Light-stimulated Absorption of Nitrate by Wolffia arrhiza

The mechanism of the light-stimulated absorption of nitrate by Wolffia arrhiza was studied. The nitrate-absorption mechanism in ammonium-grown plants is stimulated by the presence of nitrate. In a manner similar to the absorption of many other ions, the absorption of nitrate follows a typical biphasic pattern in relation to external nitrate concentration. Mechanism 1 is effective at nitrate concentrations up to 0.5 to 0.75 mM and mechanism 2 becomes operative at higher nitrate levels. Light stimulates the absorption of nitrate independently of the effect of light on the reduction of nitrate. The effects of uncouplers, inhibitors, and light of wavelengths of 700 nm or longer indicate that nitrate absorption by Wolffia cells is reduced when non-cyclic electron transport is blocked. It is postulated that under this condition, ATP in the chloroplast (produced by cyclic photophosphorylation) may be less readily transported across the chloroplast envelope than when non-cyclic electron transport is proceeding.