Biochar-Mediated Seasonal Stress Alleviation Enhances Rooting Performance in Coffea arabica F1 Hybrids

The vegetative propagation of elite Coffea arabica F1 hybrids is constrained by metabolic instability and the oxidative breakdown of auxins during periods of seasonal stress. While biochar is recognized for improving soil fertility attributes, its potential to function as a biochemical modulator of the rhizogenic enzymatic system remains unexplored. This study assessed the interactive effects of biochar and seasonal variability on the rooting physiology of Starmaya F1 hybrid coffee. The rooting success, root number and root length, enzyme activities and endogenous hormones were investigated from different biochar-amended substrates (0–40% v/v; standard river sand) across three defined climatic periods rainy season (mid-August–October), dry season (mid-November–mid-March) and a transition season (mid-March–mid-July) during the 2024-2025 cropping season. Biochar amendments at 20–30% (v/v) significantly (p < 0.01) increased rooting success from 52.3% to 84.3%, with the highest efficacy observed during the transition season (18–24°C, 65–75% humidity). Improvements were attributed to increased substrate water retention (32–45%), enhanced aeration (18–28%), and pH buffering. Biochar amendments (20–30% v/v) also significantly suppressed Indole-3-acetic acid oxidase (IAAO) activity by 31% during the critical root induction phase. This enzymatic suppression effectively preserved the auxin pool (41% increase) and optimized the auxin:cytokinin ratio, creating a favorable hormonal milieu for root development. These findings demonstrate that biochar enhances rooting not only by improving physical substrate properties but also by physiologically regulating the auxin-oxidase system, providing a robust, climate-resilient protocol for commercial coffee propagation.