Effects of Elevated Carbon Dioxide Concentration On Kentucky Bluegrass Responses to Increasing Temperatures and Drought Stress.

Global climate changes are associated with elevated CO₂ concentration in the atmosphere, elevated temperatures, and a decline in water availability.  The objective of this study was to investigate whether elevated CO₂ may mitigate the adverse effects of increasing temperature and drought stress on a cool-season turfgrass species.  Kentucky bluegrass (cv. ‘Baron’) plants were grown in growth chambers with ambient (400 ppm) or elevated (800 ppm) CO₂ and day/night temperatures of 15/12, 20/17, 25/22, 30/27, 35/32 ^oC under well-watered conditions or deficit irrigation to replace 50% evapotranspiration rate (drought stress).  Turf quality declined with increasing temperature and during drought stress, and the highest turf quality was found in the combined treatment of 20/17 ^oC, elevated CO₂, and well-watered conditions.  The lowest quality occurred at the combined treatment of 35/32 ^oC, drought stress, and ambient CO₂.  Increasing temperatures had no effects on leaf wilting score in well-watered control at both ambient and elevated CO₂ but drought stress caused severe leaf wilting, particualry at temperatures above 25/22 ^oC for both ambient and elevated CO₂.  Elevated CO₂ mitigated drought-induced leaf wilting under high temperatures.  Increasing temperatures and drought stress caused significant decline in shoot and root dry weight but plants at elevated CO₂ maintained significantly higher shoot and root biomass at the different temperatures and under drought stress compared to plants in ambient CO₂.  Single-leaf photosynthetic rate (Pn) was the highest at 20/17 ^oC and decreased with increasing temperatures.  The deline was more severe under drought stress compared to well-watered conditions.  Plants at elevated CO₂ had significantly higher Pn under all temperature regimens for both well-watered conditions and drought stress compared to plants in ambient CO₂.  Respration rate increased with increasing temperatures and elevated CO₂ suppressed the rate of increase.  Results show that elevated CO₂ mitigated the adverse effects of high temperatures and drought stress on Kentucky bluegrass as demonstrated by the imrpovement in turf quality, leaf wilting, and biomass production, which could be related to the suppression of carbohydrate consumption through respiration and the promotion of carbohydrate production through photosynthesis.