Cucurbit Genetics Cooperative Report 1:21 (article 21) 1978
L. R. Barnes, F. D. Cochran, R. L. Mott and W. R. Henderson
North Carolina State University, Raleigh, NC 27650
Plant tissue culture offers a potential tool for rapid clonal propagation of most herbaceous plants. For plant breeders, these techniques would allow the rapid multiplication of a particularly desirable genotype; a potential for cost reduction of F1 hybrids where seed costs are high due to hand labor; and specifically, in the propagation of triploid hybrid plants (e.g. seedless watermelon) and/or their diploid and tetraploid parents.
Attempts to promote adventitious shoots (i.e. shoot apices which arise de nova from differentiated cells) from callus of some species of the Cucurbitaceae has been unsuccessful (Cucumis spp. (3)) or has occurred sporadically and at a low frequency for others (Cucurbita pepo (2)). Stimulation of existing axillary meristems can provide shoots which could be used to produce rooted plantlets, or subcultured with continued growth and formation of additional axillary meristems. This repeated stimulation and subculture would allow a geometrical rate of increase for asexual propagation.
Such a system of propagation could potentially be used to reduce the expense of triploid watermelons which cost approximately $150/lb of seed (plus the cost of growing and transplanting the seedling plants to the field) compared to $6.00/lb for standard diploid cultivars (direct seeded).
Micropropagation of excised axillary buds and seedling shoot tips of triploid watermelons (Citrullus lanatus (Thumb.) Matsum. & Nakai) has been successful in our laboratory. Excised seedling shoots (1-3 mm) with attached cotyledon have been cultured on a modified Murashige and Skoog (M & S) high salts media (4). Optimal axillary bud break occurred with 4.4 5M/l Kinetin (~ 1ppm) and .28 5M/l Indole Acetic Acid (IAA) (~.05 ppm) under long day (16/8 hrs, light/dark) and low light intensities (~1000 lux). An average of 4.6 axillary shoots of sufficient size to subculture were obtained from each seedling shoot tip in 35 days. These shoot-cuttings were excised and rooted in one-to-two weeks by subculturing to M & S media with no cytokinins and higher auxin (.28 – 1.43m M/l IAA). Rooted plantlets were transferred to soil and normal growth resumed. Alternately, these shoots could be subcultured to fresh media of the original high cytokinin/ low auxin ratio, where additional precocious bud break occurred from each new leaf axil. With this continued rate of growth and subculture, 9,000 plantlets could be obtained in six months or 89 million (4.612) in one year from one original seed.
Excised buds from greenhouse grown vines have also been stimulated to provide shoots from axillary meristems. However, these grow and produce axillary shoots at only 1/3 the rate of the seedling shoots. One might expect that additional research will allow an increase in this lower multiplication rate to be more comparable with that of the seedling shoot.
The practicality of such a system has yet to be proven for watermelon. No cost estimates are presently available for watermelon. However, recent estimates for large scale production (~11,000 finished transplants/week) of broccoli plants produced by similar methods, based on a five-fold increase in five weeks, projected a total cost of 15.4″ per plantlet of transplantable size (1). After initial costs for establishing a laboratory, the highest percentage of cost is due to labor requirements. These costs could conceivably be reduced by modifying this system by using larger culture vessels with media supplied through capillary mats. Thus, large numbers of explants could be handled at one time, and nutrients and growth regulators could be easily exchanged.
Literature Cited
- Anderson, W. C., G. W. Meagler and A. G. Nelson. 1977. Cost of propagating broccoli plants through tissue culture. HortScience 12(6):543-544.
- Jelaska, S. 1974. Embryogenesis and organogenesis in pumpkin explants. Physiol. Plantarum 31:257-261.
- Fadia, V. P. and A. R. Mehta. 1976. Tissue culture studies on Cucurbits: Chlorophyll development in Cucumis callus cultures. Phytomorphology. 26(2):170- 175.
- Murashige, T. and F. Skoog. 1962. A revised medium for rapid growth and bioassays with tobacco tissue cultures. Physiol. Plantarum. 15:473-497.