Sunday 18 August 2019

Pink-flowered peas (Alderman x Salmon Flowered)

It was more than a decade ago that I made a cross between Alderman and Salmon Flowered, two heritage peas, with the vague dream of producing a good tasting culinary pea with pink flowers. But as my other three major pea-breeding projects took up more and more of my time, it kind of went by the wayside.

Just to recap: Alderman is a tall, elegant shelling pea with white flowers, introduced in 1891. Its flavour is outstanding, which is probably why it's one of the only Victorian tall peas which is still commonly available today. It's far and away my favourite shelling pea, and while there are a few others which can rival it for flavour when they're young, Alderman stays exquisitely delicious even when the peas are at full size and maturity, and I haven't found anything else that can match it. It has large, well-filled pods on large plants, its only disadvantage (if you consider it a disadvantage) is that it's quite late maturing. Salmon Flowered is a real rarity, whose seed I got from the Heritage Seed Library many years ago. It's an umbellatum-type pea, which means it has a heavily fasciated (thickened) stem with all the flower buds borne in a great clump at the top. The flowers bloom more or less all at once and the pods form in a big clump, sticking out in all directions. There were a few of these varieties around in the 19th century but they're no longer commercially available – and in terms of flavour and yield they can't really compete with modern varieties. But Salmon Flowered (not its real name, which has been lost*) has really beautiful and unusual bicolour pink flowers which I haven't seen in any 'normal' pea at all. The wing petals are a peachy salmon pink and the standard is a very pale blush pink.

*A Swedish heritage pea called Rosakrone is now available from Real Seeds and is very, very similar to Salmon Flowered. I grew some Rosakrone this year to see just how similar it was, and while I'd say it's not absolutely identical, it is similar enough that they're most likely different stocks of the same original variety. The pink flowers are very much the same colour.

So the purpose of hybridising Alderman with Salmon Flowered was to see if I could breed the pink flower trait into a crop of otherwise normal garden peas – using Alderman as the benchmark because of its exceptional flavour.

Back in 2010 I grew out the F1 seeds from my Alderman x Salmon Flowered cross and wrote about it here. The F1 plants all had bicolour purple flowers, which might seem like an absolutely bizarre thing to get from a cross between a white flowered and a pink flowered pea, but actually it's what I'd expected. Purple bicolour is the ancestral default flower colour for peas (not just culinary peas but also the sweet pea Lathyrus odoratus) and the only reason why most garden peas DON'T have purple flowers is that they've all been bred to have a recessive gene which suppresses the production of anthocyanin pigment in the plant. I went into some detail about that in my original post about this cross, so I won't repeat it all here, but suffice to say that Alderman's pure snow white blossoms are not due to any 'white flower' gene as such, they're caused by the presence of this gene which switches off the expression of purple colour so that the flowers are white by default. Salmon Flowered doesn't carry this colour-suppressing gene – it can't do, or it wouldn't have pink flowers – so when you make a cross between a variety which has the pigment-suppressing gene and one which doesn't, the F1 generation will default to the dominant condition – which is for colour to be expressed. Being the dominant ancestral trait, purple flowers prevail. But on a genetic level, the recessive half of the gene pair, which forces the flowers to turn white, is still there and ready to be passed on to a large proportion of the F2 offspring.

I only have a modest sized garden and very limited free time so the focus on my breeding for coloured pods (edible fibreless coloured pods at that) took up more attention and space for the next few years and the Alderman x Salmon Flowered went by the wayside. I had the bag of F2 seed which I produced in 2010, but hadn't sown them. So this 2019 crop was another "back from the dead" miracle story.

I found the F2 seeds in a box, and thought about how nice it would be to work on breeding for flower colour for a change, rather than pesky pod colour. But seeds from 2010 were only fit for the bin, surely? I had to do a quick test though rather than lob them straight out, so they went into a tray of water, alongside some other decrepit sideline projects.

I don't normally soak pea seeds or recommend soaking pea seeds. I was doing it here because it was just a germination test on 9-year-old seeds, and they were not expected to sprout. However, almost all of them did.

