Determining effects of elevated CO 2 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 CO 2 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 C 3 and two C 4 species at current or elevated CO 2 and three different growth temperatures (GT). We assessed photosynthetic thermotolerance in both unacclimated (basal tolerance) and preheat-stressed (preHS = acclimated) plants. In C 3 species, basal thermotolerance of net photosynthesis (P n ) was increased in high CO 2 , but in C 4 species, P n thermotlerance was decreased by high CO 2 (except Zea mays at low GT); CO 2 effects in preHS plants were mostly small or absent, though high CO 2 was detrimental in one C 3 and one C 4 species at warmer GT. Though high CO 2 generally decreased stomatal conductance, decreases in P n during heat stress were mostly due to non-stomatal effects. Photosystem II (PSII) efficiency was often decreased by high CO 2 during heat stress, especially at high GT; CO 2 effects on post-PSII electron transport were variable. Thus, high CO 2 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 CO 2 may often decrease, or not benefit as expected, tolerance of photosynthesis to acute heat stress. Therefore, interactive effects of elevated CO 2 and warmer growth temperatures on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.Key words: carbon dioxide; global climate change; photosynthesis; thermotolerance.Hamilton EW, Heckathorn SA, Joshi P, Wang D, Barua D (2008). 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 the effect of elevated CO(2) on the tolerance of photosynthesis to acute heat stress (AHS) is necessary for predicting plant responses to global warming because photosynthesis is heat sensitive and AHS and atmospheric CO(2) will increase in the future. Few studies have examined this effect, and past results were variable, which may be related to methodological variation among studies. In this study, we grew 11 species that included cool and warm season and C(3), C(4), and CAM species at current or elevated (370 or 700 ppm) CO(2) and at species-specific optimal growth temperatures and at 30°C (if optimal ≠ 30°C). We then assessed thermotolerance of net photosynthesis (P(n)), stomatal conductance (g(st)), leaf internal [CO(2)], and photosystem II (PSII) and post-PSII electron transport during AHS. Thermotolerance of P(n) in elevated (vs. ambient) CO(2) increased in C(3), but decreased in C(4) (especially) and CAM (high growth temperature only), species. In contrast, elevated CO(2) decreased electron transport in 10 of 11 species. High CO(2) decreased g(st) in five of nine species, but stomatal limitations to P(n) increased during AHS in only two cool-season C(3) species. Thus, benefits of elevated CO(2) to photosynthesis at normal temperatures may be partly offset by negative effects during AHS, especially for C(4) species, so effects of elevated CO(2) on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.

Our understanding of the effects of elevated atmospheric CO 2 , singly and in combination with other environmental changes, on plant-soil interactions is incomplete. Elevated CO 2 effects on C 4 plants, though smaller than on C 3 species, are mediated mostly via decreased stomatal conductance and thus water loss. Therefore, we characterized the interactive effect of elevated CO 2 and drought on soil microbial communities associated with a dominant C 4 prairie grass, Andropogon gerardii Vitman. Elevated CO 2 and drought both affected resources available to the soil microbial community. For example, elevated CO 2 increased the soil C:N ratio and water content during drought, whereas drought alone decreased both. Drought significantly decreased soil microbial biomass. In contrast, elevated CO 2 increased biomass while ameliorating biomass decreases that were induced under drought. Total and active direct bacterial counts and carbon substrate use (overall use and number of used sources) increased significantly under elevated CO 2 . Denaturing gradient gel electrophoresis analysis revealed that drought and elevated CO 2 , singly and combined, did not affect the soil bacteria community structure. We conclude that elevated CO 2 alone increased bacterial abundance and microbial activity and carbon use, probably in response to increased root exudation. Elevated CO 2 also limited drought-related impacts on microbial activity and biomass, which likely resulted from decreased plant water use under elevated CO 2 . These are among the first results showing that elevated CO 2 and drought work in opposition to modulate plant-associated soil-bacteria responses, which should then influence soil resources and plant and ecosystem function.Key words: denaturing gradient gel electrophoresis; drought; elevated CO 2 ; soil microbial communities. During the last nine decades, the atmospheric concentration of CO 2 has increased by 36%, primarily as a result of anthropogenic activities (IPCC 2007). Increased atmospheric CO 2 concentration and related environmental changes (e.g. warming and decreased/increased precipitation) can influence the biological processes of plants and soil microorganisms inhabiting diverse ecosystems (Zak et al. 1993(Zak et al. , 2000 Kandeler et al. 1998 C 3 and C 4 photosynthetic metabolism, though photosynthetic and growth increases are generally larger in C 3 species (e.g., Sage and Monson 1999; Ehleringer et al. 2002;Wang et al. 2008 and references therein). In C 3 species, increased growth under elevated CO 2 is primarily due to decreased photorespiration (photosynthetic fixation of O 2 rather than CO 2 ), while in C 4 species, increased growth under elevated CO 2 is primarily due to decreased stomatal conductance and transpiration, which decreases soil water use (this also occurs in C 3 species).As with most terrestrial ecosystems, grasslands will be impacted by future global environmental and climate changes, including elevated CO 2 (Fay et al. 2003), and this will have importance to the global eco...

