Elevated CO₂ mitigates chilling‐induced water stress and photosynthetic reduction during chilling

Bean, cucumber and corn plants were grown in controlled‐environment chambers at 25/18 °C day/night temperature and either ambient (350 μmol mol −1 ) or elevated (700 μmol mol −1 ) CO 2 concentration, and at 20–30 d after emergence they were exposed to a 24 h chilling treatment (6.5 ± 1.5 °C) at their growth CO 2 concentration. Whole‐plant transpiration rates (per unit leaf area basis) during the first 3 h of chilling were about 26,28 and 13% lower at elevated than at ambient CO 2 for bean, cucumber and corn, respectively. The decline in leaf water potential (ψ L ) and visible wilting of bean and cucumber during chilling were significantly less at elevated than at ambient CO 2 . Corn ψ L was not significantly affected by chilling, and corn did not exhibit any other symptoms of chilling‐induced water stress. Leaf osmotic potentials (measured before chilling only) of bean and cucumber were more negative at elevated than at ambient CO 2 , and the corresponding calculated leaf turgor potentials were significantly higher at elevated than at ambient CO 2 . Leaf relative water content (RWC) during chilling at ambient CO 2 fell to 62 and 48% for bean and cucumber, respectively. RWC during chilling at elevated CO 2 was never below 79% for bean or 63% for cucumber. Corn RWC was not measured. After 24 h of chilling at ambient CO 2 , net photosynthetic rate ( P N ) reductions were 83, 89 and 24% for bean, cucumber and corn, respectively. P N reductions during chilling were less at elevated CO 2 : 53, 40 and 4% for bean, cucumber and corn, respectively. At ambient CO 2 , none of the species fully recovered to pre‐chilling P N , but at elevated CO 2 both bean and corn recovered fully. The average percentage leaf area with visible leaf damage due to chilling was 20.6 and 9.6% at ambient and elevated CO 2 , respectively, for bean, and 32.4 and 23.6% at ambient and elevated CO 2 , respectively, for cucumber. Corn showed no significant permanent leaf damage from chilling at either CO 2 concentration. These results indicate that cucumber was most sensitive to chilling as imposed in this study, followed by bean and corn. The results support the hypothesis that, at least in young plants under controlled‐environment conditions, elevated CO 2 improves plant water relations during chilling and can mitigate photosynthetic depression and chilling damage. The implications for long‐term growth and reproductive success in managed and natural ecosystems will require testing of this hypothesis under field conditions.