Potential mechanisms of low-temperature tolerance of C4 photosynthesis in Miscanthus x giganteus: an in vivo analysis.

Miscanthus x giganteus (Greef & Deuter ex Hodkinson & Renvoize) is unique among C4 species in its remarkable ability to maintain high photosynthetic productivity at low temperature, by contrast to the related C4 NADP-malic enzyme-type species Zea mays L. In order to determine the in vivo physiological basis of this difference in photosynthesis, water vapor and CO2 exchange and modulated chlorophyll fluorescence were simultaneously monitored on attached leaf segments from plants grown and measured at 25/20 degrees C or 14/11 degrees C (day/night temperature). Analysis of the response of photosynthesis to internal CO2 concentration suggested that ribulose bisphosphate carboxylase/oxygenase (Rubisco) and/or pyruvate orthophosphate dikinase (PPDK) play a more important role in determining the response to low temperature than does phosphoenolpyruvate carboxylase (PEPc). For both species at both temperatures, the linear relationship between operating efficiency of whole-chain electron transport through photosystem II (Phi(PSII)) and the efficiency of CO2 assimilation (Phi(CO2)) was unchanged and had a zero intercept, suggesting the absence of non-photosynthetic electron sinks. The major limitation at low temperature could not be solely at Rubisco or at any other point in the Calvin cycle, since this would have increased leakage of CO2 to the mesophyll and increased Phi(PSII)/Phi(CO2). This in vivo analysis suggested that maintenance of high photosynthetic rates in M. x giganteus at low temperature, in contrast to Z. mays, is most likely the result of different properties of Rubisco and/or PPDK, reduced susceptibility to photoinhibition, and the ability to maintain high levels of leaf absorptance during growth at low temperature.