Cocoa Clones Reveal Variation in Plant Biomass, Root Nitrogen Uptake, and Apparent Nitrogen Recovery at the Seedling Stage

We aimed to investigate variation in growth, biomass, macro-nutrient uptake, and nitrogen (N) use efficiency indices of cocoa (Theobroma cocoa L.) scion clones under two different nitrogen conditions. Ten clones were top-grafted onto 4-month-old rootstocks clone MCC 02. Each seedling received either 0 or 4.6 g of nitrogen applied as urea fertilizer. The design was arranged as a two-factor experiment with four replicates in a factorial randomized block design. We measured stem diameter, biomass, nutrient concentration, and calculated nutrient uptake, nitrogen use efficiency (NUE), agronomic efficiency (AE), apparent nitrogen recovery (ANR), and physiological efficiency (PE). We found that clones significantly affected stem diameter and shoot biomass of both rootstocks and scions. Clones differed in root magnesium (Mg) and shoot calcium (Ca) concentration, root N, and shoot Mg uptake. Furthermore, a significant clone effect was observed on ANR, with local clones showing higher ANR than introduced clones. Nitrogen application significantly improved stem diameter and biomass. Compared to unfertilized plants, a significantly higher N concentration was observed in roots, resulting in higher N uptake in both roots and shoots. Nitrogen application increased shoot Ca concentration and uptake but decreased shoot phosphorus (P) and Mg concentrations. Our results suggest genotypic variability in nitrogen uptake efficiency in clonal cocoa. Clones with high nitrogen uptake efficiency are valuable genetic resources for breeding cocoa clones that efficiently use available N and, therefore, grow better under N-deficient conditions.