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We have studied the mechanism of UV protection in two duckweed species (Lemnaceae) by exploiting the UV sensitivity of photosystem II as an in situ sensor for radiation stress. A UV-tolerantSpirodela punctata G.F.W. Meyer ecotype had significantly higher indole-3-acetic acid (IAA) levels than a UV-sensitive ecotype. Parallel work on Lemna gibbamutants suggested that UV tolerance is linked to IAA degradation rather than to levels of free or conjugated IAA. This linkage is consistent with a role for class III phenolic peroxidases, which have been implicated both in the degradation of IAA and the cross-linking of various UV-absorbing phenolics. Biochemical analysis revealed increased activity of a specific peroxidase isozyme in both UV-tolerant duckweed lines. The hypothesis that peroxidases play a role in UV protection was tested in a direct manner using genetically modified tobacco (Nicotiana sylvestris). It …