Humans are increasing atmospheric CO 2 , ground-level ozone (O 3 ), and mean and acute high temperatures. Laboratory studies show that elevated CO 2 can increase thermotolerance of photosynthesis in C 3 plants. O 3 -related oxidative stress may offset benefits of elevated CO 2 during heat-waves. We determined effects of elevated CO 2 and O 3 on leaf thermotolerance of field-grown Glycine max (soybean, C 3 ). Photosynthetic electron transport ( et ) was measured in attached leaves heated in situ and detached leaves heated under ambient CO 2 and O 3 . Heating decreased et , which O 3 exacerbated. Elevated CO 2 prevented O 3 -related decreases during heating, but only increased et under ambient O 3 in the field. Heating decreased chlorophyll and carotenoids, especially under elevated CO 2 . Neither CO 2 nor O 3 affected heat-shock proteins. Heating increased catalase (except in high O 3 ) and Cu/Zn-superoxide dismutase (SOD), but not Mn-SOD; CO 2 and O 3 decreased catalase but neither SOD. Soluble carbohydrates were unaffected by heating, but increased in elevated CO 2 . Thus, protection of photosynthesis during heat stress by elevated CO 2 occurs in field-grown soybean under ambient O 3 , as in the lab, and high CO 2 limits heat damage under elevated O 3 , but this protection is likely from decreased photorespiration and stomatal conductance rather than production of heat-stress adaptations.Key words: anti-oxidants; global change; heat-shock proteins; photosynthesis. (White et al. 2001; Van Peer et al. 2004; Marchand et al. 2005 Marchand et al. , 2006Wang et al. 2008). Elevated CO 2 , relative to current CO 2 levels, has been demonstrated to affect plant tolerance to acute heat stress (most studies have focused on photosynthetic responses, as photosynthesis is among the most heat-sensitive of plant processes; Weis and Berry 1988), and such high-CO 2 effects have been positive (Faria et al. 1996; Ferris et al. 1998; Huxman et al. 1998; Faria et al. 1999; Taub et al. 2000), negative (Bassow et al. 1994; Roden and Ball 1996), or neutral (Coleman et al. 1991). However, these studies varied in the methods used to measure heat-stress effects on photosynthesis. Also, in those that compared elevated-CO 2 effects on tolerance to acute heat stress in relatively heat-sensitive vs. tolerant species, or in species with different photosynthetic pathways (Coleman et al. 1991; Bassow et al. 1994; Roden and Ball 1996; Huxman et al. 1998; Taub et al. 2000), all species were grown under identical thermal regimes, which were likely closer to optimal for some of the species examined, but supra-or sub-optimal for others. More recently, Wang et al. (2008) grew cool-season Effects of CO 2 and O 3 on Thermotolerance 1397 C 3 , warm-season C 3 , C 4 , and crassulacean acid metabolism (CAM) species at normal and elevated CO 2 , and at speciesspecific optimal growth temperatures and at a common growth temperature of 30 Mishra• C (if optimal different than 30• C); the CAM species were grown at three temperatures (25, 30, and 3...

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