Photosynthesis, Growth, and Ultraviolet Irradiance Absorbance of Cucurbita pepo L. Leaves Exposed to Ultraviolet-B Radiation (280-315 nm)

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Abstract
Net photosynthesis, growth and UV radiation absorbance were determined after for the 1st leaf of C. pepo L. exposed to 2 levels of UV-B irradiation and a UV-B radiation-free control treatment. Absorbance by extracted flavonoid pigments and other UV-B radiation-absorbing compounds from the 1st leaves increased with time and level of UV-B radiation impinging on leaf surfaces. Although absorbance of UV-B radiation by extracted pigments increased substantially, UV-B radiation attenuation apparently was insufficient to protect completely the photosynthetic apparatus or leaf growth processes. Leaf expansion was repressed by daily exposure to 1365 J/m per day of biologically effective UV-B radiation but not by exposure to 660 J/m per day. Photosynthesis measured through ontogenesis of the 1st leaf was depressed by both UV-B radiation treatments. Repression of photosynthesis by UV-B radiation was especially evident during the ontogenetic period of maximum photosynthetic activity.