*4.3. Increasing Ascorbic Acid Improves Tolerance to High Light*

As mentioned above, in addition to environmental sources, ROS is generated during exposure to high light. Excess light energy can generate triplet state chlorophyll (<sup>3</sup> Chl) which transfers its energy to ground-state O2 to produce <sup>1</sup> O2. Photosystem over reduction also produces ROS such as O2 •− and H2O2 [94] which can damage proteins, membranes, and pigments of photosystem I (PSI) and photosystem II (PSII), resulting in the inactivation of reaction centers as well as compromise their repair [95,96]. An increase in DHAR expression in tobacco resulted in less photoinhibition following exposure to high light that was likely due to an increase in the foliar levels of xanthophyll pigments and chlorophyll as well as in the electron transport rate (ETR) and CO2 assimilation, particularly at high light intensities, while ROS were reduced [97]. Thus, an increase in Asc maintains photosynthetic functioning by limiting ROS-mediated damage. Conversely, reducing Asc through suppression of DHAR results in elevated ROS and photoinhibition that is accompanied by reductions in the quantum yield of PSII and ETR [97].
