grantor:
University of TorontoCurrent surveys and paleodistributions indicate that C\sb4 species favour warm, subtropical regions. Some factors may account for the low-temperature sensitivity of C\sb4 plants: Pyruvate P\sb{\rm i}-Dikinase is inactivated below 12\sp\circC and damage to Photosystem II may also occur. The gas-exchange and biochemical results in this study demonstrated that in the C\sb4 montane grasses Bouteloua gracilis and Muhlenbergia montanum, the activity of Rubisco correlated with photosynthesis below 20\sp\circC during both short- and long-term chilling. Neither Phosphoenolpyruvate Carboxylase nor Pyruvate P\sb{\rm i}-Dikinase activities indicated that these enzymes limit CO\sb2 assimilation below 20\sp\circC. During long-term exposure to chilling conditions, the photosynthesis rate above 25\sp\circC improved in M. montanum indicating that cold-acclimation in some C\sb4 species can occur. Additionally, prolonged cold treatment had little effect on Photosystem II in this specie. Rubisco activity appears to limit C\sb4 photosynthesis at cooler temperatures, and thus may have prevented the expansion of C\sb4 flora to cooler habitats.M.Sc
