5.3Interactions Between Heat Stress

5.3Interactions Between Heat Stress and Other Global Environmental-Change Factorson RootsBoth elevated CO2 and acute heat stress individually affect roots and plant–soillinkages (e.g., C and N flow between plant and soil or microbes), but CO2 and heat stress generally have opposite effects on plants (e.g., high CO2 increases, and heatstress decreases, growth). Thus, combined effects of increases in CO2 and acuteheat stress on roots, particularly as mean temperatures increase or water availabilitychanges, are difficult to predict, and little research has been conducted on acuteheat stress CO2/water/other interactions on roots or soil.Our recent results indicate that effects of high CO2 on photosynthetic toleranceto heat stress are different from effects on roots. These studies indicate that highCO2 affects photosynthetic tolerance to acute heat stress, and the effect of CO2varies with photosynthetic pathway, plant N status, and preheat-stress growthtemperature. The main relevant results are: Effects of elevated CO2 on tolerance of photosynthesis to heat stress vary withphotosynthetic pathway [85–87]. At near-optimal growth temperatures, elevated(versus current) CO2:– increases heat tolerance of net photosynthesis and electron transport in C3plants;– decreases heat tolerance of photosynthesis and electron transport in C4 species;– can decrease heat tolerance of photosynthesis and electron transport in CAMspecies in some instances;– similar effects were observed in unacclimated/preheated plants. Effects of elevated CO2 on heat tolerance depend on background growth tempera?ture and this varied with photosynthetic pathway [85,86]:– in C3 plants, high CO2 increases photosynthetic heat tolerance at near-optimalgrowth temperatures, but at higher temperatures, high-CO2-related benefitsdecline or disappear;– inC4 plants, high-CO2 effects on photosynthetic heattolerance can be negative orpositive at suboptimal growth temperatures, but are negative at near- or supra?optimal temperatures;– in CAM plants, high CO2 has little effect on photosynthetic heat tolerance atmedium growth temperatures, but at higher temperatures, high CO2 decreasesheat tolerance.