UC Davis

Cacao (Theobroma cacao) is a tropical understory tree predominantly harvested for its beans which provide the raw materials for chocolate. Increasing production of this crop through improvements in pest and disease resistance has been a primary breeding target for decades. However, characterization and development of cacao with tolerance to abiotic stressors such as water deficit has been neglected. As a rainfed tree with minimal fertilizer inputs, cacao is grown in areas that experience months-long periods without any rainfall creating an urgent need to assess the diversity of cacao responses to water deficit stress. Furthermore, large amounts of nitrogen removed from cacao via harvest, has created production systems with nitrogen deficits. Improving cacao's ability to handle abiotic stress by evaluating whole-plant traits helps to better understand its complex stress responses.

The main objectives of this work were to identify morphophysiological responses, microbes, and genes that enhance cacao survival and growth under water deficit stress and to study the effects of nitrogen deficiency on cacao physiology, growth, and its interactions with other vital nutrients. The two main hypotheses are as follows: (1) distinct populations of cacao will have measurable disparate responses to water deficit stress with respect to rhizosphere community, growth and physiology responses, and gene expression patterns and (2) in regard to nitrogen alone, cacao growth and physiological responses would be stunted as a result to nitrogen deficit. A water stress study was performed using 120 trees derived from open-pollinated pods of four maternal genotypes. This study was conducted with subsets of 20 trees in six experiments that introduced seasonal variation within the study. Morphological and physiological traits were collected and compared between the water treatments and genotypes. Trunk growth rate and total biomass growth rate were most affected by the seasonal variation and treatment with no strong differences among maternal genotypes. In addition to the effects of experiment and treatment, the physiological data was also impacted by maternal genotype. Transcriptomic data was collected from leaf tissue to identify genes potentially involved in the water deficit response in cacao. This data further established strong seasonal variation in the strength of response experienced by cacao plants. Experiments that were conducted during higher temperatures and longer daylengths faced an amplified stress to water deficit. Soil bacterial abundance and diversity were assessed with amplicon sequencing. The results from this portion of the study identified no significant impact of the water deficit treatment on the community dynamics of soil microbes. Seasonal weather variation yielded the strongest response to soil bacteria. Finally, through a nitrogen gradient experiment, cacao response to nitrogen application was assessed in two differing maternal genotypes. Decreased growth and depressed physiological performance were observed in plants under nitrogen deficient conditions demonstrating that the deficiency of a single resource strongly limits cacao productivity. This response, however, was not found to be different between genotypes. In conclusion, these experiments provide evidence that (1) cacao water stress response is impacted by seasonal weather conditions and (2) cacao is a nitrogen responsive crop with limited genotypic differentiation between nitrogen responsiveness.