Abstract
Three cucumber cultivars (cv. Suyo, Shimoshirazu-jibaikyuuri and Tsukemidori) were grown for 3 weeks in sun-lit environmental glass chambers under UV-B (290-320nm) irradiation from six 20W fluorescent sunlamps using 0.10mm polyvinyl chloride sheeting and UV-B free control conditions using 0.10mm polyester film. Plants were irradiated with three levels (0, 244 and 381mW m-2) of biologically effective UV-B (UV-BBE) for 8h (8 a.m. to 4 p.m.) a day. The levels of UV-BBE irradiance at Tsukuba (36°N latitude) were about 250mW m-2 at noon in summer under clear sky conditions. Visible foliar injury (yellow lesions and glazing) due to UV-B irradiation occurred on the leaf surface. The leaf area and dry weight of whole plants of the three cultivars of cucumber were significantly reduced and decreased with increasing intensities of UV-BBE irradiation. Growth parameters of the relative growth rate (RGR), the net assimilation rate (NAR) and leaf area ratio (LAR) were reduced by medium and high UV-BBE irradiation throughout irradiation periods. Specific leaf area (SLA) decreased remarkably due to UV-B, which indicates that plants exposed to UV-B increase their leaf thickness. This increase of leaf thickness might be an adaptation mechanism against increased UV-B radiation.
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