NADH Oxidase Periodicity of Spinach Leaves Synchronized by Light.

The plasma membrane-associated NADH oxidase (NOX) of spinach leaf disks demonstrates a regular periodicity of 24.3+/-0.5 min (NADH = reduced nicotinamide adenine dinucleotide). Within a single population of plants, all exhibited a similar period. Experiments were carried out to determine what stimulus in the environment was responsible for this synchronization. The findings demonstrate that when plants are transferred from darkness to light, light exposure initiates a period maximum 12 min following the light exposure, independent of the point in the NOX cycle at which the light was given. Red light (650 nm, 10 min, 50 µM m-2 s-1) was equivalent to white light. Green or yellow light at a similar level as red light was ineffective. Far-red illumination (730 nm, 5 min) appeared to be equivalent to darkness and was sufficient to condition the plants to respond to light by resetting the NOX cycle. Involvement of phytochrome as a possible light-detecting molecule is indicated.