Monday, 23 October 2006

Update on the purple pea project

My hand-pollinated hybrid pea reveals its flower colour. Innit lovely?

Well, plant breeding may be a complicated pain in the neck, involving a lot of poking of flowers with scalpels and potentially years of patience and careful study, but I reckon there's no greater gardening thrill than to see one of your home-made hybrids burst into flower for the first time ... and find out what colour it is.

I'm getting immense pleasure just from looking at these large flamboyant purple-pink flowers, created by me, on a garden vegetable which normally has quite unexciting flowers. They are so beautiful.

The pods are turning colourful too, after a fashion. They're not as decisively purple as a true purple variety, but they're purple nonetheless. The flower buds start off a pale creamy pink with the maroon inner petal just showing through, then open out into a lovely two-tone pink which gradually matures to blue.

And so far my hybrid is following the pattern I predicted from what I know about the genes involved in pea colouration.


Just to recap on what this project is all about: I'm trying to breed my ideal garden pea. The basic spec is for all the fine qualities of a 19th century variety called Alderman, but with pink or purple flowers and purple pods instead of the usual white and green. Alderman has large white flowers, grows from between 6 to 8 feet tall and has the most exquisitely sweet and juicy peas (when eaten raw) that I've ever tasted. I couldn't tell you what they taste like cooked, because I've never got any as far as the kitchen. They taste too good straight off the plant. The peas are also huge in size and stay sweet even when fully mature. It's a fabulous variety and well worth seeking out.

To start my quest for a colourful version of Alderman, I did some crosses earlier in the year using Alderman flowers hand-pollinated with pollen from Mr Bethell's Purple Podded, an heirloom pea with pretty two-tone pink flowers and purple pods, which I got from the Heritage Seed Library. Its actual peas are green and reasonably large and pleasant tasting, but I'd be lying if I said they had anything like the ambrosial sweetness of a good green-podded pea like Alderman.

Usually when you do a plant breeding experiment you just have to try things out and see what happens, because very little is known about most of the genes in most plants. Peas have been better studied than most because their genetic patterns are very uniform and simple ... which is why they were responsible for the discovery of the fundamental laws of genetics. But even so, not all the genes have been identified. I have no idea what genes (or groups of genes) are involved in the production of Alderman's sweet flavour, so all I can do is grow my hybrid for several years and do loads of very thorough taste tests in each generation to find the flavour I want (it's a hard life).

But things are slightly easier when breeding for colour, because those genes have been identified and named. If I want plants with purple pods and a purple flash or ring in the leaf axils, there are four genes I need and they're all dominant. Gene A is the crucial one that gives the plant the ability to synthesise anthocyanin, the pigment responsible for purple colouring in most vegetables. A is effectively the 'on' switch for purple, but it doesn't control where the colour is expressed within the plant. To get purple pods, I need two additional genes, Pu and Pur (no I'm not making these up), and to get the purple splash at the base of the leaves I need another gene called D. A plant may carry genes Pu, Pur and D and have no purple colouring at all, because these genes can only express themselves in the presence of gene A.

I've made the assumption that Mr Bethell's Purple Podded has all four genes, A, Pu, Pur and D, because it has purple pods and purple leaf axils. And I also assume that Alderman, being green podded and green leaved, doesn't have A, although it may or may not have any of the other three genes. Therefore, when I make a cross between these varieties, I expect the F1 hybrid (that's the first generation of seed from the cross) to have purple pods and purple leaf axils. Why? Because they will inherit one half of their genome from the green podded variety and one half from the purple podded, and in this case the genes in the purple variety are dominant.

And so far that's exactly what I've got, which suggests that my hybrid pea, as well as being beautiful in its own right, has all four of the genes I want.


At first it looked as though the plants were going to produce green pods, because they were quite slow to change colour. With most purple podded peas, the baby pods start off green with a purple strip along the top edge, and then the rest of the pod colours up when it's a couple of days old. My hybrid variety is taking a couple of days longer to turn purple than I would normally expect, and the colour is slightly patchy on some of them. It's too early to draw any conclusions about why that's the case ... but it may be that the dominant purple pod genes are competing with another dominant gene for green pods, resulting in co-dominance and a colour part way between the two.

I haven't yet found out what genes are involved in the flower colour, but my guess is that they too are dominant but reliant on the presence of A. I'll know more about that when I grow the next generation. The flowers on my hybrid are basically the same colour as the 'father' plant, Mr Bethell's Purple Podded, but they're larger (an Alderman trait) and they have a very flat standard petal (that's the wide one at the back) which is a characteristic not seen in either parent. Most intriguing.

More intriguing still, one of the plants has started producing two flowers at each node, when both of the parent varieties produce only one. (There is a purple podded pea which does have paired blooms at each node, Ezetha's Krombek Blauwschok, but I didn't use that variety in my cross.) Presumably there is a gene for two-flowers-per-node which was being carried, but not expressed, by one or both of the parents.

But anyway ... aside from the joy of seeing a new hybrid come into flower, it doesn't really matter how beautiful the plants of this F1 generation are ... they will not be the same in the next (F2) generation. Those two halves of the genome will be broken up and randomly recombined, so that different genes from either of the original parents will start to show up in the offspring, with a huge number of different combinations possible. That's where you get the opportunity to choose the ones you like as the basis for a new variety. So the purpose of my growing these plants is just to obtain as many (self-pollinated) seeds as possible for the next generation, and those seeds will show a mixture of green pods and purple pods, from which I will select the best purple ones.


And while we're on the subject I must publicly thank Silvia (of the Windywillow blog) for the beautiful artwork she did based on my purple podded peas and my complaint that the faeries were coming along in the night and stealing the gold threads I was using to tag my hand-pollinated buds. Though in fairness to the faeries, they rarely actually steal things ... things disappear but nearly always turn up again, albeit in odd places. My missing gold threads turned up about three days later on a different plant, so it'll be interesting to see what happens when I plant the seeds from that batch. Silvia's artwork shows a couple of very smug-looking faeries tugging the threads off the flowers and is really wonderful ... she's very talented.

5 comments:

I need orange said...

I love reading about all this. It's the sort of thing I might do, in another life, so getting to learn about it (without needing to *do* it) is truly excellent. Thank you.

I hope your husband and cat are doing better?

-- Vicki in Michigan

ps -- a boring gray or tan thread might be less interesting to the fairies?

Rebsie Fairholm said...

Thanks for the words of encouragement, Vicki!

My husband is a little better, but sadly the cat is in decline.

Mmm, faery-proof thread ... I must look into that.

Patrick said...

It is great watching you do this. I hope to do this too, when I have more time, and it'll be great to have someone to ask questions from. Keep us posted!

All the best to both husband and cat.

Rebsie Fairholm said...

Thanks Patrick, much appreciated!

I'm glad you're finding it interesting and I'll gladly share whatever knowledge I gain from doing this.

Dan said...

Rebsie, I have a good purple podded pea - actually there are several: a purple snap and a purple sugar are now homozygous, and there are a couple of bicolors almost ready too.

There are a few photos on purplepeas.net - but of course the appearance is only half the battle, the taste and texture were the most difficult to get right. Contact me if you're interested in knowing more about them.