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Anthocyanins are red and purple pigments commonly found in plant leaves. The function of anthocyanins in leaves is largely unknown. They may working in concert with another class of pigments called the xanthophylls to protect leaves from excess light, a function called photoprotection. Anthocyanins may also reduce damage from insect herbivory and disease. However, under certain conditions, anthocyanins may block light that could otherwise be used for photosynthesis. The goal of this project was to explore the hypotheses that anthocyanins protect plants from excess light, that they decrease damage from disease and insects, and that they impose costs on photosynthesis. We tested these hypotheses in green-leafed and red-leafed (anthocyanin-rich) varieties of the crop plants lettuce (Lactuca sativa), kale (Brassica oleracea), and shiso (Perilla frutescens) grown in two outdoor garden plots and in a greenhouse. After light stress, red shiso from the greenhouse sustained less damage from excess light than green shiso. Red leaves also had lower quantity and activity of protective xanthophylls than green leaves, suggesting that red leaves compensated for the photoprotective effects of anthocyanins. In some cases, red-leafed varieties showed increased resource allocation to light capture and less efficient use of light for photosynthesis, suggesting potential costs. In field populations, plants with red leaves had reduced damage from some insect herbivores, but the disease resistance hypothesis was not supported. In plant leaves, anthocyanins seem to serve a role in photoprotection and may reduce damage caused by insect herbivores, but their accumulation might represent a tradeoff with photosynthesis.
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