Photoinduced changes in the chlorophyll a to chlorophyll B ratio in young bean plants.

An alternating light-dark system is described under which etiolated bean (Phaseolus vulgaris) leaves form selectively chlorophyll a.This system consists of cycles of 2 minutes of white light alternating with 98-minute dark periods.Etiolated tissue exposed to a series of such light-dark cycles accumulates chlorophyll a predominantly, with no lag phase in its biosynthesis. A very high ratio of chlorophyll a:chlorophyll b is thus established. The ratio increases with the number of light exposures up to about 45 such exposures and then it declines. Transfer of the etiolated tissue to continuous illumination after a number of such light exposures results in a dramatic drop of the chlorophyll a:chlorophyll b ratio to values normally found in green plants.