Photoinhibition in C4 and C3 grasses native to high latitudes

Abstract

Low growing season temperatures limit the geographic distribution of C4 plants, but the reasons for this are unclear. We tested the hypothesis that C4 plants are more susceptible to photoinhibition at chilling temperatures than their C3 competitors using Muhlenbergia glomerata (C4) and Calamogrostis canadensis (C3). These grasses are native to northern Canada and regularly experience chilling temperatures during the growing season. Plants were grown in controlled environment chambers under 16-hour photoperiods at 14/10oC (cool-grown) or 26/22oC (warm-grown). Excised leaves were photoinhibited under a PPFD of 1900 mmol m-2 s-1 at 8oC for two hours. Leaves recovered for 2 hours under 4 combinations of light (0 or 10 mmol m-2 s-1) and temperature (8 or 20oC), followed by an additional 22 hours of darkness at room temperature. A PAM fluoremeter (OS-500, Opti-Science) was used to measure the efficiency of excitation capture by open PSII centres (Fv/Fm, Fv'/Fm' in illuminated samples). The pre-treatment Fv/Fm value of the cool-grown C4 averaged 0.783, which was at least 5% lower than the other species/growth temperature combinations. The photoinhibitory treatment reduced Fv'/Fm' to roughly 35% of Fv/Fm in all cases. The cool-grown C4 recovered more slowly than the other combinations during the initial 2-hour recovery phase under low light, suggesting a quenching mechanism that remains active under even low irradiance. In all cases, Fv/Fm after 24 hours of recovery was between 85 and 88% of the pre-treatment value. Our data indicate that C4 plants are not more susceptible to photoinhibition at chilling temperatures than C3 species, but low growth temperatures may affect changes in the kinetics of fluorescence quenching in the C4 species. Photoinhibition does not appear to be a complete explanation for the low temperature limitation on C4 plant distribution.