Photooxidative sunburn is the consequence of photosystem overexcitations. It results in tissue color changes as a result of chlorophyll degradation and accumulation of oxidized polymeric phenolics (OPPs), resulting from scavenging of reactive oxygen species (ROS). From a productive point of view, OPPs should be considered as damages, decreasing the economical and esthetical values of plants and crops. However, from a physiological perspective, OPPs could be also play a screening role against excessive photosynthetically active radiation (PAR), because they follow the criteria proposed for the identification of photoprotectors, as follows: (i) As a result of the complex conjugated double bond systems, OPPs absorb and, thus, screen the visible PAR. (ii) The accumulation of brown OPPs is well-known to be stimulated by light exposure, resulting in sunburn symptoms. (iii) OPPs induce PAR resistance; for example, the sunburned brown skin allows the fruit ripening to proceed without further interferences. (iv) The screen provided by the accumulated OPPs in death cells protect underlying tissues, demonstrating an increased resistance to radiation when other physiological processes are not functioning.
Keywords: PAR screen; brown pigments; photooxidation; radiative excess; sunburn.