Temperature and CO2Responses of Leaf and Canopy Photosynthesis: a Clarification using the Non-rectangular Hyperbola Model of Photosynthesis

The responses of C3leaf and canopy gross photosynthesis to increasing temperature and CO2can be readily understood in terms of the temperature and CO2dependencies of quantum yield (φi) and light-saturated photosynthesis (Asat), the two principal parameters in the non-rectangular hyperbola model of photosynthesis. Here, we define these dependencies within the mid-range for C3herbaceous plants, based on a review of the literature. Then, using illustrative parameter values, we deduce leaf and canopy photosynthesis responses to temperature and CO2in different environmental conditions (including shifts in the temperature optimum) from the assumed sensitivities of φiandAsatto temperature and CO2. We show that: (1) elevated CO2increases photosynthesis more at warm than at cool temperatures because of the large combined CO2-responses of both φiandAsatat high temperatures; (2) elevated CO2may substantially raise the temperature optimum of photosynthesis at warm temperatures, but not at the cool temperatures which prevail for much of the time at temperate and high latitudes; (3) large upward shifts in the temperature optimum of canopy gross photosynthesis occur at high irradiances, following the response ofAsat, and are probably important for global carbon fixation; (4) canopy gross photosynthesis shows smaller CO2-temperature interactions than leaf photosynthesis, because leaves in canopies receive lower average irradiances and so more strongly follow the dependencies of φi; and (5) at very low irradiances, the temperature optimum of photosynthesis is low and is raised very little by increasing CO2.