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- Author or Editor: Xingping Zhang x
- HortScience x
Tetraploids are needed to synthesize triploid watermelons, which produce “seedless” fruit with improved quality. Traditionally, the tetraploids are induced by applying colchicine to the growing apex of seedlings or soaking the seeds with colchicine solution. This method often produces low frequency of tetraploids and high frequency of chimeras. Breeding tetraploids takes much longer time than breeding diploids because of the low female fertility. We developed a tissue culture approach that allows breeders to develop desirable tetraploids with commercially acceptable volume of seed in 2 years. This tissue culture approach includes: 1) regenerate plants via shoot organogenesis from cotyledon tissue; 2) screen tetraploids based on leaf morphology (more serrated leaf margin and wider leaf shape) before transplanting, and confirm tetraploids based on pollen morphology (larger pollen with four copi) and/or seed characteristics; 3) self-pollinate tetraploids or cross the tetraploids with diploids to accurately estimate the female fertility; 4) micropropagate the best tetraploid(s) using axillary buds during the off-season; and 5) produce tetraploid seed from the cloned tetraploids in an isolation plot and evaluate the triploids derived from the tetraploid(s) in the following season. This approach has been practiced on more than 20 genotypes over the past 4 years.
A spontaneous watermelon mutant, previously named branch less, was re-evaluated in this study. The mutant watermelon plants from genetic stock Bl-91 and derived from F2 and BC1 populations, did not produce tendrils under field or greenhouse conditions. The mutants stopped producing branches after the fifth or sixth node. Leaf shape changed during development of the mutants. Early leaves were normal, but later leaves had fewer and fewer lobes, finally becoming triangular toward the end of the shoot. The most distinct effect of the mutant gene was to convert vegetative meristems into floral meristems; tendrils and axillary buds were replaced by flowers at the node. The mutant plants were determinate. A grafting experiment showed that the rootstock had no effect on the mutant phenotype. Genetic analysis of F1, F2, and BC1 populations suggested that the mutant is inherited as a single, recessive nuclear gene. Based on the phenotype, a new name is suggested for this mutant: tendrilless, with a new gene symbol tl.
Genetic diversity and relatedness were assessed among 46 American cultivars of watermelon (Citrullus lanatus var. lanatus), and 12 U.S. Plant Introduction accessions (PIs) of Citrullus sp. using 25 randomly amplified polymorphic DNA (RAPD) primers. These primers produced 288 distinct reproducible bands that could be scored with high confidence among cultivars and PIs. Based on the RAPD data, genetic similarity coefficients were calculated and a dendrogram was constructed using the unweighted pair-group method with arithmetic average (UPGMA). The cultivars and C. lanatus var. lanatus PIs differentiated at the level of 92% to 99.6% and 88% to 95% genetic similarity, respectively. In contrast, the C. lanatus var. citroides, and C. colocynthis PIs were more divergent and differentiated at the level of 65% to 82.5% and 70.5% genetic similarity, respectively. The low genetic diversity among watermelon cultivars in this study emphasizes the need to expand the genetic base of cultivated watermelon.
A selection of Congo produced fruit that were not infected by blotch (pathogen Acidovorax avenae subsp. citrulli) in a replicated trial interplanted with infected seedlings. Ninety percent of Congo fruit not infected with the bacterial pathogen had a darker green background than those infected. PI 295843 and PI 299318 selections were also not infected. Infection rates in susceptible checks ranged from 22.5% to 47.6% and from 0 to 13.9% among triploids. Both ploidy level and genotype significantly affected infection rates. Infestation rates in triploid seeds were reduced but not eliminated by dry heat up to 75C. Heat treatment necessary to kill the pathogen was detrimental to germination.
Vegetable grafting began in the 1920s using resistant rootstock to control soilborne diseases. This process is now common in Asia, parts of Europe, and the Middle East. In Japan and Korea, most of the cucurbits and tomatoes (Lycopersicon esculentum Mill.) grown are grafted. This practice is rare in the United States, and there have been few experiments to determine optimal grafting production practices for different geographical and climatic regions in America. This is beginning to change as a result of the phase out of methyl bromide. The U.S. cucurbit and tomato industries are evaluating grafting as a viable option for disease control. Because reports indicate that type of rootstock alters yield and quality attributes of the scion fruit, some seed companies are investigating grafting as a means to improve quality. It has been reported that pH, flavor, sugar, color, carotenoid content, and texture can be affected by grafting and the type of rootstock used. Reports vary on whether grafting effects are advantageous or deleterious, but it is usually agreed that the rootstock/scion combination must be carefully chosen for optimal fruit quality. Additionally, it is important to study rootstock/scion combinations under multiple climatic and geographic conditions because many rootstocks have optimal temperature and moisture ranges. This report gives an overview of the effect of grafting on vegetable quality.