Short-term thermal photosynthetic responses of C4 grasses are independent of the biochemical subtype

C-4 photosynthesis evolved independently many times, resulting in multiple biochemical pathways; however, little is known about how these different pathways respond to temperature. We investigated the photosynthetic responses of eight C-4 grasses belonging to three biochemical subtypes (NADP-ME, PEP-CK, and NAD-ME) to four leaf temperatures (18, 25, 32, and 40 degrees C). We also explored whether the biochemical subtype influences the thermal responses of (i) in vitro PEPC (V-pmax) and Rubisco (V-cmax) maximal activities, (ii) initial slope (IS) and CO2-saturated rate (CSR) derived from the A-C-i curves, and (iii) CO2 leakage out of the bundle sheath estimated from carbon isotope discrimination. We focussed on leakiness and the two carboxylases because they determine the coordination of the CO2 concentrating mechanism and are important for parameterizing the semi-mechanistic C-4 photosynthesis model. We found that the thermal responses of V-pmax and V-cmax, IS, CSR, and leakiness varied among the C-4 species independently of the biochemical subtype. No correlation was observed between V-pmax and IS or between V-cmax and CSR; while the ratios V-pmax/V-cmax and IS/CSR did not correlate with leakiness among the C-4 grasses. Determining mesophyll and bundle sheath conductances in diverse C-4 grasses is required to further elucidate how C-4 photosynthesis responds to temperature.