Elevated CO2 at 1000 ppm Maintains Growth and Mitigates Drought Stress in Tabernaemontana divaricata Under High Growing Temperatures

This greenhouse study examined whether the drought-mitigating effects of elevated CO2 (eCO2), reported under favorable temperatures, persist at high temperatures in the subtropical desert climate of the United Arab Emirates (UAE). Tabernaemontana divaricata (L.) R. Br. ex Roem. & Schult. (Apocynaceae), a tropical C3 plant, was grown at 35.5/26 °C (day/night) under three CO2 levels (ambient, 700, and 1000 ppm) and three water regimes (well-watered, mild, and moderate drought). Significant CO2× drought interactions were observed for all variables except stomatal density. Physiological responses to eCO2 relative to respective ambient CO2 (aCO2) varied across variables and water regimes. Contrary to the expected decline, eCO2 increased stomatal conductance at 700 and 1000 ppm under mild (+77.8% and +243.4%) and moderate drought (+65.5% and +305.7%), respectively. At 1000 ppm, stomatal area and density showed the expected inverse relationship under well-watered (area +34.7%, density −29.1%) and moderate drought conditions (area +5.4%, density −19.4%). Under both drought levels, 700 and 1000 ppm increased abscisic acid (1.4-fold to 7.7-fold), glycine betaine (2.4-fold to 96.2-fold), and leaf water potential (+9.0% to +42.6%). Proline and chlorophyll b increased only at 700 ppm under mild (+28.8% and +15.3%) and moderate drought (+29.1% and +10.7%), whereas 1000 ppm reduced proline (−7.9% and −15.1%) and, under moderate drought, chlorophyll b (−6.6%). Chlorophyll a increased at 700 and 1000 ppm under mild drought (+35.4% and +16.3%) but decreased at 1000 ppm under moderate drought (−21.8%). Effective quantum yield of photosystem II decreased at both CO2 levels (−14.4% to −19.2%), except at 700 ppm under moderate drought. Relative to well-watered aCO2 plants, 1000 ppm countered mild and moderate drought impacts on biomass and increased biomass under mild drought (+8.3%); however, 700 ppm only countered mild drought impacts. Yet these growth benefits were not directly explained by the measured physiological variables. Overall, similar to eCO2 responses reported under favorable temperatures, 1000 ppm mitigated drought effects on the growth of T. divaricata under high growing temperatures.