Abstract/Summary

Cacao (Theobroma cacao L.) is widely farmed throughout the humid tropics where it is an important crop to smallholder farmers. Climate change projections suggest increased temperatures and altered rainfall patterns in these regions. The effects of two key climate change variables; elevated carbon dioxide (CO2) concentrations and water deficit were investigated on the reproductive development of six genotypes of cacao (CL19/10, ICS 1, IMC 47, Pound 7/B, SCA 6, and SPEC 54/1). Genotypes showed variability in their response to treatments; however general observations were also made. At elevated CO2 (700ppm) the mean length of pollen tubes decreased and the degree of fluctuation in flowering intensity increased over time without any overall reduction in flower production. Final pod size and maximum rate of growth increased in response to elevated CO2 in the second year of study. Additionally in the second year there was an increase in husk weight and thickness, and individual bean weight. There was no increase in bean number or shell percentage. Total cocoa butter content and percentage unsaturated fat was lower under elevated CO2 along with reductions in stearic, oleic and linoleic fatty acid content; however these responses varied between genotypes and sampling period. Final woody biomass increased under elevated CO2 whereas leaf biomass was unaffected. Water deficit stress had no observed effect on pollen performance, flowering behaviour, or cocoa butter content and composition. Final pod size and rate of growth was reduced under water stress in the first year of study only. The reduced pod size was reflected in reduced husk and bean weights. Final wood and leaf biomass were both reduced in response to the water stress treatment. Across all parameters measured, no evidence of interaction between elevated CO2 and water deficit stress was observed. Overall, these results demonstrate the potential for yield improvement in cacao through breeding in preparation for future climate scenarios.