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

Abstract

The responses of C8 leaf and canopy gross photosynthesis to increasing temperature and CO2 can be readily understood in terms of the temperature and CO2 dependencies of quantum yield (φ1) and light-saturated photosynthesis (A(sat)), the two principal parameters in the non-rectangular hyperbola model of photosynthesis. Here, we define these dependencies within the mid-range for C3 herbaceous plants, based on a review of the literature. Then, using illustrative parameter values, we deduce leaf and canopy photosynthesis responses to temperature and CO2 in different environmental conditions (including shifts in the temperature optimum) from the assumed sensitivities of φ1 and A(sat) to temperature and CO2. We show that: (1) elevated CO2 increases photosynthesis more at warm than at cool temperatures because of the large combined CO2-responses of both φ1 and A(sat) at high temperatures; (2) elevated CO2 may 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 of A(sat), 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 φ1; and (5) at very low irradiances, the temperature optimum of photosynthesis is low and is raised very little by increasing CO2.