INFLUENCE OF PHOTON FLUX DENSITY IN THE 400–700 nm WAVEBAND ON INHIBITION OF PHOTOSYNTHESIS BY UV‐B (280–320 nm) IRRADIATION IN SOYBEAN LEAVES: SEPARATION OF INDIRECT AND IMMEDIATE EFFECTS

Abstract

Abstract— Visible radiation can substantially influence the degree to which plant photosynthesis is inhibited by UV‐B radiation. This study was designed to separate the immediate effects of visible radiation on UV‐B photosynthetic inhibition from the indirect influence of visible irradiation on morphological and physiological properties of leaves during leaf development. Soybean plants were pretreated in growth chambers with either high or low visible irradiance (750 and 70 μmol m ‐2 s ‐1 quantum flux in the 400–700 nm waveband, respectively) during the development of leaves used subsequently for UV irradiation. Test leaves still attached to the plant were exposed to 5 h of polychromatic UV‐B irradiation and the photosynthetic capacity (net CO 2 exchange) was determined before and after the UV irradiation. During the UV irradiation, plants from both pretreatment groups received either high or low visible flux. Development of leaves in the high visible flux pretreatment conditions resulted in thicker leaves, higher chlorophyll a/b ratios, more UV‐absorbing pigments, and reduced sensitivity to the UV‐B irradiation. However, higher visible flux during the UV‐B irradiation resulted in greater depression of photosynthesis by the UV‐B irradiation. The relative magnitude of photosynthetic depression under these treatment combinations was the same when photosynthesis was measured under either light‐limited or light‐saturated conditions.