Evaluating Water Deficit Responses with a Detached Leaf Method in Cacao (Theobroma cacao)

Cacao (cocoa, Theobroma cacao) yield and seedling establishment are highly sensitive to water deficit, and in the face of climate change and the increasing effects of drought on agricultural production, a thorough understanding of water deficit responses is necessary to secure cocoa production through breeding for climate-resilient genotypes. The purpose of thisresearch was to develop a method to efficiently screen for water deficit tolerance traits in cacao using detached leaves to simulate conditions of limited water availability. To assess suitability of the method in reflecting whole-plant responses to water deficit, comparisons of several traits were made in parallel between detached leaves of seedlings and these seedlings exposed to water deficit as whole plants. A high-performance thin-layer chromatography method to quantify leaf pigments was developed, calculations of leaf minimum conductance (gmin) were made, and metabolomics analyses were conducted to assess comparability between the methods. β-carotene, chlorophyll a, and chlorophyll b concentrations showed similar responses in detached and water deficit seedling leaves; however, these responses were inconsistent in a repeated experiment. While gmin estimates showed agreement between detached leaves and whole plants, results were somewhat inconsistent between repeated experiments. Metabolomics analyses revealed that amino acids and related compounds were well-represented among the metabolites with the greatest changes in abundance due to detached leaf and water deficit treatments. These responses were generally similar between treatments as well, suggesting potential indicators for use to study whole-plant responses in broader studies. Additional studies are needed to refine the detached leaf assay by evaluating the effects of ambient conditions and the rate of water loss on water deficit responses. Overall, this research enables future water deficit experiments and provides hypotheses for metabolic mechanisms driving water deficit tolerance