Field analysis of photoprotection in co-occurring cool climate C(3) and C(4) grasses.

C(4) 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°C. We tested the hypothesis that rapidly reversible photoprotection enables some C(4) species to tolerate cool climates, by focusing on two questions: (1) Do chlorophyll fluorescence responses differ seasonally between co-occurring C(3) and C(4) grasses in the field? (2) Does xanthophyll-mediated photoprotection differ between the two pathways? Spartina pectinata (C(4) ) and Calamogrostis canadensis (C(3) ) were sampled in a herbaceous fresh-water meadow in New Brunswick, Canada (45°N 66°W). Non-photochemical thermal energy dissipation (Φ(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 C(3) and C(4) 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 Φ(NPQ) and lower EPS during seasonal and daily temperature minima. The low Rubisco capacity of C(4) species is a potential limiting factor to C(4) success at high latitudes, but our findings suggest that it is unlikely via a photoinhibitory feedback mechanism.