Net Photosynthesis, Electron Transport Capacity, and Ultrastructure of Pisum sativum L. Exposed to Ultraviolet-B Radiation

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Abstract
P. sativum L. was exposed to ultraviolet-B (UV-B) radiation (280-315 nm) in greenhouse and controlled environment chambers to examine the effect of this radiation on photosynthetic processes. Net photosynthetic rates of intact leaves were reduced by UV-B irradiation. Stable leaf diffusion resistances indicated that the impairment of photosynthesis did not involve the simple limitation of CO2 diffusion into the leaf. Dark respiration rates were increased by previous exposure to this radiation. Electron transport capacity, as indicated by methylviologen reduction, was also sensitive to UV-B irradiation. The ability of ascorbate-reduced 2,6-dichlorophenolindophenol to restore much of the electron transport capacity of the UV-B-irradiated plant material suggested that inhibition by this radiation was more closely associated with photosystem II than photosystem I. EM indicated structural damage to chloroplasts as well as other organelles. Plant tissue irradiated for only 15 min exhibited dilation of thylakoid membranes of the chloroplast in some cells. Some reduction in Hill reaction activity was evidenced in the plant materials which were irradiated for periods as short as 15 min.