Evaluating the combined effects of climate change parameters on growth and physiology of Theobroma cacao L.

Climate change scenarios predict increases in the atmospheric carbon dioxide concentration [CO2] leading to global warming and changes in rainfall patterns in tropical regions. This will potentially impact the sustainability of cacao production and the livelihoods of millions of smallholder cacao farmers. Through a series of experiments, carried out under controlled environment facilities (growth cabinets and greenhouses), this research aimed to examine the interactive effects of elevated [CO2], temperature and water deficit on the growth and physiology of juvenile and mature plants of different cacao genotypes. Elevated [CO2] improved photosynthesis and growth parameters in cacao plants. However, the enhancement of growth was more evident in seedlings than matures trees. In seedlings, elevated [CO2] shifted the optimal temperature of photosynthesis by 2.5°C under warming conditions, suggesting potentially increased resilience of cacao to increased temperatures under higher [CO2] when air humidity and soil water is not limited. However, above 36/27°C (day/night) the compensatory effect of elevated [CO2] diminished. The negative effect of increased temperatures on growth and leaf area in juvenile and mature cacao plants, as well as on aspects of reproductive development (pollen viability and fertilisation success), pod growth and pod and bean biomass, were alleviated by elevated [CO2]. However, the apparent susceptibility of some genotypes to increases in temperature seemed to regulate the extent to which elevated [CO2] alleviated the negative impacts of temperature. Under the water-limited treatment, the compensatory effect of elevated [CO2] on photosynthesis and growth of seedlings was still observed but at a lower magnitude. However, temperature increases above 36/27°C exacerbated the adverse effect of water deficit. Genotypic variation in response to the different climate parameters demonstrated the potential for breeding cacao to cope with future scenarios. The results have shown that the impact of climate parameters on cacao are dynamic and interactive in nature, and the effect of a single climate variable may be modulated by others when they occur in combination