Photoinhibition of photosynthesis: effect of O2 and selective excitation of the photosystems in intact Lemna gibba plants.

Intact Lemna gibba plants were photoinhibited under anaerobic conditions on illumination with monochromatic light which selectively excited the photosystems. Photoinhibition was less when PS 1 was excited and greatest when mainly PS 2 was excited, which suggests that PS 2 was most damaged by photoinhibition induced in complete absence of O2 and CO2.The illumination of plants with monochromatic light exciting PS 1, at different O2 concentrations (in CO2 deficient conditions), showed that PS 1 photoinhibition was increased at the low O2 concentrations. The damage to PS 1 was more evident at 2% O2 than at the higher O2 concentrations.CO2 as well as O2 at atmospheric concentration, (air), was necessary for complete protection of the plant from photoinhibition when both photosystems were excited either separately or together.