Ultraviolet‐ B component of sunlight stimulates photosynthesis and flavonoid accumulation in variegated P lectranthus coleoides leaves depending on background light

We used variegated P lectranthus coleoides as a model plant with the aim of clarifying whether the effects of realistic ultraviolet‐B ( UV ‐ B ) doses on phenolic metabolism in leaves are mediated by photosynthesis. Plants were exposed to UV ‐ B radiation (0.90 W m −2 ) combined with two photosynthetically active radiation ( PAR ) intensities [395 and 1350 μ mol m −2 s −1 , low light ( LL ) and high light ( HL )] for 9 d in sun simulators. Our study indicates that UV ‐ B component of sunlight stimulates CO 2 assimilation and stomatal conductance, depending on background light. UV ‐ B ‐specific induction of apigenin and cyanidin glycosides was observed in both green and white tissues. However, all the other phenolic subclasses were up to four times more abundant in green leaf tissue. Caffeic and rosmarinic acids, catechin and epicatechin, which are endogenous peroxidase substrates, were depleted at HL in green tissue. This was correlated with increased peroxidase and ascorbate peroxidase activities and increased ascorbate content. The UV ‐ B supplement to HL attenuated antioxidative metabolism and partly recovered the phenolic pool indicating stimulation of the phenylpropanoid pathway. In summary, we propose that ortho ‐dihydroxy phenolics are involved in antioxidative defence in chlorophyllous tissue upon light excess, while apigenin and cyanidin in white tissue have preferentially UV ‐screening function. Variegated P lectranthus coleoides was used to clarify whether the effects of realistic UV‐B doses on phenolic metabolism in leaves are mediated by photosynthesis. UV‐B radiation stimulated CO 2 assimilation and stomatal conductance especially under low light. UV‐B ‐specific induction of apigenin and cyanidin glycosides was observed in the whole leaf, especially in the white tissue. Differential responses to high PAR and UV‐B radiation in both ‘source’ and ‘sink’ tissues are shown.