Cotton seeds (Gossypium hirsutum L.) were planted in soil in 15 liter pots in unshaded greenhouses, April 20, 1992. Carbon dioxide (650 ± 10 µmol mol^-1) was injected into the air flowing through two greenhouses beginning April 26, 1992 (when 50% percent of the seedlings had emerged) and continued until July 21, 1992 (24 hours per day). Two additional greenhouses had the same airflow but at ambient CO₂ concentration (370 µmol mol^-1 during day time). One each of the high and low CO₂ greenhouses was maintained at a temperature regimes of 40 C maximum during the day and 32 C minimum at night, the other two greenhouses were maintained at 32 C max, 22 C minimum. Each day the temperature in each greenhouse was changed in 2-hour time steps to mimic day and night temperatures of Phoenix, AZ during the cotton growing season. ABA was determined in lyophilized leaf samples. Separation of ABA was done using a SAX 250x4.6 mm strong anion exchange column in 80% methanol-0.1 N acetic acid followed by a C-18 adsorbshere HS column (250 mm x 4.6 mm) with an eluent consisting of 60% methanol-0.0.2 N acetic acid. During ABA extractions known additions of (14)C-ABA were used as internal standards and the recovered radioactivity was determined using Beckman LS7500 Liquid Scintillation Counter.

Plant biomass increased significantly in the high CO₂-high temperature (HTHC) environment compared to low temperature-high CO₂ (LTHC). However, production of squares and bolls per plant increased in LTHC compared to HTHC. Diurnal changes in ABA showed a gradual decrease in all treatments from 8:00 to 15:00 h except in LTHC. ABA level remained significantly lower in LTHC than in the rest of the treatments. Stomatal conductance decreased in plants grown in LTHC and remained lower in this environment than in HTHC. These results suggest that effects of enriched CO₂ did not mediate through ABA and perhaps benefits of high CO₂ on plants will depend on the rise of temperature in the atmosphere.