Anthocyanins are a class of water-soluble, flavonoid pigments responsible for most of the blue, red, pink, and purple coloration in plants. Among species that synthesize anthocyanins, there is much heterogeneity as to when, where, and why they are produced. For example, in some species, anthocyanins accumulate in the upper epidermis of leaves experiencing light and/or nutrient stress. However, they also accumulate in the lower epidermis of leaves of other species growing in the deep shade where light stress is typically low. In yet other species, anthocyanins are produced in developing leaves but they can also occur in mature and/or senescent leaves. This variability in the spatial and temporal patterns of anthocyanin production, particularly in leaves, has greatly complicated their study and led to different hypotheses about their functions in plants. In this study, we explored how anthocyanins influence photosynthesis and photoprotection in herbaceous plants using varieties of Perilla frutescens (Lamiaceae) that produce leaves that are either green (acyanic), red (cyanic) with anthocyanins in the upper and lower epidermal layers, or bicolored with anthocyanins in only the lower epidermis grown in a common garden experiment.
Red-leafed shiso sustained less photo-oxidative damage from experimental light stress than either the green or bicolor-leafed varieties. Red-leafed shiso also had higher leaf chlorophyll contents, smaller Chla:Chlb ratios, and lower levels of xanthophyll cycle mediated photoprotection than green- and bicolor-leafed plants, classical signatures of shade acclimation. Collectively these results suggest that anthocyanins in red-leafed shiso complement photoprotection typically provided by the xanthophyll cycle by shading chloroplasts, thus reducing potential excess light. While this shading may provide photoprotection, red-leafed shiso also had reduced net photosynthetic rates under low light, which could limit photosynthetic carbon gain during periods of low irradiance in this variety. Like red-leafed shiso, bicolor-leafed shiso had pigment profiles suggestive of shade acclimation despite the fact this variety accumulates anthocyanins in its lower epidermis, not the upper. In addition, like the red-leafed shiso, the bicolor variety had lower levels of xanthophyll cycle mediated photoprotection during light stress. These results suggest that anthocyanins in the lower epidermis may also provide some degree of photoprotection. Additional results of this study are presented and discussed.