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

The cold acclimations of mesophyll conductance (g_m ), bundle-sheath conductance (g_bs ) and the CO₂ concentrating mechanism (CCM) of C₄ plants have not been well studied. Here, we estimated the temperature response of g_m , g_bs and leakiness (ϕ), the amount of concentrated CO₂ that escapes the bundle-sheath cells, for the chilling-tolerant C₄ plant Miscanthus × giganteus grown at 14 and 25°C. To estimate these parameters, we combined the C₄ -enzyme-limited photosynthesis model and the Δ¹³ C 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 ϕ 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.