Abstract/Summary

Climate change is leading to increased temperature worldwide, however the impact of this on crops needs to be considered in conjunction with accompanied increases in CO2 concentration ([CO2]). This study examined the combined effects of elevated [CO2] and increased temperature on pollen viability, pod set, pod growth and bean yield in cacao. All pods were established by hand pollination. Mature cacao trees of three genotypes (CCN 51, SCA 6, and T 85/799) were grown in a six compartment, controlled environment glasshouse adapted for cacao studies under non-limiting water and nutrient supply. Trees were exposed to three day/night temperatures; 31/22°C (Tc), 33.5/24.5°C (Tc+2.5°C) and 36/27°C (Tc+5.0°C) combined with two [CO2] treatments; ambient (ca. 437 ppm) and elevated (ca. 693 ppm) in a factorial design for 553 days. The control temperature regime (Tc) was based on temperatures that are common for West African cacao regions. Pollen germination and pollen tube length were negatively affected by a 5°C increase in temperature; this was particularly evident for SCA 6 and T 85/799. However, elevated [CO2] largely compensated the negative effect of high temperature in all three genotypes. The percentage of pods set decreased with an increase in temperature, and the highest percentage of wilted pods was observed at 33.5/24.5°C. However, elevated [CO2] enhanced pollination success and pod set, and reduced the percentage of pods wilted across all temperatures. At ambient [CO2], pod size declined with increases in temperatures, whereas at elevated [CO2] an increase in temperature resulted in larger pods in CCN 51 and little change for SCA 6. A similar trend was observed for pod dry weight. A positive effect of elevated [CO2] on individual bean dry weight was observed at the higher temperatures for CCN 51, whereas little effect was seen in SCA 6. For most of the reproductive components studied here, it can be concluded that elevated [CO2] mitigates to a greater or lesser extent the negative effect of elevated temperatures. Pod growth and bean yield were more responsive to elevated [CO2] under warm conditions in CCN 51 than SCA 6 suggesting there is some scope for selecting genotypes better suited to a changing climate.