The Disappearing Watermelon Seed

Watermelon seeds are an icon of summer, but maybe not for much longer. Plant scientist Tracy Kahn of the University of California-Riverside talks about the emergence of seedless fruit.

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ANDREA SEABROOK, host:

Spitting watermelon seeds. It's a sound that defined summer. No more. At the grocery store, at the farmer's market, even at the roadside fruit stand, it's tough to even find a watermelon with seeds these days. Toss in grapes, oranges and you've got a whole fruit salad of seedless wonders. That got us wondering what we're missing. It's Science Out of the Box.

(Soundbite of music)

SEABROOK: Meet Tracy Kahn. She's a plant scientist at the University of California Riverside, and she's curator of the school's citrus variety collection. Welcome to the show, Tracy Kahn.

Ms. TRACY KAHN (Plant Scientist, University of California Riverside; Curator, School's Citrus Variety Collection): Thank you.

SEABROOK: So, how do you make a seedless fruit?

Ms. KAHN: There are some that are just naturally seedless or you can actually do things scientifically to encourage them to be seedless.

SEABROOK: You just graft it over and over again or something?

Ms. KAHN: Well, in the case of citrus you actually - all commercial citrus trees, they take a bud out of the branch next to a leaf and they put it on a seedling root stock. They make a cut into the bark of a seedling root stock and they insert that bud and then that bud that you took from the type that you want to eat, the variety that you want to propagate, becomes the new top of that tree and then that's a commercial tree, which has two different types sandwiched together.

With the top part being the type that we're interested in growing and the bottom part being the root stock, which confers all sorts of protection mechanisms to that part of the tree.

SEABROOK: Okay. I understand now that you can - there is these genetic variations. You look for genetic variation that happens to produce seedless fruit. You propagate with grafts, say, onto another plant. What about seedless seeds? I heard you can buy watermelon seeds that produce seedless watermelons.

Ms. KAHN: Okay. You know, we have chromosomes in us. We have two copies - one that came from one parent; one that came from the other.

SEABROOK: Right.

Ms. KAHN: And that's true with most plants as well. Most plants have two copies of chromosomes. And when the plants go to make either pollen - that carries the sperm cells or ovules - that carry the egg cell - the chromosome number that goes to half. And then when the sperm and the egg come together again it goes back to two copies again.

SEABROOK: Just like us.

Ms. KAHN: Just like us.

SEABROOK: So, plant sex.

Ms. KAHN: Plant sex. And in fact those watermelons that you were talking about - in that case what they did is they took - first of all, they started out with a watermelon plant that had two copies of the chromosomes. They treated it with a chemical that caused the chromosome number to double to four copies, okay?

SEABROOK: Mm-hmm.

Ms. KAHN: And then that became one of the parents. And then so you had, you crossed a watermelon that had four copies of the chromosomes with one that had two copies of the chromosomes. And what you end up with is plants that have three copies of the chromosomes…

SEABROOK: So, it's…

Ms. KAHN: …which is a triploid.

SEABROOK: …it's the watermelon version of a mule.

Ms. KAHN: It's the watermelon version of the mule.

(Soundbite of laughter)

SEABROOK: It can't reproduce but it exists.

Ms. KAHN: It can't reproduce. But in fact in order to - so, when you go out and you buy watermelon seeds, you would buy seeds that have three copies of the chromosomes, but you would also need to buy - and it would usually be included with it - some that produce the normal two copies. Because in fact, watermelons need to be cross-pollinated in order for that fruit to develop.

SEABROOK: A few years ago I think I would have never believed there would be a seedless watermelon. I mean, my God, watermelon seeds are, like, iconic.

(Soundbite of laughter)

Ms. KAHN: We really don't like seeds. And in fact even among researchers and growers, we've had discussions with it. It'd be valuable to have a seedless lemon given what you do with lemons. You know, you use them in drinks and you use them…

SEABROOK: Sure.

Ms. KAHN: …for cooking and…

SEABROOK: Go for the seedless lemon. A lot easier to squeeze into my tacos.

(Soundbite of laughter)

Ms. KAHN: It's true, yeah. You'd have to sort of dig them out if you're squeezing them into the taco.

SEABROOK: Tracy Kahn is the curator of UC-Riverside Citrus Variety Collection. Thanks so much for speaking to us.

Ms. KAHN: Okay. Thank you so much.

SEABROOK: Really interesting.

Ms. KAHN: Thank you.

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