Gas Exchange and Co-regulation of Photochemical and Nonphotochemical Quenching in Bean during Chilling at Ambient and Elevated Carbon Dioxide

The effects of elevated (700 micromol mol(-1)) and ambient (350 micromol mol(-1)) CO(2) on gas exchange parameters and chlorophyll fluorescence were measured on bean (Phaseolus vulgaris) during 24 h chilling treatments at 6.5 degrees C. Consistent with previous research on this cultivar, photosynthetic decline during chilling was not significantly affected by CO(2) while post-chilling recovery was more rapid at elevated compared to ambient CO(2). Our primary focus was whether there were also CO(2)-mediated differences in demand on nonphotochemical quenching (NPQ) processes during the chilling treatments. We found that photosystem II quantum yield and total NPQ were similar between the CO(2) treatments during chilling. In both CO(2) treatments, chilling caused a shift from total NPQ largely composed of q(E), the protective, rapidly responding component of NPQ, to total NPQ dominated by the more slowly relaxing q(I), related to both protective and damage processes. The switch from q(E) to q(I) during chilling was more pronounced in the elevated CO(2) plants. Using complementary plots of the quantum yields of photochemistry and NPQ we demonstrate that, despite CO(2) effects on the partitioning of NPQ into q(E) and q(I) during chilling, total NPQ was regulated at both CO(2) levels to maximize photochemical utilization of absorbed light energy and dissipate only that fraction of light energy that was in excess of the capacity of photosynthesis. Photodamage did occur during chilling but was repaired within 3 h recovery from chilling in both CO(2) treatments.