How do farmers propagate seedless crops?

We put this question to Stephen Tomkins, from Homerton College, Cambridge, and Jennifer Schultz Nelson, Horticulture Educator with the University of Illinois:

Stephen - You need to know something about why they have no seeds and that's because they're often triploids. That is, they've got three sets of chromosomes and you and me have two. And when it comes to sexual cell divisions in meiosis, they can't organize their chromosomes properly and they fail, and therefore, they have no babies.

So the question that you want answered is how do you get to a triploid fruit? And the answer to that is a little bit complicated. Most plants have two sets of genes, two sets of chromosomes and they divide equally when cell divisions take place. But plants are very tolerant of having multiple sets and that's called polyploidy.

Polyploid plants can be triploid, tetraploid, pentaploid, and up the numbers go! And it doesn't often badly affect the viability of the plant. They grow. They photosynthesise. They can produce fruits. But if they have odd numbers of chromosomes, they are stuffed. They can't have any babies at all. And that happens to the triploids and the pentaploids because you can't divide odd numbers evenly.

If you want to multiply up that plant, you can do it by cloning or vegetative propagation. And that's an important thing for people setting out to grow large orchards of triploid fruit.

Diana - So you can propagate plants which are gameteless so they make seedless fruit. And here's a little more on how it's done.

Jennifer - When humans get involved, they try to manipulate things and they can do things like treat a plant with a chemical called 'colchicine' which disrupts the meiotic process. So when the plant is producing pollen grains and ovules, they will produce a gamete that has double the amount of chromosomes. So then when it is pollinated by the original plant that provides one copy of chromosomes, the result is a triploid plant.

We talk about cloning being a really modern method, but actually, horticulturists have been using it for quite some time. Any time you're taking a cutting of a plant and rooting it using different plant hormones or simply sticking it in a glass of water, watching the roots grow and then planting that plant, that's cloning, also called 'vegetative propagation.'

I actually found some information that when you talk about polyploidy in plants and it sounds like a really rare thing, but it's actually not that rare. Scientists think that anywhere from 30 to 70% of angiosperms from the plant kingdom are polyploidy and polyploidy plants tend to be larger. And so, maybe they're more competitive in nature and so that might be some advantage for that to be selected for in nature.

Diana - Triploid plants can be reproduced using cloning and vegetative propagation as well as chemical mutagens like 'colchicine.' But many farms will grow the fertile parent plants nearby so that more seedless offspring can be produced later.