Effect of BAP and NAA on callus proliferation and characteristics of papaya (Carica papaya L.) cv. Sekaki explants

Efficient in vitro systems are essential for rapid multiplication and genetic improvement of papaya (Carica papaya L.), a commercially important tropical fruit crop. This study evaluated the effects of different concentrations and ratios of 6-benzylaminopurine (BAP) and α-naphthaleneacetic acid (NAA) on callus proliferation from stem and petiole explants of papaya cv. Sekaki. Explants from 45-day-old seedlings were cultured on Murashige and Skoog medium supplemented with BAP (0–2.0 mg L−1) and NAA (0–4.0 mg L−1), and the four best induction treatments for each explant were further assessed for callus fresh weight, dry weight, length, morphology, and callus growth rate in a randomized complete block design. In stem explants, 2.0 mg L−1 BAP + 2.0 mg L−1 NAA produced the highest biomass (8.13 ± 3.64 g fresh weight; 0.33 ± 0.08 g dry weight), longest callus (3.88 ± 0.61 cm), and highest growth rate (1.27 ± 0.57 g/week), with light yellow, soft, intact tissues, whereas lower NAA concentrations resulted in significantly reduced biomass, growth rate (0.18 ± 0.07 g/week), and friable, browned callus. In petiole explants, 2.0 mg L−1 BAP + 3.0 mg L−1 NAA gave the greatest biomass (10.56 ± 2.07 g fresh weight; 0.37 ± 0.05 g dry weight), length (4.12 ± 0.19 cm), and growth rate (1.68 ± 0.33 g/week) while maintaining light yellow, compact callus structure. Growth rate analysis revealed strong correlations with final fresh weight (r2 = 0.98 for stem; r2 = 0.99 for petiole), validating this parameter as a reliable indicator of proliferation efficiency. Across optimized treatments, petiole explants consistently produced higher biomass, longer callus, and superior growth rates than stem explants, indicating petiole as the preferred tissue for large-scale callus production in cv. Sekaki. These explant-specific, optimized BAP–NAA combinations provide a reproducible protocol for high-quality callus proliferation in papaya cv. Sekaki that can be directly applied as a starting point for micropropagation, somatic embryogenesis, genetic transformation, and in vitro production of bioactive compounds in horticultural breeding and propagation programmes. Optimal BAP: NAA combinations (2:2 mg/L for stem; 2:3 mg/L for petiole) maximized papayacallus biomass and quality, with petiole explants ac hieving the highest growth rate (1.68 g/week) ,providing explant specific protocols for micropropagation and genetic improvement.