The other day, my daughter mentioned how surprised she was to realize grapes could have seeds. We usually buy seedless grapes, but it did make me wonder – how do grapes grow without any seeds?
Seedlessness or parthenocarpy occurs once in a while when a natural mutation prevents seeds from maturing and developing a hard coat. This kind of aborted seed development is called stenospermocarpy. The seeds remain as tiny specks, with its size dependant on the variety and climate.
Normally when seeds develop, they secrete a hormone called gibberellin, which encourages the fruit to grow. Over 120 forms of gibberellin have been identified in plants thus far. Seedless grapes tend to grow small, so farmers will apply more gibberellin to help them grow bigger.
The Thompson seedless grape is one of the most common types of seedless table grapes. It was named after William Thompson, a Scottish immigrant who promoted this variety in California during the mid-1870s. The grapes were derived from a strain called Lady de Coverly which he obtained from a nursery in New York.
Over time, the cultivation of these seedless varieties became more and more sophisticated. In the 1950s, research horticulturalist with the US Agriculture Department, John Weinberger, developed the Flame – a red seedless grape that is runner-up to the Thompson in popularity. Weinberger and his team tested the viability of over 100,000 seedlings, involving crosses with five different varieties including the Thompson seedless.
From the 1980s, breeders have been able to remove the developing embryo seeds before they abort, through a process known as embryo rescue. These rescued seeds can be grown in petri dishes or test tubes to cross with other strains. The identification of genes for seedlessness, along with techniques for inserting genetic material directly into developing plants, has further broadened the possibilities for genetic modification. Although genetic modification through artificial selection has occurred for centuries, the more recent development of genetically modified organisms (GMOs) enables the addition of characteristics related to robustness or productivity from different species altogether. This allows for a more efficient process for selecting desirable traits, but the social and political ramifications of commercializing biological properties can be a concern.
Without knowledgeable farmers, seedless plants could quickly become a thing of the past. Fortunately, plants can be propagated through cuttings as well. A cutting is when you cut off a piece of plant, dip it in some rooting hormone (such as auxin, along with antibacterials), and plant it. The descendants of this process are genetically identical to the parent, which means they are clones. This works for seedless grapes, and many other plants as well. Most commercial fruit is grown from cuttings because the crop is more likely to turn out the way you expect. The risk with this is that if something unexpected were to happen, like a disease infecting your field of identical grapes, all of your grapes will be wiped out in short order.
Top Graft Pick
Another way grapes and other plants, seedless or not, can be propagated is by grafting. This is where you take the cutting and stick it to another plant. They then carry on as if they were meant to be that way. This allows the characteristics of desirable fruit to be combined with a rootstock that is resistant to pests and diseases, and able to grow well under local conditions.
Have a Grape Day!
I hope you’ve enjoyed learning all about seedless grapes with me! Don’t forget to share your questions, thoughts and experiences in the comment box below!