The importance of non-photosynthetic pigments and cinnamic acid derivatives in photoprotection

Excess light and UV-radiation are hazardous natural stress factors, and plants have evolved a range of avoidance and tolerance strategies employing versatile tools against these constraints. The paper outlines the contribution of non-photosynthetic pigments to the protection of plants from excess light and UV-radiation, as well as the mechanisms involved. A large pool of secondary metabolites, belonging mainly to the highly diversified array of flavonoids (C6-C3-C6 types), and the closely related anthocyanins (flavylium salts, C6-C3-C6+ types), as well as betacyanins are often referred to as non-photosynthetic pigments. In addition to screening out incoming visible and UV-radiation by absorbtion, these pigments can dissipate excess photon energy, while the antioxidant and reactive oxygen species (ROS)-scavenging ability of non-photosynthetic pigments can also protect against light- and UV-induced oxidative stress. Their role in adjustment of source activity to reduced sink strength is examined as a possible mechanism of protection in conditions where excess light stress is aggravated by other constraints limiting the CO2 assimilation. Evidence for the photoprotective role involving UV-screening, antioxidant activity, ROS-scavenging and energy-dissipation of another group of secondary metabolites, cinnamic acid derivatives (C6-C3 types), is also discussed.