Ecophysiological response of cacao (Theobroma cacao cv. Criollo and cv. Trinitario) in a Southern Belizean rainforest agroforestry system across light regimes

Abstract

Theobroma cacao is a tropical crop of economic and cultural importance in Southern Belize. Cacao plants, like coffee, are commonly grown under shade trees to reduce physiological stress affecting plant longevity. However, few studies have explicitly compared ecophysiological performance and plant stress responses of commonly cultivated varieties across a gradient of light. In this study, a suite of physiological responses linked to light-use (i.e., capture and processing) and plant stress among cacao agroforestry plants were investigated comparing multiple cultivars, wild-grown cacao, and cacao saplings exposed to a light gradient from full sun to nurse-plant shaded. Comparisons between cultivars demonstrated that ‘Trinitario’ performs similarly or higher than ‘Criollo’ as evidenced by similar photosynthetic rates relative to cultivated Criollo (6.3 ± 1.4 vs. 3.5 ± 1.3 μmol m⁻² s⁻¹) and higher linear electron flow rates than wild Criollo (58 ± 32 vs. 47 ± 34 μmol e⁻ m⁻² s⁻¹), though at the trade-off of more physiological stress. For example, the photochemical reflectance index of Trinitario was higher compared to cultivated Criollo (0.02 ± 0.007 vs. 0.04 ± 0.01). Mature agroforestry-grown Criollo cacao and wild grown Criollo showed similarities for both light-use and stress responses. Cacao plants grown under nurse-plant shaded conditions exhibited reduced light-use and stress responses relative to those grown under partial- to no shade. Across cacao stands, increased light capture and processing was coupled with increased plant stress, and reduced leaf temperatures were associated with reduced plant stress. These results reinforce the importance of shade optimization for sustainable cacao agroforestry.