Field analysis of photoprotection in co-occurring cool climate C3 and C4 grasses

C4 photosynthesis is particularly successful at high light intensities and high temperatures, but is relatively rare when the average growing season temperature is less than about 15 degrees C. We tested the hypothesis that rapidly reversible photoprotection enables some C4 species to tolerate cool climates, by focusing on two questions: (1) Do chlorophyll fluorescence responses differ seasonally between co-occurring C3 and C4 grasses in the field? (2) Does xanthophyll-mediated photoprotection differ between the two pathways? Spartina pectinata (C4) and Calamogrostis canadensis (C3) were sampled in a herbaceous fresh-water meadow in New Brunswick, Canada (45 degrees N 66 degrees W). Non-photochemical thermal energy dissipation (phi NPQ) and the epoxidation state of the xanthophyll cycle (EPS) were used as indicators of photoprotection. We observed no differential susceptibility to chronic photoinhibition (i.e. photodamage) between the C3 and C4 species, except potentially during spring emergence. On average, C. canadensis showed higher levels of protective dynamic photoinhibition throughout the growing season, but S. pectinata had greater phi NPQ and lower EPS during seasonal and daily temperature minima. The low Rubisco capacity of C4 species is a potential limiting factor to C4 success at high latitudes, but our findings suggest that it is unlikely via a photoinhibitory feedback mechanism.