Diploid Watermelon Research Articles

Identification of tetraploid regenerants from cotyledons of diploid watermelon cultured in vitro

Adventitious shoots were obtained from the diploid watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] cultivars Dixielee, Jubilee II, Mickylee, Minilee, and Royal Sweet by culturing excised cotyledons on shoot regeneration medium for six weeks. Tetraploid and diploid regenerants were identified by counting the number of chloroplasts per guard cell pair from leaves of regenerated plants. Cross fertilization of putative tetraploids with diploid pollinators and the production of triploid seed confirmed the efficacy of this approach. The mean number of chloroplasts for tetraploid regenerants was 19.1 whereas diploids averaged 11.2. These values were similar to tetraploid and diploid plants from seed. Ovary diameter, petal, and anther diameter of male flowers, and leaf length by width ratio were also good indicators of plant ploidy. Progeny obtained from self-fertile tetraploids of ‘Mickylee’ were crossed with various diploid pollinators to produce triploid hybrid seed. All triploid plants from tissue culture-derived tetraploids produced fruit comparable in quality to fruit produced by currently-available triploid hybrids, demonstrating that in vitro tetraploid induction can be used to produce high quality tetraploid plants for use in triploid hybrid seed production.

Watermelon Seedling Growth, Fruit Yield, and Quality following Pretransplant Nutritional Conditioning

Pretransplant nutritional conditioning (PNC) of transplants during greenhouse production may improve recovery from transplanting stress and enhance earliness and yield of watermelon [Citrullus lanatus (Thumb.) Matsum. & Nakai]. Two greenhouse experiments (Expts. 1 and 2) and field experiments in South Carolina and North Carolina (Expt. 3) were conducted to evaluate N and P PNC effects on watermelon seedling growth and their effects on fruit yield and quality. `Queen of Hearts' triploid and `Crimson Sweet' diploid watermelon seedlings were fertilized with N from calcium nitrate at 25, 75, or 225 mg·liter–1 and P from calcium phosphate at 5, 15, or 45 mg·liter–1. In the greenhouse, most variation in the shoot fresh and dry weights, leaf count, leaf area, transplant height, and root dry weight in `Queen of Hearts' and `Crimson Sweet' was attributed to N. Cultivar interacted with N, affecting all seedling growth variables, but not leaf area in Expt. 2. To a lesser extent, in Expt. 1, but not in Expt. 2, P interacted with cultivar, N, or cultivar × N and affected shoot fresh and dry weights, leaf count and leaf area. In the field, transplant shock increased linearly with N, regardless of cultivar or field location. The effect of PNC on plant growth diminished as the growing season progressed. For both cultivars at both locations, N and P PNC did not affect time to first staminate flower, fruit set, fruit width or length, soluble solids concentration, or yield. Vining at Charleston for both cultivars was 2 days earlier when N was at 75 rather than 25 mg·liter–1, without further change with the high N rate. At Clinton, the first pistillate flower was delayed linearly the higher the N rate for `Crimson Sweet'. At Charleston, hollow heart in the `Queen of Hearts' increased nearly 3 times when N PNC rate was tripled (from 75 or 225 mg·liter–1), while N had no effect on hollow heart in `Crimson Sweet'. In contrast, at Clinton, hollow heart in either cultivar was affected by P PNC, not N. PNC with 25N–5P (in mg·liter–1) can be used to reduce seedling growth and produce a more compact plant for easier handling, yet not reduce fruit quality or yield.

150 THE IDENTIFICATION OF TETRAPLOID REGENERANTS FROM COTYLEDONS OF DIPLOID WATERMELON AND THEIR USE IN BREEDING TRIPLOID HYBRIDS

Tetraploid individuals were identified among plants regenerated from cotyledons of diploid watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] cultured in vitro. Tetraploid and diploid plants were distinguished by counting the number of chloroplast per guard cell pair. The mean number of chloroplasts was 19 and 11 for tetraploid and diploid plants, respectively. Self-fertile tetraploids were obtained from the diploid cultivars Mickylee, Jubilee II and Royal Sweet. `Dixielee' and `Minilee' tetraploids failed to set fruit. Progeny obtained from self-fertile tetraploids were crossed with diploid pollinators to produce triploid hybrid seed. All triploid plants produced seedless fruit that was superior or equal to fruit produced by currently available triploid hybrids. This demonstrates that tissue culture can be used to produce high quality tetraploid plants for use in triploid hybrid seed production.

YIELD OF TRIPLOID WATERMELONS IS INFLUENCED BY POLLENIZERS AND INCIDENCE OF MATURE SEEDS BY PARENTS

Mature seeds occur occasionally in triploid watermelon fruit. In one trial, the average number varied from 0.3 to 28.7 seeds per fruit in 30 entries and from 0.5 to 8.6 seeds per fruit in the cultivars within this group. The frequency of mature seed in triploid fruit with the same tetraploid parent ranged from 0.3 to 3.0 and from 1.25 to 5.0 seeds per fruit in triploid fruit having the same diploid parent. Tetra A, with 151 seeds per fruit, produced triploids with 6 seeds per fruit; whereas Tetra B, with 74 seeds per fruit produced triploids with only 1.3 seeds per fruit. Date-of-flowering of diploid watermelon cultivars used as pollenizers for triploids affected maturity date of the triploids. Icebox-types that flower early produced higher early yields of triploid fruit; whereas standard cultivars that flower later produced higher yields late in the season.

In vitro propagation of watermelon

A 3-stage, in vitro propagation system for diploid watermelon ( Citrullus lanatus (Thumb.) Matsum. and Nakai) has been developed which is also applicable to the economic production of triploid (seedless) watermelon transplants. Stage I involves the stimulation of axillary-bud development from excised seedling shoot tips (1–3 mm) by a high cytokinin (K)/low auxin (IAA) ratio (4.6 μmol/l K 0.28 μmol/l IAA) on a modified Linsmaier and Skoog medium. An average of 4.5 axillary shoots of sufficient size to subculture (> 15 mm) were obtained from each explant in 5 weeks. These shoots were induced to root during 3 weeks subculture on Stage II medium containing 11.5 μmol/l IAA. Well-rooted plantlets were successfully transplanted to a 1 : 1 peat : sand (v/v) substrate and gradually “hardened-off” to greenhouse conditions (Stage III) for 3 weeks, after which they could be transferred to field conditions. Alternatively, axillary shoots from Stage I could be returned to fresh media with 9.3 μmol/l K and 0.28 μmol/l IAA, where an average of 10.3 axillary shoots could be obtained after 5 weeks. Cost estimates for producing 10 000 finished transplants per week project an approximate cost of 16 dollar cents per transplant.

Studies on the Characteristics of Triploid Water-Melon. I.On some morphological characters of the triploids compared with its diploid water-melons

In order to confirm some morphological characterirtics of triploid weter-melon, the present author h, as tried a comparison between the triploid plants (F1 hybrid obtained from the cross 4 X- Asahi X2X-Yamato Improyed No. 2) and their original diploid plants (2X-Asahi and 2 X- Yarnato Improved No. 2), in respect to several characters of their vegetative and reproductive organs and tissues. Thus some important results defined are briefly described in the following.