UV-B radiation constrains the photosynthesis of Quercus robur through impacts on the abundance of Microsphaera alphitoides

Quercus robur saplings were exposed at an outdoor facility in the UK to sup plemental levels of UV-B radiation (280-315 nm) under arrays of cellulose d iacetate-filtered fluorescent lamps which also produced UV-A radiation(315- 400 nm). Saplings were also exposed to supplemental UV-A radiation under ar rays of polyester-filtered lamps and to ambient levels of solar radiation u nder arrays of unenergized lamps. The UV-B treatment was modulated to maint ain a 30% elevation above the ambient level of erythemally weighted UV-B ra diation. Naturally occurring infections by oak powdery mildew (Microsphaera alphitoides) were more abundant, and developed more rapidly, on lammas lea ves of saplings which were exposed to treatment levels of UV-B radiation th an on leaves of saplings exposed to supplemental UV-A or to ambient levels of solar radiation over 12 weeks in summer and autumn 1996. An analysis of leaf photosynthetic reduced the quantum efficiency of photosystem capacitie s revealed that M. alphitoides infection reduced the quantum efficiency of photosynthetic (PS) II by 14% at moderate irradiance. Although there was no direct effect of UV-B radiation on PSII photochemistry, exposure of saplin gs to supple mental UV-A radiation under polyester-filtered lamps resulted in a 17.5% decrease in PSII quantum efficiency, compared with saplings expo sed to ambient solar radiation. The results from our study suggest th at ph otosynthesis of Q. robur may be constrained by exposure to UV-B radiation i n the natural environment through impacts on the abundance of M. alphitoide s.