Comparative micropropagation efficiency of diploid and triploid mulberry (Morus alba cv. S1) from axillary bud explants
Micropropagation ability of two cytotypes (2n = 2x = 28 and 2n = 3x = 42) of mulberry (variety S1) was tested from axillary buds through organogenesis. Both shoot and rootlets development periods varied considerably with ploidy variations. The triploid shoots generally grew more vigorously in Murashige and Skoog (MS) medium fortified with 6-benzyl amino purine (6-BAP) than those of diploids. Maximum shoot initiation frequency (80%) was obtained with 4.4 μM of 6-BAP on 4.2 days of culture in triploid; whereas, the axillary buds sprouted within 3.5 days with 100% shoot initiation frequency in diploid cultures. Highest shoot length (4.8 and 5.6 cm per explant) was obtained at 8.8 and 4.4 μM 6-BAP supplementations in diploid and triploid cytotypes, respectively. Subsequently, regenerated shoots were transferred to auxin rich rooting media supplemented with either the different doses of indole acetic acid (IAA) or naphthalene acetic acid (NAA). Root initiation was more vigorous in diploid than triploids with both the tested growth regulators. Maximum number of rootlets per shoot (15) and root length (4.2 cm) were observed in MS basal medium supplemented with 4.0 μM NAA after 21 days of culture of diploid plants; while in triploid cytotype, maximum rootlets per shoot (8.3) were noticed with 4.4 μM NAA after 21 days of culture. Rooted plantlets were hardened in plastic cups containing sterile sand and soil mix (1:1; w/w) for 21 days and subsequently transferred to clay pots (l x b x h: 12 x 12 x 12 cm) in shade with a survival of 65 and 52% for diploid and triploid cytotypes, respectively. The time required for field establishment of micro-propagated triploid was about 60 days.
Key words: Morus alba, diploid, axillary bud, organogenesis, shoot regeneration, triploid, hardening.