A cyclic series of interconversions involving certain xanthophylls, which plays a vital role in the photoprotection of plant chloroplasts by quenching reactive oxygen species (ROS) such as hydrogen peroxide and singlet oxygen. Under intense illumination, violaxanthin is converted, via an intermediate, to zeaxanthin. The latter acts as a quenching agent by dissipating excess energy from chlorophyll as heat, thereby avoiding possible damage to the photosynthetic apparatus. In dim light, the zeaxanthin is reconverted to violaxanthin, so that all incident light energy is used for photosynthesis. Xanthophylls also ‘mop up’ ROS formed by other plant stressors such as drought, heat, and salinity.