Elevated CO2-mitigation of high temperature stress associated with maintenance of positive carbon balance and carbohydrate accumulation in Kentucky bluegrass

Abstract

Elevated CO2 concentration may promote plant growth while high temperature is inhibitory for C3 plant species. The interactive effects of elevated CO2 and high temperatures on C3 perennial grass growth and carbon metabolism are not well documented. Kentucky bluegrass (Poa pratensis) plants were exposed to two CO2 levels (400 and 800 μmol mol-1) and five temperatures (15/12, 20/17, 25/22, 30/27, 35/32°C, day/night) in growth chambers. Increasing temperatures to 25°C and above inhibited leaf photosynthetic rate (Pn) and shoot and root growth, but increased leaf respiration rate (R), leading to a negative carbon balance and a decline in soluble sugar content under ambient CO2. Elevated CO2 did not cause shift of optimal temperatures in Kentucky bluegrass, but promoted Pn, shoot and root growth under all levels of temperature (15, 20, 25, 30, and 35°C) and mitigated the adverse effects of severe high temperatures (30 and 35°C). Elevated CO2-mitigation of adverse effects of high temperatures on Kentucky bluegrass growth could be associated with the maintenance of a positive carbon balance and the accumulation of soluble sugars and total nonstructural carbohydrates through stimulation of Pn and suppression of R and respiratory organic acid metabolism.

Figure 1. Effects of elevated CO₂ on turf quality responses to increasing temperatures at 7 d (a), 14 d (b), 21 d (c), and 28 d (d) of temperature treatment.

Vertical bars represent the values of least significant difference at p = 0.05 for comparison of CO₂ treatment effects at a given temperature. The LSD value for comparisons between temperature treatments was 0.1198 and 0.0284 under ambient and elevated CO₂ concentration, respectively, at 7 d, 0.0283 and 0.0229 at 14 d, 0.1909 and 0.1685 at 21 d, and 0.1928 and 0.1732 at 28 d.

Figure 2. Effects of elevated CO₂ on the responses of shoot dry weight (a), root dry weight (b), and root/shoot dry weight ratio (c) to increasing temperatures at 28 d of temperature treatment.

Vertical bars represent the values of least significant difference at p = 0.05 for comparison of CO₂ treatment effects at a given temperature. The LSD value for comparisons between temperature treatments was 0.7078 and 0.9462 under ambient and elevated CO₂ concentration, respectively.

Figure 3. Effects of elevated CO₂ on the responses of leaf photosynthetic rate (Pn) and leaf respiration rate (R) to increasing temperatures at 7 d (a), 14 d (b), 21 d (c), and 28 d (d) of temperature treatment, primary vertical axis for Pn, secondary vertical axis for R.

Vertical bars represent the values of least significant difference at p = 0.05 for comparison of CO2 treatment effects at a given temperature. The LSD value for comparisons between temperature treatments was 0.2885 and 0.3068 under ambient and elevated CO₂ concentration, respectively, at 7 d, 0.3812 and 0.2818 at 14 d, 0.5131 and 0.3814 at 21 d, and 0.4642 and 0.4347 at 28 d.

Figure 4. Effects of elevated CO₂ on leaf photosynthetic rate (Pn) to leaf respiration rate (R) ratio (Pn/R) at different temperatures at 7 d (a), 14 d (b), 21 d (c), and 28 d (d) of temperature treatment, and the dotted line represents Pn/R ratio was 1.0.

Vertical bars represent the values of least significant difference at p = 0.05 for comparison of CO₂ treatment effects at a given temperature.

Figure 5. Effects of elevated CO₂ on total non-structural carbohydrates in leaves at 28 d of different temperature treatments.

Vertical bars represent the values of least significant difference at p = 0.05 for comparison of CO₂ treatment effects at a given temperature. The LSD value for comparisons between temperature treatments was 0.3392 and 0.3993 under ambient and elevated CO₂ concentration, respectively.

Figure 6. Effects of elevated CO₂ on the relative soluble sugars content in leaves at 28 d of different temperature treatments.

Vertical bars represent the values of least significant difference at p = 0.05 for comparison of CO₂ treatment effects at a given temperature. The LSD value for comparisons between temperature treatments was 0.0183 and 0.0223 under ambient and elevated CO₂ concentration, respectively.

Figure 7. Effects of elevated CO₂ on the relative organic acids content in leaves at 28 d of different temperature treatments.

Vertical bars represent the values of least significant difference at p = 0.05 for comparison of CO₂ treatment effects at a given temperature. The LSD value for comparisons between temperature treatments was 0.0318 and 0.0171 under ambient and elevated CO₂ concentration, respectively.