Billy B. Rhodes

Nutrient utilization in liquid/membrane system for watermelon micropropagation

Watermelon [Citrullus lanatus (Thunberg) Matsumura and Nakai] proliferating shoot meristems from established shoot cultures were inoculated on modified Murashige and Skoog salts medium supplemented with 10 μM 6-benzyladenine (BA) for shoot proliferation and on similar medium supplemented with 1 μM BA and 10 μM gibberellic acid (GA3) for shoot elongation. Agar-solidified medium and microporous polypropylene membrane rafts in liquid medium were used to support the tissues. Growth over culture time of proliferating and elongating tissues in liquid and agar-solidified media were compared. Nutrient depletion in liquid medium was monitored and quantified using ion selective electrodes. Tissue fresh weights in both proliferation and shoot elongation media were greater in liquid than in agar-solidified medium. Relative dry matter content, however, was greater in agar-solidified than in liquid medium. More shoots elongated in agar-solidified than in liquid medium. The numbers of buds or unelongated shoot meristems, however, were comparable for both the liquid and agar-solidified medium. Proliferating and elongating tissues in liquid medium used Ca++ and K+ minimally. NO3− was utilized but not depleted by proliferating tissues. NH4+, however, was depleted. Most of the NH4+ was utilized by the proliferating tissues within 21 days of culture when growth rate was greatest. At 35 days, residual Ca++, K+, NO3−, and NH4+ in proliferation medium were 81.0%, 67.8%, 55.7%, and 1.2% of initial levels, respectively. NO3− and NH4+ in shoot elongation medium were depleted. The greatest NO3− and NH4+ utilization was observed during the first 14 days of culture when the largest growth rate was obtained. The residual Ca++, K+, NO3− , and NH4+ in shoot elongation medium at 38 days were 63.5%, 37.9%, 21.2%, and 24.3% of initial concentrations, respectively. At the end of experiment, 72.3% and 42.8% of initial sugars were still remaining in the shoot proliferation and shoot elongation medium, respectively.

Picolinic acid-induced direct somatic embryogenesis in sweet potato

Somatic embryos are being considered as an alternative material for in vitro germplasm conservation of sweet potato [(Ipomoea batatas (L.) Lam.)]. Picolinic acid was tested for somatic embryo production in sweet potato apical meristem tip cultures. Low level (0.2 mgl-1) of picolinic acid combined with kinetin or 6-benzylamino purine (6-BAP) (1.0 and 2.0 mgl-1) suppressed shoot growth and induced callus proliferation. Increased amount of picolinic acid (2 and 3 mgl-1) in combination with kinetin (0.25 and 1.0 mgl-1) induced direct somatic embryogenesis from apical meristem tips of variety Regal but not in Jewel. The primary embryos matured and germinated bipolarly yielding whole plantlets and unipolarly producing embryogenic hyperhydrated-fasciated shoots. The hyperhydrated-fasciated shoots, when cultured in picolinic and kinetin-enriched medium, produced secondary embryos. The secondary embryos also germinated bipolarly and unipolarly, resulting in subsequent cycles of embryogenesis. This recurrent embryogenesis ensures maintenance and proliferation of embryogenic tissues. Somatic embryos were also formed in mannitol-induced hyperhydrated shoots in response to picolinic acid and kinetin or 6-BAP treatment. Embryogenesis did not occur in non-hyperhydrated leaf, petiole, and internode sections.

Micropropagation of Citrullus lanatus (Thunb.) Matsum. and Nakai (Watermelon)

The watermelon is documented in hieroglyphs on the walls of Egyptian tombs that are at least 4000 years old. In 1857, David Livingstone found watermelon growing in the Kalahari Desert. In 1959, archeologists found watermelon seeds in a prehistoric cave in Hang-Zhou, China which dated back to 3000 B.C. From its origin in central Africa, it must have first been carried to northern Africa, through Persia, to China and India. European explorers brought the watermelon home with them in the 1500s. Watermelon seeds were carried to North America by colonial settlers from Europe and from Africa by slaves.