A SIMPLE PROTOCOL FOR MICROPROPAGATING DIPLOID AND TETRAPLOID WATERMELON USING SHOOT-TIP EXPLANTS

Shoot-tip explants from 21-day-old aseptically-germinated watermelon seedlings were incubated on solidified MS medium containing test concentrations of benzyladenine (BA) and kinetin (each at 0, 1, 5 or 10 muM), and thidiazuron (TDZ; 0, 0. 1, 1 or 5 muM) for 8 weeks. Approximately 1.5x-2.8x more axillary shoots formed at the optimum BA level (1 muM) compared to the best TDZ (0.1 muM) or kinetin (10 muM) concentration. The ability of various diploid and tetraploid genotypes to undergo prolonged axillary shoot proliferation on medium with 1 muM BA was examined. Among the genotypes tested, the number of axillary shoots per explant was greater for 'Bush Jubilee' and 'Jubilee II' than for 'Minilee', 'Dixielee', and the tetraploid genotypes. For a majority of the genotypes tested, the number of shoots per explant was low (2.7-4.0) during the first month of culture, peaked (5.3-12.5) at 2 to 3 months, and then declined (3.7-7.7) at 6 months. In contrast, the number of shoots per explant was greatest (11.7) for 'Bush Jubilee' during the first month of culture and declined to 7.7 by the sixth subculture. The percentage of rooted shoots varied from 60% to 100% and the percentage of acclimatized plants ranged from 21% to 96% depending on the genotype and the length of time in culture. Using this procedure, 13,200 finished plants could be produced in 3 months from 250 seedlings.