Seedless grapes, a popular and convenient choice, are often mistakenly thought to be a recent invention of genetic engineering. In fact, their origins lie in natural biological processes and ancient cultivation.
Historical Origins
The journey of seedless grapes began centuries ago with natural genetic mutations. Early cultivators, particularly in the Middle East and Mediterranean regions, discovered grapevines that naturally produced berries without fully developed seeds. For instance, the ‘Sultana’ grape, widely known as ‘Thompson Seedless’ in the United States, is believed to be of ancient origin from the region between Turkey and Greece. Ancient cultures, including the Greeks and Romans, valued these naturally seedless varieties for their smooth texture and ease of consumption, especially for making raisins.
Farmers recognized the desirable trait of seedlessness and learned to propagate these unique vines. Instead of planting seeds, which would revert to seeded varieties, they used cuttings from the original seedless plants, a method of vegetative propagation that created genetically identical clones. William Thompson, a farmer in California, notably popularized the ‘Thompson Seedless’ variety in the mid-1870s, successfully propagating it through cuttings from a naturally occurring seedless vine.
The Science of Seedlessness
Seedlessness in grapes primarily results from two natural biological mechanisms: parthenocarpy and stenospermocarpy.
Parthenocarpy describes fruit development without fertilization, leading to truly seedless berries that are often smaller. While some fruits like bananas exhibit natural parthenocarpy, it is less common in commercial seedless grape varieties.
Most commercial seedless grapes, including ‘Thompson Seedless’, result from stenospermocarpy. In this process, pollination and fertilization occur, and a tiny embryo begins to form but aborts prematurely. This interruption prevents the development of a hard seed coat, leaving only small, soft, often unnoticeable seed traces. Stenospermocarpy allows for larger fruit size compared to parthenocarpy because initial fertilization stimulates fruit growth.
Modern Cultivation and Development
Current cultivation of seedless grapes heavily relies on vegetative propagation, as planting seeds from seedless varieties would not yield consistent results. Growers take cuttings, typically sections of dormant canes, from existing seedless grapevines. These cuttings are then rooted, often with the aid of rooting hormones, to grow new, genetically identical clones of the parent vine. Grafting, where a cutting from a desirable seedless variety is joined to the rootstock of another grapevine, is also a common practice.
Modern breeding programs further enhance seedless grape varieties through selective breeding. Researchers cross existing seedless varieties, or cross seeded and seedless varieties, to develop new types with improved traits such as disease resistance, flavor, or texture. A technique called embryo rescue is frequently employed in these programs, allowing breeders to grow viable plants from the tiny, aborted embryos of seedless grapes. This enables the creation of new seedless varieties that combine desirable characteristics from both parent plants.