Interactive Effects of Elevated CO_2 and Growth Temperature on the Tolerance of Photosynthesis to Acute Heat Stress in C_3 and C_4 Species

Determining effects of elevated CO2 on the tolerance of photosynthesis to acute heat-stress （heat wave） is necessary for predicting plant responses to global warming, as photosynthesis is thermolabile and acute heat-stress and atmospheric CO2 will increase in the future. Few studies have examined this, and past results are variable, which may be due to methodological variation. To address this, we grew two C3 and two C4 species at current or elevated CO2 and three different growth temperatures （GT）. We assessed photosynthetic thermotolerance in both unacclimated （basal tolerance） and preheat-stressed （preHS = acclimated） plants. In C3 species, basal thermotolerance of net photosynthesis （Pn） was increased in high CO2, but in C4 species, Pn thermotlerance was decreased by high CO2 （except Zea mays at low GT）; CO2 effects in preHS plants were mostly small or absent, though high CO2 was detrimental in one C3 and one C4 species at warmer GT. Though high CO2 generally decreased stomatal conductance, decreases in P, during heat stress were mostly due to non-stomatal effects. Photosystem Ⅱ （PSⅡ） efficiency was often decreased by high CO2 during heat stress, especially at high GT; CO2 effects on post-PSⅡ electron transport were variable. Thus, high CO2 often affected photosynthetic theromotolerance, and the effects varied with photosynthetic pathway, growth temperature, and acclimation state. Most importantly, in heat-stressed plants at normal or warmer growth temperatures, high CO2 may often decrease, or not benefit as expected, tolerance of photosynthesis to acute heat stress. Therefore, interactive effects of elevated CO2 and warmer growth temperatures on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.

Variation in Heat-shock Proteins and Photosynthetic Thermotolerance among Natural Populations of Chenopodium album L.from Contrasting Thermal Environments:Implications for Plant Responses to Global Warming

Production of heat-shock proteins （Hsps） is a key adaptation to acute heat stress and will be important in determining plant responses to climate change. Further, intraspecifc variation in Hsps, which will influence species-level response to global warming, has rarely been examined in naturally occurring plants. To understand intraspecific variation in plant Hsps and its relevance to global warming, we examined Hsp content and thermotolerance in five naturally occurring populations of Chenopodium album L. from contrasting thermal environments grown at low and high temperatures. As expected, Hsp accumulation varied between populations, but this was related more to habitat variability than to mean temperature. Unexpectedly, Hsp accumulation decreased with increasing variability of habitat temperatures. Hsp accumulation also decreased with increased experimental growth temperatures. Physiological thermotolerance was partitioned into basal and induced components. As with Hsps, induced thermotolerance decreased with increasing temperature variability. Thus, populations native to the more stressful habitats, or grown at higher temperatures, had lower Hsp levels and induced thermotolerance, suggesting a greater reliance on basal mechanisms for thermotolerance. These results suggest that future global climate change will differentially impact ecotypes within species, possibly by selecting for increased basal versus inducible thermotolerance.

Interactive Effects of Elevated CO_2 and Ozone on Leaf Thermotolerance in Field-grown Glycine max

Humans are increasing atmospheric CO2, ground-level ozone （O3）, and mean and acute high temperatures. Laboratory studies show that elevated CO2 can increase thermotolerance of photosynthesis in C3 plants. O3-related oxidative stress may offset benefits of elevated CO2 during heat-waves. We determined effects of elevated CO2 and O3 on leaf thermotolerance of field-grown Glycine max （soybean, C3）. Photosynthetic electron transport （Фet） was measured in attached leaves heated in situ and detached leaves heated under ambient CO2 and O3. Heating decreased Фet, which O3 exacerbated. Elevated CO2 prevented O3-related decreases during heating, but only increased Фet under ambient O3 in the field. Heating decreased chlorophyll and carotenoids, especially under elevated CO2. Neither CO2 nor O3 affected heat-shock proteins. Heating increased catalase （except in high O3） and Cu/Zn-superoxide dismutase （SOD）, but not Mn- SOD; CO2 and O3 decreased catalase but neither SOD. Soluble carbohydrates were unaffected by heating, but increased in elevated CO2. Thus, protection of photosynthesis during heat stress by elevated CO2 occurs in field-grown soybean under ambient O3, as in the lab, and high CO2 limits heat damage under elevated O3, but this protection is likely from decreased photorespiration and stomatal conductance rather than production of heat-stress adaptations.