Effect of elevated CO2, high temperature, and water deficit on growth, photosynthesis, and whole plant water use efficiency of cocoa (Theobroma cacao L.)

In this study, the response of 6-month-old cocoa (Theobroma cacao L.) seedlings to elevated CO2 concentration [ECO2], elevated temperature [ET], and their interaction with water deficit stress was studied in an open top chamber (OTC). Each OTC was maintained at chamber control (400 ppm CO2), [ECO2] 550 ppm, [ECO2] 700 ppm, ET 3 degrees C above chamber control, and ET 3 degrees C + [ECO2] 550 ppm. Inside each OTC, a set of plants received moisture at 100% FC, while the other set was at 50% FC, which was the water deficit stress treatment. Increasing the CO2 concentration in cocoa increased photosynthesis (Pn) by 27%, which resulted in high biomass accumulation, thus improving the whole plant water use efficiency (WUE). The impact of high temperature (T-max), around 39 degrees C in ET treatment against 36 degrees C in chamber control, is quite severe on Pn, leaf psi, and biomass accumulation. Similarly, water deficit at 50% FC resulted in the leaf psi reducing to - 14.06 bars at which Pn, leaf area, and biomass were significantly reduced. [ECO2] could ameliorate the negative effect of high temperature and water deficit stress to certain extent. However, the relative response of cocoa seedlings to [ECO2] in improving Pn, leaf psi, biomass, and WUE was greater under 50% FC compared to plants at 100% FC suggested additional advantage of [ECO2] to cocoa under water limited conditions.