Watermelon is an important horticultural crop that accounts for 2% of the world area devoted to vegetable crops. There were 6,024,000 acres of watermelon grown in the world and 187,000 acres of watermelons grown in the United States in 1997. Asia is by far the most important watermelon production site with 72% of the world area and 77% of the world production (FAO Production Yearbook 51, 1998). The estimated annual world watermelon value exceeded $7.6 billion when using the United States average price for 1995-1997. The United States watermelon crop amounted to over 41 million cwt, from over 174,000 harvested acres, and a farm value of over $266 million, accounted for 9.2% of the harvested acres, 10.0% of the production, and 3.5% of the value of the United States fresh vegetable industry in 1999 (USDA Agricultural Statistics 2001). California was the leading state in watermelon farm gate value, exceeded $72 million in 2000, due to high percentage of triploid seedless watermelon grown in California. Seedless watermelon receives well above the average price for seeded watermelons in the market. Triploid seedless watermelon also produces higher yields than the diploid seeded watermelons.
Triploid seedless watermelon is a true F1 hybrid between a tetraploid watermelon, as the female parent, and a diploid watermelon, as the male parent (Kihara, H. 1951, Triploid Watermelons, Proceedings of American Society for Horticultural Science, 58:217-230). Regular watermelons, the seeded diploid watermelons, have 22 chromosomes (2N=2X=22) in their somatic cells. The tetraploid watermelons have 44 chromosomes (2N=4X=44) in their somatic cells. When female flowers of tetraploid plants are cross-pollinated by the male flowers of normal diploid plants, the seeds produced in the fruit of tetraploid plants are triploid seeds. Triploid seeds produce triploid plants. When the triploids plants are grown with the normal diploid plants in the same field, the triploid plants produce fruits that are seedless. The seedless condition in triploid watermelon is the result of the presence of three homologous sets of chromosome per somatic cell rather than the usual two. Cells with three sets of homologous chromosomes are said to be triploid and are designated as 3X. The triploid seedless watermelons have 33 chromosomes (2N=3X=33) in their somatic cells. The inability of the triploid zygote to produce normal viable gametes (pollen and egg cells) causes the absence of seeds in triploid fruits. Typically, seedless watermelons contain small edible white ovules, similar to those in immature cucumbers.
Triploid seedless watermelons have been commercially grown in the United States since the late 1980's. The popularity of seedless watermelon has increased since its commercial introduction in the United States. Most of the watermelons produced in California in 2001 were triploid seedless watermelons. Triploid varieties produce higher yields than the diploid seeded varieties, due to more fruit per plant and longer harvest period. The triploid seedless watermelon receives premium prices because of the high quality flesh virtually free of seeds.
The present invention addresses the need for triploid watermelons producing smaller seedless fruits while maintaining high standards of fruit taste and texture. However, when the size of the fruit is reduced, the proportion of the rind in the fruit increases, leaving only little space for the edible flesh. Thus, it is also an objective of the present invention to develop watermelons producing small seedless fruits with high flesh content. The present invention also addresses the need for tetraploid watermelon plants, which can be used as parents for triploid watermelons producing such small seedless fruits.