NADH oxidase periodicity of spinach leaves synchronized by light

The plasma membrane-associated NADH oxidase (NOX) of spinach leaf disks dem onstrates a regular periodicity of 24.3 +/- 0.5 min (NADH = reduced nicotin amide adenine dinucleotide). Within a single population of plants, all exhi bited a similar period. Experiments were carried out to determine what stim ulus in the environment was responsible for this synchronization. The findi ngs demonstrate that when plants are transferred from darkness to light, li ght 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. R ed light (650 nm, 10 min, 50 mu M m(-2) s(-1)) was equivalent to white ligh t. Green or yellow light at a similar level as red light was ineffective. F ar-red illumination (730 nm, 5 min) appeared to be equivalent to darkness a nd was sufficient to condition the plants to respond to light by resetting the NOX cycle. involvement of phytochrome as a possible light-detecting mol ecule is indicated.