FAST FLUORESCENCE QUENCHING FROM ISOLATED GUARD-CELL CHLOROPLASTS OF VICIA-FABA IS INDUCED BY BLUE-LIGHT AND NOT BY RED-LIGHT

Chlorophyll a fluorescence transients from isolated Vicia faba guard cell chloroplasts were used to probe the response of these organelles to light quality. Guard cell chloroplasts were isolated from protoplasts by passing them through a 10-mum nylon net. Intact chloroplasts were purified on a Percoll gradient. Chlorophyll a fluorescence transients induced by actinic red or blue light were measured with a fluorometer equipped with a measuring beam. Actinic red light induced a monophasic quenching, and transients induced by blue light showed biphasic kinetics having a slow and a fast component. The difference between the red and blue light-induced transients could be observed over a range of fluence rates tested (200-800 mumol m-2 s-1). The threshold fluence rate of blue light for the induction of the fast component of quenching was 200 mumol m-2 s-1, but in the presence of saturating red light, fluence rates as low as 25 mumol m-2 s-1 induced the fast quenching. These results indicate that guard cell chloroplasts have a specific response to blue light.