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Cotyledon explants of five watermelon cultivars (`Desert King', `Mickylee', `Sangria', `Sweet Princess', and `Male Sterile') were prepared from 7-day-old in vitro-germinated seedlings. Explants were incubated on shoot regeneration medium for 6 weeks, followed by several 3-week cycles on shoot elongation medium. The five cultivars differed in their ability to form shoots within 9 weeks on the selected media. Shoot regeneration frequency was about 1.5to 2.9-fold greater for `Mickylee' (60%) than `Sangria' (47%), `Sweet Princess' (27%), `Male Sterile' (26%), and `Desert King' (24%). Rooting of elongated shoots (>2 cm) occurred within 2 weeks on medium containing 1 μM IBA and ranged from 25% (`Desert King') to 92% (`Sangria'). Plantlets were transferred to six-pack containers filled with soilless medium (1 Sunshine Mix: 1 coarse perlite) and covered with a transparent plastic lid. Plants were acclimatized to ambient conditions by gradually removing the lid over a period of 3 days after new growth was observed. The percentage of acclimatized plants ranged from 50% (`Sweet Princess' and `Mickylee') to 100% (`Male Sterile'). Acclimatized plants were transferred to the greenhouse and grown for at least 4 weeks before screening for ploidy variants. Ploidy of regenerated plants was estimated by counting the number of chloroplasts per guard cell pair. Plants with an average of 18 or more chloroplasts per guard cell pair were declared tetraploids. Plants with fewer chloroplasts per guard cell pair were declared diploids and discarded. Tetraploid plants were transferred to the field, grown to maturity, and self-pollinated for seed increase.
For certain crops, the high cost of stand establishment and/or maintenance has stimulated the search for alternative propagation systems. This presentation discusses the possible applications of these systems to high value crops. Germplasm conservation of vegetatively propagated or recalcitrant seeded crops is difficult due to the high cost of stand maintenance. Utilization of synthetic seed technology (i.e., somatic embryos) would allow clonal germplasm to be economically conserved in seed repositories. Many ornamental crops with high per-plant values are now laboriously micropropagated via organ culture. Substitution of synthetic seed technology for propagation of such crops would decrease production costs by reducing labor. Similarly, field establishment of seedless watermelon can cost $0.40 or more per plant, whereas synthetic seed cost has been calculated (for alfalfa) at $0.04 per plant; approximately 10% less than that of conventional seed. In seedless watermelon variety development, simple micropropagation can be used to reduce the time required for commercial seed increase of new tetraploid parental lines from ten years to less than one year.
Experiments were conducted to determine the effect of varying solution N concentrations on fruit yield and NO3-N concentration in leachate from rockwool-grown `Midal' peppers (Capsicum annuum L.) in Florida. Treatment 1 plants received a series of nutrient solutions containing N at 60, 90, and 120 mg·liter–1 (60–90–120 mg·liter–1) during their growth cycle. Plants in treatments 2 and 3 were grown with N at 120 or 175 mg·liter–1, respectively, throughout their entire growth cycle. Two trials were conducted; trial 1 from 17 Nov. 1991 to 1 July 1992, and trial 2 from 31 July 1992 to 23 Feb. 1993. In both trials, total marketable fruit weight was significantly (P ≤ 0.05) higher (16% to 67%) for plants grown with N at 175 than with 60–90–120 mg·liter–1. In trial 2, plants receiving N at 175 mg·liter–1 produced significantly more fruit (8%) and 14% higher total fruit weight than plants receiving N at 120 mg·liter–1. The trend toward higher yield with N at 175 rather than 120 mg·liter–1 also occurred during trial 1, but differences were not significant. Nitrogen concentration did not significantly affect the percentage of total fruit having blossom-end rot in either trial (41% in trial 1; 13% in trial 2). Nitrogen at 175 mg·liter–1 resulted in 10% to 40% increases in total nutrient solution use and 2.5- to 3.5-fold increases in leachate NO3-N concentration compared to N at 120 mg·liter–1.
Many nutrient recommendations for greenhouse production of vegetable crops were developed in northern climates and may not be optimum for Florida production. Experiments were designed to determine nitrogen (N) levels that would maximize yield of rockwool-grown peppers (Capsicum annuum `Midal') in Florida, while reducing nitrate leaching. Treatment 1 plants were fed 60, 90, and 120 ppm N during vegetative, early fruit, and late fruit stages, respectively. Plants in Treatments 2 and 3 were grown at 120 and 175 ppm N, respectively, throughout their entire growth cycle.
In Trial 1, increasing N did not affect the number of marketable fruit produced, but increased fruit size. Marketable fruit weight was significantly greater for plants in Treatment 3 compared to Treatment 1. However, there was not a significant difference in marketable yield between plants grown at 120 ppm N and 175 ppm N. Excess N provided by the 175 ppm N treatment caused a 10% increase in total water use and a 250% increase in nitrate-N in the leachate compared to the 120 ppm N treatment. Nitrogen level did not affect blossom end rot (BER) occurrence. Early results of Trial 2 indicate higher occurrence of BER with increasing N concentration and are again showing that 120 ppm N will maximize yield and reduce environmental impact of greenhouse pepper production in Florida.
Design modification of a particle inflow-type gun for particle bombardment significantly simplifies construction and reduces fabrication time. The gun consists of a high-speed electric solenoid valve mounted on and through a vacuum jar. DNA-coated tungsten particles are placed on the support grid of a filter housing and accellerated by a burst of pressurized helium, which is controlled by a timer. Specimens are held between plastic screens and their distance from the particle support grid is adjusted with a miniature laboratory apparatus positioner. Transient expression of GUS in cantaloupe cotyledons and grape somatic embryos was equivalent to that obtained with a conventional particle inflow gun. The device was constructed with locally-available hardware in 40 minutes using a hand drill, some thread taps and a thread die.
Cotyledon explants were established from 5- to 6-day-old `Minilee' seedlings germinated on MS medium with 2% sucrose and 0.7% TC agar, and precultured for 2 days on modified MS as above but with 3% sucrose and 5 μM BA before bombardment. Plasmid DNA [pUC221 and pUC472, which contain the β-glucuronidase (GUS) and NPTII (kanamycin resistance) genes, respectively] was delivered to the explants using a modified particle inflow gun following precipitation on to 1.1 μm tungsten particles. Explants infected with Agrobacterium (strain LBA4404 with the binary vector pBI121, which contains genes for GUS and kanamycin resistance) were not precultured prior to cocultivation for 4 days. GUS expression was measured 1 week after transfer to selection medium. Infection with Agrobacterium was the best method for delivering foreign DNA to watermelon cotyledons. Up to 300 GUS-expressing colonies were observed per explant following infection with Agrobacterium compared to 128 after bombardment.