Leaves under stressful conditions usually show downregulated maximum quantum efficiency of photosystem II [inferred from variable to maximum chlorophyll (Chl) a fluorescence (F-v/F-m), usually lower than 0.8], indicating photoinhibition. The usual method to evaluate the degree of photoinhibition in winter red leaves is generally by measuring the F-v/F-m on the red adaxial surface. Two phenotypes of overwintering Buxus microphylla 'Wintergreen' red leaves, with different measuring site and leaf thickness, were investigated in order to elucidate how red pigments in the outer leaf layer affected the Chl a fluorescence (F-v/F-m) and photochemical reflectance index. Our results showed that the F-v/F-m measured on leaves with the same red surface, but different leaf thickness, exhibited a slightly lower value in half leaf (separated upper and lower layers of leaves by removing the leaf edge similarly as affected by winter freezing and thawing) than that in the intact leaf (without removing the leaf edge), and the F-v/F-m measured on the red surface was significantly lower than that on the inner or backlighted green surface of the same thickness. Our results suggest that the usual measurement of F-v/F-m on red adaxial surface overestimates the actual degree of photoinhibition compared with that of the whole leaf in the winter.
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