From these results for photosynthes

From these results for photosynthesis, we predict that there should be interactiveeffects of acute heat stress and CO2 on roots and plant–soil links (e.g., root growthand C loss, which should then impact soil microbes and soil N). Interestingly, whenwe conducted a meta-analysis of available data (literature plus our own), we foundthat the benefits of elevated CO2 to whole-plant biomass were different than forphotosynthesis alone [19]. Specifically, in C3 plants, the benefits of high CO2 to heattolerance were less for whole-plant mass (especially roots) than for photosynthesis,while in C4 plants, high CO2 decreased photosynthesis, but increased plant growth(especially roots) during acute heat stress. We also found that elevated CO2benefited root mass in non-legumes during acute heat stress, but not in legumes,while elevated CO2 benefited photosynthesis during acute heat stress more in legumes than non-legumes (not shown). Consistent with the biomass effects, in C3species, shoot %N decreased more with elevated CO2 during acute heat stress thanin controls, while no such heat effect was observed for C4 species, suggesting aheat CO2 effect on root N uptake in C3 but not C4 species.Thus, our results indicate that effects of elevated CO2 on photosynthetic heattolerance are different from effects on roots, and heat CO2 effects vary withphotosynthetic pathway and between legumes and non-legumes. Given that effectsof CO2 on photosynthesis during acute heat stress depend on preheat-stress growthtemperature and plant N status [85,86], and photosynthetic and growth responsesto heat stress depend on N (e.g., [88,89]), then it is likely that the effects of elevatedCO2 during acute heat stress on root and soil function depend on plant N andpreheat-stress growth temperature too. Interactive effects of acute heat stress, CO2,and N on legumes, as in the above meta-analysis, should also be expected. The Ccost of symbiotic N fixation is high, and fixation is restricted at high soil N [90] andaffected by high CO2 [91]. Nodulation and N fixation are also sensitive to increasesin mean growth temperature (e.g., decreasing at 35 versus 25
C) [92]. Thus,nodulated legumes may be very sensitive to high-CO2-caused effects on photo?synthesis during heat stress.