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Herbicides are the most commonly used means of controlling weeds. Recently, there has been growing concern over the potential impacts of global climate change, specifically, increasing temperatures and elevated carbon dioxide (CO 2 ) concentrations, on the sensitivity of weeds to herbicides. Here, glyphosate response of both Conyza canadensis and Chenopodium album was evaluated under different environmental conditions. Reduced glyphosate sensitivity was observed in both species in response to increased temperature, elevated CO 2 level, and the combination of both factors. Increased temperature had greater effect on plant survival than elevated CO 2 level. In combination, high temperature and elevated CO 2 level resulted in loss of apical dominance and rapid necrosis in glyphosate-treated plants. To investigate the mechanistic basis of reduced glyphosate sensitivity, translocation was examined using 14 C-glyphosate. In plants that were subjected to high temperatures and elevated CO 2 level, glyphosate was more rapidly translocated out of the treated leaf to shoot meristems and roots than in plants grown under control conditions. These results suggest that altered glyphosate translocation and tissue-specific sequestration may be the basis of reduced plant sensitivity. Therefore, overreliance on glyphosate for weed control under changing climatic conditions may result in more weed control failures.

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.

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 pre-heat-stressed (preHS = acclimated) plants. In C3 species, basal thermotolerance of net photosynthesis (P(n)) was increased in high CO2, but in C4 species, P(n) 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(n) 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.

Temperature dependence of photoinhibition and photoprotective mechanisms (10-35 degrees C) was investigated for Chenopodium album leaves grown at 25 degrees C under 500 micro mol quanta m(-2) s(-1). The fraction of active photosystem II (PSII) was determined after photoinhibitory treatment at different temperatures in the presence and absence of lincomycin, an inhibitor of chloroplast-encoded protein synthesis. In the absence of lincomycin, leaves were more tolerant to photoinhibition at high (25-35 degrees C) than at low (11-15 degrees C) temperatures. In the presence of lincomycin, the variation in the tolerance to photoinactivation became relatively small. The rate constant of photoinactivation (k(pi)) was stable at 25-35 degrees C and increased by 50% with temperature decrease from 25 to 11 degrees C. The rate constant of recovery of inactivated PSII (k(rec)) was more sensitive to temperature; it was very low at 11 degrees C and increased by an order of magnitude at 35 degrees C. We conclude that the recovery of photoinactivated PSII plays an essential role in photoprotection at 11-35 degrees C. Partitioning of light energy to various photoprotective mechanisms was further analyzed to reveal the factor responsible for k(pi). The fraction of energy utilized in photochemistry was lower at lower temperatures. Although the fraction of heat dissipation increased with decreasing temperatures, the excess energy that is neither utilized by photochemistry nor dissipated by heat dissipation was found to be greater at lower temperatures. The k(pi) value was strongly correlated with the excess energy, suggesting that the excess energy determines the rate of photoinactivation.