PHOTOSYNTHESIS AND HIGH LIGHT STRESS

Summary. Exposure of plants to irradiances far above the light saturation point of photosynthesis, known as high-light stress, induces various responses including light adaptation of the photosynthetic apparatus and chloroplast ultrastructure by formation of sun-type chloroplasts. The latter possess a lower cross section for light absorption (less light-harvesting chlorophyll proteins) and higher rates of photosynthetic quantum conversion than shade-type chloroplasts. De-epoxidation of violaxanthin to zeaxanthin, increase in heat emission, rise of non-photochemical de-excitation of absorbed light quanta (qN) and photoinhibition of the photosynthetic pigment apparatus are further high-light stress responses. The degree of photoinhibition can clearly be determined via measurements of the chlorophyll fluorescence relaxation kinetics and depends on the photon flux density of the high-light stress as is shown here with soybean leaves. A decrease in photochemical quenching qP and variable chlorophyll fluorescence ratios F′ v /F′ m or ΔF/F′ m , and an increase of non-photochemical quenching qN are not yet an indication of a photoinhibition. One always has to determine the photoinhibitory quenching coefficient qI which is one of three qN components. However, in soybean leaves a photoinhibition (increase in qI) of the chloroplasts, as measured at the adaxial light exposed upper leaf side, did not affect the photosynthetic CO 2 assimilation rates (P N ) of the whole leaf. Thus, chlorophyll fluorescence measurements, usually carried out with low irradiance light, taken at the upper leaf-side alone are not representative for the physiological situation of the whole leaf. Chlorophyll fluorescence signatures have to be determined as well from the abaxial lower leaf side in order to correctly judge the physiological state of the leaf.