Effect of elevated [CO₂] on plant thermotolerance.

Determining the effect of elevated CO₂ on the tolerance of photosynthesis to acute heat-stress (AHS) is necessary for predicting plant responses to global warming, as photosynthesis is heat-sensitive and AHS and atmospheric CO₂ will increase in the future. In this study, we grew 11 species (including: cool- & warm-season, and all 3 photosynthetic pathways- C₃, C₄, CAM) at current or elevated (370 or 700 ppm) CO₂.  Thermotolerance of P_n in elevated (vs. ambient) CO₂ increased in C₃, but decreased in C₄ (especially) and CAM (high growth temperature only), species.  In contrast, elevated CO₂ decreased electron transport in 10-of-11 species.  High CO₂ decreased g_st in 5 of 9 species, but stomatal limitations to P_n increased during AHS in only 2 cool-season C₃ species.  Thus, benefits of elevated CO₂ to photosynthesis at normal temperatures may be partly offset by negative effects during AHS, especially for C₄ species, so effects of elevated CO₂ on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.

How availability of N affects plant thermotolerance at different [CO₂].

We grew Hordeum vulgare (barley, C₃) and Zea mays (corn, C₄) at current or elevated CO₂ (370, 700 ppm) and limiting or optimal soil N (0.5, 7.5 mM).  For barley, thermotolerance of P_n, F_v'/F_m', and q_p was decreased slightly by elevated CO₂ at low N, but increased slightly at high N.  However, for corn, P_n, F_v'/F_m', and q_p were decreased substantially by elevated CO₂ under high and low N.  Negative effects of high CO₂ were associated with decreased CE and rubisco activase (except high-N barley) and HSPs (especially HSP70).  Therefore, stimulatory effects of elevated CO₂ at normal temperatures on photosynthesis and growth may be partly offset by negative effects during heat stress, especially for C₄ species and low-N conditions.  Thus, CO₂ and N effects on photosynthetic thermotolerance may contribute to changes in plant productivity, distribution, and diversity.

Impact of heat stress on plant performance and plant-soil links.

Extreme events, in spite of their ephemeral nature, can also cause shifts in the structure of plant communities. But how N affects community responses to acute heat stress remains unknown. Evaluating the effect of N on the changes of vegetation to acute heat stress requires insight into how stress physiology and community structure interact.

Plant physiological and growth responses to elevated [CO₂] at different global change scenarios.

Our results showed that elevated CO₂ had no effect on net photosynthesis (A) under heat stress (HS), but increased enhanced A at ambient (AT) and elevated (ET) temperature. Plants with C₃ and C₄ plants photosynthetic pathways responded differently to increase in CO₂ and temperature. It was also found that for C₃ species.  Elevated CO₂ increased A by a greater magnitude in legumes than in non-legumes at HS, suggesting that N status might be an important factor in affecting plant tolerance for HS under elevated CO₂.  Total plant biomass (WT) increased by 42.5% at AT, 36.7% at ET and 25.9% at HS for C₃ species, but no significant CO₂ effect was observed for C₄ species, regardless of temperature treatments due to small sample sizes and thus large variances. Overall, our results demonstrate that elevated CO₂ affects plant physiology and growth to varying degrees under different temperature regimes. These findings have important implications for biomass accumulation and ecosystem functioning in the future when CO₂ is higher and more climate extremes, e.g., heat waves, will become likely become more frequent.