Seedless grapes are a popular fruit enjoyed worldwide for their convenience and sweet flavor. Lacking the hard, noticeable seeds found in traditional grapes, these varieties have become a staple in fruit bowls and lunchboxes, making them a preferred choice for many consumers.
The Biology of Seedless Grapes
Seedlessness in grapes primarily results from two biological mechanisms: parthenocarpy and stenospermocarpy. Parthenocarpy involves the development of fruit without fertilization, leading to truly seedless fruit that is typically smaller in size. While some grape varieties exhibit this, it is less common for commercial seedless grapes.
Stenospermocarpy is the more prevalent mechanism in commercially grown seedless grapes. In this process, fertilization occurs, and an embryo begins to form, but its development is aborted at an early stage. This early abortion means that while tiny, undeveloped seed traces might be present, they remain soft and are usually undetectable.
The genetic basis for seedlessness, particularly stenospermocarpy, is linked to specific genes. Research indicates that a missense mutation in the MADS-box gene VviAGL11 is a major factor causing seed abortion in most cultivated seedless grape varieties. This genetic characteristic prevents the full development of the seed coat and endosperm, even after initial fertilization. Ongoing studies continue to explore the polygenic nature of seedlessness, identifying other genes and genetic regions that contribute to this trait.
Cultivating Seedless Varieties
The development of new seedless grape varieties involves traditional breeding programs. Horticulturists cross-pollinate existing grape varieties, aiming to combine desirable traits from parent plants. This process is long-term, requiring the screening of many seedlings over multiple generations to identify new strains that consistently produce seedless fruit.
A technique called “embryo rescue” is employed in these breeding efforts. Since seedless grapes produce non-viable or rudimentary seeds, traditional propagation from seed is not feasible. Embryo rescue involves extracting the tiny, undeveloped embryos from the fruit and growing them in a sterile laboratory environment until they are large enough to be planted. This method allows breeders to make crosses between two seedless grape varieties, accelerating the development of new seedless cultivars.
Another technique used to enhance seedless grapes is the application of gibberellic acid (GA3). This naturally occurring plant hormone promotes cell elongation and division, leading to larger fruit size. Since seedless grapes lack fully developed seeds, they produce less natural gibberellin, resulting in smaller berries. Commercial growers spray gibberellic acid on grape clusters to increase berry size and to loosen the clusters, which can improve air circulation and reduce the risk of rot.
How Seedless Grapes Continue
Because seedless grapes do not produce viable seeds, they cannot be reproduced through conventional planting methods. Instead, growers rely on vegetative propagation to ensure new plants are genetically identical to the parent vine. This method allows for the consistent production of desired seedless varieties.
The primary technique for vegetative propagation is through cuttings. A section of stem from an existing seedless grapevine is cut and rooted, either directly in the soil or in a nursery setting. This cutting then grows into a new plant that is a clone of the original, maintaining all its characteristics, including seedlessness. This practice has been used for centuries to propagate plants.
Grafting is another related technique used in grape cultivation. It involves joining a cutting (called a scion) from a desirable grape variety, such as a seedless type, onto the rootstock of another grapevine. This rootstock is chosen for its disease resistance, adaptability to specific soil conditions, or vigor. Grafting allows growers to combine the fruit quality of the seedless scion with the beneficial root characteristics of the rootstock, ensuring the long-term health and productivity of the vineyard.