Temperature and CO2 responses of leaf and canopy photosynthesis: A clarification using the non-rectangular hyperbola model of photosynthesis

The responses of C-3 leaf and canopy gross photosynthesis to increasing tem perature and CO2 can be readily understood in terms of the temperature and CO2 dependencies of quantum yield (phi(i)) and light-saturated photosynthes is (A(sat)) the two principal parameters in the non-rectangular hyperbola m odel of photosynthesis. Here, we define these dependencies within the mid-r ange for C-3 herbaceous plants, based on a review of the literature. Then, using illustrative parameter values, we deduce leaf and canopy photosynthes is responses to temperature and CO, in different environmental conditions ( including shifts in the temperature optimum) from the assumed sensitivities of phi(i) and A(sat) to temperature and CO2. We show that: (1) elevated CO 2 increases photosynthesis more at warm than at cool temperatures because o f the large combined CO2-responses of both phi(i) and A(sat) at high temper atures; (2) elevated CO2 may substantially raise the temperature optimum of photosynthesis at warm temperatures, but not at the cool temperatures whic h prevail for much of the time at temperate and high latitudes; (3) large u pward shifts in the temperature optimum of canopy gross photosynthesis occu r at high irradiances, following the response of A(sat), and are probably i mportant for global carbon fixation; (4) canopy gross photosynthesis shows smaller CO2-temperature interactions than leaf photosynthesis, because leav es in canopies receive lower average irradiances and sep more strongly foll ow the dependencies of phi(i); and (5) at very low irradiances, the tempera ture optimum of photosynthesis is low and is raised very little by increasi ng CO2 .(C) 1998 Annals of Botany Company.