitrate in dark. Conco mitantly, NADPH phosphatase activity was also increased in nitrate tre ated, dark incubated leaves. It is proposed that nitrate regulates dar k nitrate reduction by facilitating generation of NADH from NADPH by N ADPH phosphatase. High amounts of ammonia accumulated in MSX treated, but not in control leaves, upon dark incubation. Relative activities o f NR and NADPH phosphatase, and amounts of soluble sugar and NADH were low in MSX fed samples compared to that of control. So, high amount o f ammonia might partially repress NADPH phosphatase and consequently d eprive NR of reducing equivalents.