Cold acclimation of mesophyll conductance, bundle-sheath conductance and leakiness in Miscanthus x giganteus

The cold acclimations of mesophyll conductance (g(m)), bundle-sheath conductance (g(bs)) and the CO2 concentrating mechanism (CCM) of C-4 plants have not been well studied. Here, we estimated the temperature response of g(m), g(bs) and leakiness (phi), the amount of concentrated CO2 that escapes the bundle-sheath cells, for the chilling-tolerant C-4 plant Miscanthus x giganteus grown at 14 and 25 degrees C. To estimate these parameters, we combined the C-4-enzyme-limited photosynthesis model and the Delta C-13 discrimination model. These combined models were parameterised using in vitro activities of carbonic anhydrase (CA), pyruvate, phosphate dikinase (PPDK), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and phosphoenolpyruvate carboxylase (PEPc). Cold-grown Miscanthus plants increased in vitro activities of RuBisCO and PPDK but decreased PEPc activity compared with warm-grown plants. Mesophyll conductance and g(bs) responded strongly to measurement temperatures but did not differ between plants from the two growth temperatures. Furthermore, modelling showed that phi increased with measurement temperatures for both cold-grown and warm-grown plants, but was only marginally larger in cold-grown compared with warm-grown plants. Our results in Miscanthus support that g(m) and g(bs) are unresponsive to growth temperature and that the CCM is able to acclimate to cold through increased activity of PPDK and RuBisCO.