Foliar anthocyanins in coleus and ornamental grasses: accumulation, localization, and function

Abstract

Anthocyanins provide red coloration in plants. The research objectives were to (1) investigate the influence of environmental factors on anthocyanin accumulation, (2) compare photosynthetic rates of red and green leaves, (3) determine anthocyanin localization in leaves, and (4) determine anthocyanin function(s) in leaves. Coleus (Solenostemon scutellarioides) and ornamental grasses, commonly cultivated for their foliage coloration, were selected as model plants. Irradiance and temperature influenced anthocyanin content in red coleus. Anthocyanin content increased with increasing irradiance, although photobleaching occurred in some cultivars at the highest irradiance. Exposure to low temperature (12 °C) resulted in maximum anthocyanin content in two cultivars but minimal anthocyanin content in another. In switchgrass and purple fountaingrass, anthocyanin content in individual leaves and the percent red leaves increased with increasing irradiance. Intensified seasonal leaf coloration in red-leaved grasses (Imperata cylindrica, Panicum virgatum, Pennisetum advena, Pennisetum purpureum, and Schizachyrium scoparium) resulted from increased anthocyanins and decreased chlorophyll. Anthocyanins were negatively correlated with average daily temperature and daily light integral (DLI) and positively correlated with total growing degree days and total DLI. Annual and non-native grasses had minimal seasonal fluctuations in pigmentation relative to native grasses (P. virgatum and S. scoparium), and this seasonal increase in anthocyanins might be an adaptive mechanism. At saturating irradiance, neither leaf color had a distinct advantage. Maximum photosynthetic rates (Amax) in red and green coleus were similar per area, higher in red per fresh or dry weight, and higher in green per unit chlorophyll. Amax in switchgrass was higher in green leaves per area, fresh, or dry weight, and similar in red and green leaves per unit chlorophyll. Anthocyanins in coleus and switchgrass localized in epidermal cells. They were ideally situated to provide a photoprotection role as light attenuators. Anthocyanins offered minimal photoprotection in switchgrass and their presence may simply be due to selection for desired ornamental attributes. Photoprotection by anthocyanins was most evident in coleus during low temperature/high irradiance stress. Red coleus exhibited less of a decline in Fv/Fm, photosynthesis, electron transport rate, and effective quantum yield than green coleus, and Fv/Fm and photosynthetic rate recovered to pre-stress levels more quickly.