This goes against everything I've been taught about pea seeds. 1-2 years is commonly given as a pea seed lifespan. Some people report them lasting a bit longer if you store them in the fridge or the freezer. But mine hadn't had any special storage conditions. They were in plastic bags inside a cat food box on top of my bookcase. The 16 seeds (of quite diverse size and colour, being F2s) in the top right of the photo above became 16 sprightly little plants in a frame in the garden.

I didn't see any loss of vigour from the seed being nine years old. In fact one of the plants turned into the most productive pea I've ever grown, producing large numbers of pods on a multitude of sideshoots. That one will be covered in another blog post later, along with a couple of other interesting things which emerged from this growout.

I labelled the plants individually and numbered them from 1 to 16, and took notes on them as they grew, paying particular attention to the colour and density of the axillary pigmentation (the pinky purple splodges where the leaves join the stem) as I've increasingly noticed a significant correlation between axil splodges and the eventual flower colour and/or pod colour.

Well, I already knew that the F1 had produced purple flowers, so what was I expecting to get from the F2? In a sample size of only 16, I wouldn't expect to get perfect Mendelian ratios for anything, but still there are general trends to look out for. The first would be the colour-suppressing gene discussed above. That should, in theory, turn up as white flowers in about one in four of the F2 plants. In the event, I only had one plant with white flowers (plant no.7), but that's enough to show that the inheritance of that trait is working as expected. In this project, the colour-suppressing gene is not what I want, and I will have to select against it in future years as well, as it will be lurking as a hidden recessive in many of the other plants even though they didn't have white flowers.

Of those which showed coloured flowers, I was expecting a majority to have purple bicolour flowers like the F1, as that's controlled by dominant genes and is the trait most likely to express itself. I know very little about the genetics behind pink flowers in peas, if I'm honest, but I was pretty sure it would be a recessive trait and so I had my fingers crossed that it would turn up in a proportion (maybe one in four) of the non-white plants.

And voilà!

It actually turned up in exactly four plants, numbers 3, 6, 10 and 15, which is close to being a Mendelian ratio. These all produced flowers which were a very consistent shade of pink, that is, they were all bicolour pinks with peachy salmon wings and a pale blush pink standard. There was no variation in the expression of the pink colour, other than some differences in how the colour changed over time as the pigment broke down in the fading flowers, where some turned a more dusky, rosy pink than others. But in terms of the essential flower colour, they were identical to one another and identical to the colour of the parent plant, Salmon Flowered, which provided the pink gene. They did all have somewhat larger flowers than Salmon Flowered – flower size was more consistent with that of Alderman – but that's controlled by a different genetic locus.

They were absolutely beautiful and I was thrilled with them.

A second recessive trait which I was expecting to show up in one in four plants was the umbellatum form – which as far as I know is a simple recessive gene which causes the flowers and pods all to bunch together in a cluster on top of a fattened stem. But having never done breeding work with umbellatum-type peas before, I couldn't be sure how this would work out in practice. Again though, it turned out much as predicted. Only two plants, plants 5 and 9, were of the umbellatum type, which is a bit short of a Mendelian ratio but close enough to show the principle of it. There were no intermediate types, all the plants were either umbellatum or non-umbellatum. As far as this project is concerned, the umbellatum trait is not a desirable one and so I'll be having to select against it until all the hidden recessives are eliminated from future generations. The two plants which had this trait were both purple flowered anyway. It may not be a desired trait in this cross but it does look quite spectacular!

All of the pink-flowered plants had what you might call 'normal' form. They produced two large flowers per node, which gave rise to large green pods, much like Alderman. The umbellatum types did tend towards very slightly smaller pods, whereas all the non-umbellatum types had normal size pods, which suggests that smaller pods are a byproduct of the fasciation trait in umbellatum peas. It may simply be that the plant doesn't have enough energy to produce the flowers and pods all together at the same time without compromising on size a little bit. If there was a genetic cause, i.e. a gene in Salmon Flowered which made its pods smaller (as they ARE quite small) then I would expect that trait to segregate randomly through the F2 plants – but it didn't.

