Optimization of Natural Plant Growth Regulators for Enhancing Early Growth of Wedge-Grafted Robusta Coffee (Coffea robusta L.) Seedlings

The availability of uniform and high-quality robusta coffee seedlings remains a major constraint in nursery production, particularly due to suboptimal shoot initiation following wedge grafting. The use of environmentally friendly natural plant growth regulators (PGRs) is urgently needed to improve early graft performance while reducing dependence on synthetic hormones. This study aimed to evaluate the effects of different types and concentrations of natural PGRs on the growth of wedge-grafted robusta coffee (Coffea robusta L.) seedlings. The experiment was conducted using a factorial Completely Randomized Design with three natural PGR sources (shallot extract, bean sprout extract, and coconut water) and four concentration levels (25%, 50%, 75%, and 100%), with three replications. Growth parameters observed at 90 days after grafting included graft survival percentage, shoot emergence time, shoot number, longest shoot length, leaf number, and leaf area. Results showed that PGR type significantly affected leaf area and had a highly significant effect on shoot number, with coconut water producing the highest shoot number (2.54 shoots) and shallot extract resulting in the largest leaf area (42.29 cm²). PGR concentration significantly influenced shoot emergence time, shoot length, and shoot number, with optimal values observed at 25% (23.76 days), 48% (3.6 cm), and 68.75% (2.5 shoots), respectively. Significant interaction effects indicated that bean sprout extract at 25% accelerated shoot emergence, shallot extract at 55% enhanced shoot length, and coconut water at 60.7% increased leaf number. In conclusion, natural PGR effectiveness in grafted robusta coffee seedlings is strongly dependent on both type and concentration, highlighting the importance of optimized hormonal management in sustainable coffee propagation systems.