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Net Photosynthesis, Electron Transport Capacity, and Ultrastructure of Pisum sativum L. Exposed to Ultraviolet-B Radiation ¹

^a Department of Range Science, Department of Plant Science and The Ecology Center, Utah State University, Logan, Utah 84322

Pisum 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 with photosystem I. Electron micrographs indicated structural damage to chloroplasts as well as other organelles. Plant tissue irradiated for only 15 minutes exhibited dilation of thylakoid membranes of the chloroplast in some cells. Some reduction in Hill reaction activity was also evidenced in these plant materials which had been irradiated for periods as short as 15 minutes.

³ Present address: Jornada Experimental Range, New Mexico State University, P.O. Box 698, Las Cruces, New Mexico 88001.

¹ Research supported in part by the Utah Agriculture Experiment Station and by the United States Department of Transportation.

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