Axillary pigmentation is another interesting one. It occurs in all (in my experience) peas with coloured flowers. A purple-pink splodge at the point where leaf meets stem is invariably seen in a plant with purple bicolour flowers. The purple blotch is made from anthocyanin pigment, so it's subject to the action of the same colour-suppressing gene which causes white flowers. A pea with white flowers will have no pigment in the axils at all, because this gene switches off all production of anthocyanin pigment throughout the whole plant. So in a cross between a white-flowered and a coloured-flowered pea, like this one, the presence or absence of colour in the leaf axils is a very early indicator of whether the plant will eventually have white or coloured flowers. The axil pigment usually shows up quite early in the seedling stage, when they've produced their first couple of sets of true leaves.

In the case of this hybrid, the Salmon Flowered parent variety has an unusual kind of axillary pigmentation. It's lighter than the usual type – in fact it's a dusky rose pink, and very soft and subtle. I was thinking it's probably not a coincidence that a variety with unusual pink flowers also has unusual pink axillary pigmentation – there's probably a meaningful correlation between the two. So I was watching my F2 seedlings to see if there was any sign of this correlation, and there was.

The photo shows the subtle dusky pink pigment in the axil of ASF 06, one of the pink-flowered phenotypes. All the other pink-flowered ones had this as well, while the purple-flowered ones had the more normal blotch of purple colour in the leaf axils, and the white-flowered plant had none at all, as expected.

This is bloody useful, actually. If the colour in the leaf axil is a reliable indicator of flower colour, which it does seem to be, subject to errors of interpretation when the shade is a bit ambiguous, then it means you can identify the flower colour a good month or two before they flower.

Right then, so the appearance of pink flowers in roughly a quarter of the F2 plants suggests a fairly straightforward recessive gene at work. I'm sure there is some info out there somewhere about how this works, but the details of which genes control which traits are often buried in papers in academic journals which I don't readily have access to – and even when I do get hold of them, I struggle to make sense of the scientific jargon and it makes my brain hurt. So in my layperson ignorance I'm going to make a speculative guess about what's happening with this pink flower business.

I think that a pink-flowered pea is essentially a purple-flowered pea which has come under the influence of a modifier gene – probably just a single, recessive modifier gene. I think this modifier gene acts on the chemical makeup of the anthocyanin pigment, suppressing the production of blue pigment while leaving red pigment unaffected, so that the flower comes out pink instead of purple.

There are two reasons why I think that. The first is to do with the axillary pigmentation. If there was a gene specifically coding for pink flowers, I can't see why it would affect the colour of the axils as well. But clearly it does, because pink flowers and pink axils go together. Which suggests a modifier gene having a blanket effect on anthocyanin production throughout the whole plant.

The second reason I think this is the case is because of a study which has been done on sweet peas, which are a different genus from edible peas but have a lot in common with them. As I mentioned earlier, the default ancestral colour for sweet peas is a purple bicolour, but some time in the 18th century a mutation occurred which gave us the lovely pink-and-white bicolour known as Painted Lady, which is still widely available today. A study was published in 2017 in the Canadian Journal of Plant Science on the genetic basis of this mutation, and although I don't have access to the paper itself there was enough information in the abstract to tell me what I needed to know: a single base pair mutation means that the flower is lacking the blue pigment known as delphinidin, which is one of the anthocyanins which make up the purple colour in sweet peas. In the absence of delphinidin, the flower becomes pink. In simple terms, if you imagine that the colour of the purple flower is made from layers of translucent blue and pink, removing the blue layer leaves you with just pink.

So that's what I think is probably happening in Pisum sativum as well. If you look at the pink flowers in my F2 plants, they are all bicolours. They are all essentially the same, there's no variation in the colouring. So it seems quite plausible that they are meant to be the default purple bicolours, and that a recessive gene has come along and deleted the production of delphinidin (or whatever blue pigment they're supposed to have) and this salmon-pink bicolour is the result.

As I said, this is just my speculation! I'm sure there are people out there who know more about it than me.

