Genotypic differences in water deficit effects on leaf and crown traits in mature field-grown cocoa

Abstract

Cocoa trees are highly sensitive to water stress but these negative effects may differ genetically and may be mitigated by potassium (K) application. We studied these effects on six-year-old adult trees, five genotypes produced by somatic-embryogenesis, and one by cross pollination (Hybrid), grown under sub-optimal field conditions in central Côte d'Ivoire. Trees were subjected to two soil water treatments (with or without dry season irrigation) and two K fertilization levels (with and without K). We assessed interactive effects of treatments on several leaf and crown traits: sap flux density (SF), leaf water potential (LWP), stomatal conductance (Gs), leaf size (LS), specific leaf area (SLA), leaf water content (LWC), leaf area index (LAI), light interception, litterfall and a visual index of whole-plant water stress (WSI). Mixed-effects model results revealed that withholding irrigation negatively affected SF, LWP, Gs, LS, SLA, LAI and light capture and increased WSI scores. Potassium application did not significantly mitigate the negative effects of withholding irrigation. A significant effect of genotype was observed for most of the monitored leaf physiological, morphological and crown traits. Furthermore, we found significant, positive interactions between genotype and irrigation for SF, LWP, leaf area, LS, SLA, LWC, LAI and intercepted light, suggesting that the differences in observed responses to drought conditions are genotype-specific. These results provide insights into the acclimation strategies of cocoa and genetic variation therein, and can be used to select drought-tolerant genotypes.