Evaluation of Optical Chlorophyll Meters as Proxy for Nitrogen in Theobroma cacao L.

Leaf nitrogen (N) and leaf chlorophyll concentrations are key indicators of a plant’s nutritional status. For many crops, readings from simple optical methods for estimating chlorophyll correlate well with laboratory-based chlorophyll extraction methods. Yet, in cacao (Theobroma cacao L.), an important tree commodity crop, optical measuring devices for chlorophyll are widely used but are not yet validated against actual leaf chlorophyll concentrations. Moreover, cacao leaf chlorophyll has not been demonstrated to be a good proxy for cacao tree N status. Closing these knowledge gaps to support cacao farmers in determining the N status of their trees was the goal of this study. The experiment was conducted with leaves from mature trees raised under five N fertigation treatments ranging from 10 to 150 mg·L^–1 N (10, 25, 50, 75, and 150 mg·L^–1 N). The two tested optical meters (SPAD-502 and MC-100) were found to be suitable for chlorophyll content determination, showing good linear correlations with in vitro chlorophyll concentrations (R² 5 0.87 for the SPAD-502 and R² 5 0.83 for the MC-100). Leaf chlorophyll concentrations by all three types of measurement increased as leaf N increased up to a value of 3.2% leaf N (75 mg·L^–1 N fertigation). None of the chlorophyll measurement techniques were sensitive to higher levels of leaf N (3.4%, 150 mg·L^–1 N fertigation). Nitrogen concentrations in the leaves of the two lowest N treatments (N10 and N25) are considered deficient and low, respectively. Leaf N concentrations in the N50 and N75 treatments are in the normal to high ranges. Leaf N in the N150 treatment is considered excessive. Trees in the different treatments indeed displayed many signs characteristic of their nutritional status, including influence on timing and intensity of flowering, and fruit set. These findings confirm that optical meters can be a dependable and easy-to-use tool for assessing the nutritional status of cacao trees, and can guide fertilization reliably to minimize N deficiencies.