This is turning into a very long post, but I have learned such a lot from growing these sixteen F2 plants!

So let's finish up with a bit about the pods and peas. This project is not seeking to produce edible pods or coloured pods: both parents are green-podded shelling peas, and all the offspring are green-podded shelling peas as well. My aim was to get the kind of big, plump green pods and fat peas found in Alderman, and not so much of the small pods and small peas of Salmon Flowered. In this, the F2 generation has given me what I wanted, because all the non-umbellatum type plants produced pretty good pods and most had good sized peas.

I did some taste tests as well. I was hoping to get as close as possible to the sweet and complex flavour of Alderman and not so much of the pleasant but rather mealy taste of Salmon Flowered. In this I was also very lucky. I tasted three out of the four pink-flowered phenotypes and they all had very good tasting peas, with ASF 06 being the best. Unfortunately ASF 10, which was a lovely plant with beautiful flowers, died prematurely after getting its main stem damaged in a storm. It had only just begun setting pods at that stage and the peas inside were still very immature. I thought I had nothing to lose by leaving the pods on the plant as long as possible in the hope that they would use the residual energy of the plant to carry on maturing a bit. And they did. When I finally harvested the pods, the peas were still quite small but they look like they might, just might, be mature enough to germinate. I didn't eat any of these – I wanted to conserve as many as I possibly could.

Here are the seeds from the four pink-flowered plants after being harvested and dried. As you can see there are a few differences between them. They all have subtle purple speckles on them, except for the salvaged seeds of ASF 10, which were not fully mature. ASF 15 has more of a green colour to its seeds, while ASF 06 and ASF 15 have a mixture of green and tan. The tan seeds are a trait inherited from Salmon Flowered, which also seems to be related, albeit loosely, to the pink flower trait. You may also notice that ASF 06 in particular has some variation between wrinkled seeds and rounded, dimpled seeds. The wrinkled ones are a rule-of-thumb indication of sweetness in peas, because sugar shrinks more than starch does. I probably won't select out the wrinkled ones next year though, I'll grow a bit of both, but I might possibly separate them out into different halves of the seed tray so that I can keep track of whether there's any correlation between wrinkled seeds and sweeter flavour.

That's it now, until next year when I can grow the F3. I would expect all four of the pink-flowered peas to breed true for the pink colour, as it's a recessive trait and they must be homozygous for that trait. It's likely that some of the purple-flowered phenotypes are heterozygous for pink flowers, and will produce a few of them in their offspring – so I will probably grow out some of the best of those to see if I can get some more pinks. But either way, I'm extremely pleased with what has come out of this F2 crop and I'm feeling quite optimistic about the prospect of getting peas with pink blossoms and lovely flavour, within a couple more years.


Rhizowen said...

Welcome back!

I'm looking forward to your continuing adventures in Peaseland.

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keen101 said...

Nice post rebsie! I really do like the pink flowers, though I find in my often dry hot climate the pink color washes out too much most years.

But you are right about the pink flower inheritance! It is controlled by the "B" modifier gene. I was told so by Mike Ambrose of the JIC before he retired. The old "PGENE" database is still there but has lots of broken links and hard to find.

I wrote about pink flowers several years back after being inspired by your blog! My old posts are a bit dated now and may contain some errors, lol.

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Karl K said...

I thought you might be interested in a paper that deals with the pink-flower trait in peas.
Plant Physiol. 159(2): 759–768 (June 2012)
The b Gene of Pea Encodes a Defective Flavonoid 3',5'-Hydroxylase, and Confers Pink Flower Color
Carol Moreau, Mike J. Ambrose, Lynda Turner, Lionel Hill, T.H. Noel Ellis, and Julie M.I. Hofer

The "wild type" peas are colored by delphinidin-, petunidin-, and malvidin-3-rhamnoside-5-glucosides.
The pinks have only pelargonidin-, cyanidin-, and peonidin-3-rhamnoside-5-glucosides.
A single defective gene knocks out the delphinidin/petunidin/malvidin pathway, leaving the other three pigments that accumulate because of the left-over materials.

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