Abstract/Summary

Despite multiple studies of the impact of climate change on temperate tree species, experiments on tropical and economically important tree crops such as cacao (Theobroma cacao L.) are still limited. Here, we investigated the combined effects of increased temperature and [CO2] on the growth, photosynthesis, and development of juvenile plants of two contrasting cacao genotypes: SCA 6 and PA 107. The factorial growth chamber experiment combined two [CO2] treatments (410 and 700 ppm) and three day/night temperature regimes (control: 31/22°C, control+2.5°C: 33.5/24.5°C, and control+5.0°C: 36/27°C) at a constant vapour pressure deficit of 0.9 kPa. At elevated [CO2], final dry weight, total and individual leaf area increased in both genotypes, whilst duration for individual leaf expansion declined in PA 107. For both genotypes, elevated [CO2] also improved light-saturated net photosynthesis (Pn) and intrinsic water-use efficiency (iWUE), whereas leaf transpiration (E) and stomatal conductance (gs) decreased. Under a constant low vapour pressure deficit, increasing temperatures above 31/22°C enhanced rates of Pn, E, gs, in both genotypes suggesting that photosynthesis responds positively to higher temperatures than previously reported for cacao. However, dry weight, total and individual leaf area declined with increases in temperature being more evident in SCA 6 than PA 107, suggesting the latter genotype was more tolerant to elevated temperature. Our results suggest that the combined effect of elevated [CO2] and temperature is likely to improve the early growth of high temperature-tolerant genotypes, while elevated [CO2] appeared to ameliorate the negative effects of increased temperatures on growth parameters on more sensitive material. The evident genotypic variation observed in this study, demonstrates scope to select and breed cacao varieties capable of adapting to future climate change scenarios.