Low growth temperatures modify the efficiency of light use by photosystem II for CO₂ assimilation in leaves of two chilling-tolerant C ₄ species, Cyperus longus L. and Miscanthus x giganteus

Two C₄ plants, Miscanthus x giganteus and Cyperus longus L., were grown at suboptimal growth temperatures and the relationships between the quantum efficiencies of photosynthetic electron transport through photosystem II (PSII) (PSII operating efficiency; F_q′/F_m′) and CO₂ assimilation (φ_CO2) in leaves were examined. When M. x giganteus was grown at 10°C, the ratio of the PSII operating efficiency to φ_CO2 increased relative to that found in leaves grown at 14 and 25°C. Similar increases in the F_q′/F _m′ : φ_CO2 occurred in the leaves of two C. longus ecotypes when the plants were grown at 17°C, compared to 25°C. These elevations of F_q′/F_m′ : φ_CO2 at low growth temperatures were not attributable to the development of anthocyanins, as has been suggested for maize, and were indicative of the operation of an alternative sink to CO₂ assimilation for photosynthetic reducing equivalents, possibly oxygen reduction via a Mehler reaction, which would act as a mechanism for protection of PSII from photoinactivation and damage. Furthermore, in M. x giganteus grown at 10°C, further protection of PSII was effected by a 20-fold increase in zeaxanthin content in dark-adapted leaves, which was associated with much higher levels of non-photochemical quenching of excitation energy, compared to that observed in leaves grown at 14 and 25°C. These differences may explain the long growing season and remarkable productivity of this C₄ plant in cool climates, even in comparison to other C₄ species such as C. longus, which occur naturally in such climates.