In coffee (Coffea arabica L.), vegetative and reproductive growth occur simultaneously, creating strong competition for assimilates and constraining biomass allocation. Although fruits are the dominant sink, quantitative relationships between fruit load, vegetative growth, and harvest index (HI) remain poorly understood in perennial crops. We evaluated how fruit load regulates vegetative biomass accumulation, yield, and HI in field-grown coffee trees and proposed a practical framework to quantify biomass partitioning under contrasting reproductive demand. Three field experiments were conducted in southeastern Brazil over two growing seasons, where six fruit load levels (100%, 80%, 60%, 40%, 20%, and 0%) were manually imposed at the pinhead stage. Vegetative growth, leaf area, fruit yield, and aboveground biomass were measured. Increasing fruit load markedly reduced vegetative biomass by up to 79% compared with trees without fruits, suppressing leaf and branch development. Each kilogram of fruit dry matter produced was associated with a reduction of approximately 392 g of vegetative dry matter. HI increased with fruit load but stabilized at approximately 0.75 once fruit dry yield exceeded 1.4 kg tree−1, indicating a physiological upper limit to reproductive allocation. These values, based on branch and leaf biomass, represent an upper estimate because whole-plant HI including roots and perennial woody tissue would be lower. The plateau likely reflects the need to maintain sufficient vegetative structure to sustain photosynthesis and assimilate transport despite high reproductive demand. These results clarify source-sink regulation in coffee and provide a robust framework for estimating biomass partitioning with applications in crop management and growth modelling.
