03 August 2016

Sorghum breeding

Strictly speaking I work in corn breeding.  But that's just what I do at work.  (I also have some at-home corn breeding experiments planned for next year.)  At home, I breed pumpkins and sorghum.

Here is some sorghum.
This is a mix of things.  The paper bags on two of the plants are there mainly to keep the birds from eating all the grain (although they're really pollination bags).  You may notice a single very tall plant in the back (it's hard to see because it's so narrow and the grain head hasn't emerged yet).  That is a breed of popping sorghum called Allu Jola, which I've never grown before.  But in front of that are six plants of the variety I call Smitty's Dwarf.  You may notice that the three on the left are not especially short.  They're a bit shorter than the grain sorghum I started with, but they're nothing special.  The three on the right are a bit shorter.  None of these are great, though.  (Still, I want the grain this year.  Last year the birds ate almost everything.)

Last year I planted about 40 plants of a standard white grain sorghum originally from Kansas.  The average plant was about five feet tall.  I want a dwarf plant.  (Why?  I don't know.  I just wanted a project.  I tell people my goal is to breed up a high-yielding dwarf that I could grow to sell to brewers for a gluten-free malt.  But that would probably require that I actually malt the grain, and I don't have the capacity to do that.  If you'd like to donate to a Kickstarter that would allow me to buy both a set of commercial ovens and 25 acres of farmland....)
After last season I selected the two shortest plants, and retained the grain from them.  I would have preferred four or eight plants, but have I mentioned the birds?  Grosbeaks LOVE sorghum.

 This sorghum is a good bit shorter than that in the first picture.  You can see one of the regular-height plants here on the far right, and the six shorter plants in the plot to the left.  You may also see the popcorn that's growing behind the six shorter plants.  I strongly suspect proximity to the much more vigorous popcorn might have something to do with how short these plants are relative to the ones in the first picture.  It's tough to say, and I'll include these plants among the total when I select the shortest ones for next year, but it's impossible to say whether this is the genetics or the environment at work.  (Notice also the big batch of river oats intruding from the left side of the picture; this sorghum is hemmed in on all sides.  Next year the sorghum is getting a big plot all to itself.)

So.  Sorghum is self-fertile, like most plants.  This means that I could plant a single sorghum plant and, assuming there was a bit of wind while it was shedding pollen, the pollen from that one plant would pollinate the ovaries on that plant and I'd get fertile seed.  The fertilization rate wouldn't be great, although I could slip a bag over it and capture the pollen and hope to get better fertilization.
When multiple plants are around, though, sorghum plants can cross-pollinate.  Pollen from one plant may get blown around and fertilize ovaries on another plant.  There's no way for the plant breeder to know when that happens (hence the paper bags).  Plants in the field like mine, left uncovered, are referred to as "open-pollinated."  I'm not deliberately trying to self- or cross-pollinate them.  In an open situation, the percentage of seeds that arise from pollen from a different plant is referred to as the "outcrossing rate."  In a large field of grain sorghum outcrossing rates may range from about 7% to 35%, although research has reported outcrossing in certain varieties and environments all the way from 0% to 100%.  The 100% rate seems impossible and I'm suspicious of the methods of the researchers who reported it.
Anyway, in my yard, since the sorghum isn't terribly close together, I expect a fairly low outcrossing rate.  And since I'm putting bags up before the last 1/3 of the seeds are pollinated, that's reducing my total outcrossing rate.  (If I'm being diligent, and I don't want to ensure selfed plants, I'll select seeds for next year exclusively from the top 2/3 of the seedheads, which pollinated before the bags went on.  Or, if I do want selfed plants, I'll select from the bottom 1/3.  I haven't decided which I'm going for yet.)
So, I'll select the shortest 10 or 20% of plants, and take 20 or so seeds from each one (this is easy, because sorghum makes hundred or thousands of seeds per plant).  I'll put the seeds in labelled packets and sow them in blocks next year.  Then I can both compare how the blocks perform against each other, and how the individual plants within each block perform.  I'll self-pollinate the ones I like best and continue.  Because of the way plant genomes work, it takes several generations of self-pollination to get a batch of plants that are truly genetically identical If I had acre upon acre (and days upon days, and funding from an interested party) I'd self ALL of them and plant a whole huge field with 150 or 200 plots and try to get several different lines out of it.  But this is a side project.

Now here's an interesting candidate plant.  You can't quite tell in this picture, but this plant is only a foot tall.  It's extraordinarily short, shorter than even dwarf rice and wheat are.  Possibly this is a mutant, or it has a disease (it is yellowing early), or a virus, or some other condition that's causing this.  If it's a mutant, hooray!  By self-pollinating it, and then selecting the dwarfiest of its progeny and selfing them, and so on for four or five generations, I should be able to isolate a population of foot-high sorghum.  Provided it also actually produces a decent yield (this plant has a surprisingly large grain head considering how small and yellow it is) and is reasonably disease tolerant, this would constitute the end goal of the Smitty's Dwarf project.  I might even apply for a patent on it.  But that would be several years down the road.

That stunted dwarf is only one of the interesting plants I got in this year's batch.
This picture has several interesting things going on.  (Okay, I know that when I say "interesting," what I mean is, "interesting to me."  But you're reading this.  Whoever you are.)

If you click on the picture to blow it up, you'll see a few markings.  The two plants with the brown bags are about 3 feet high or so, which is pretty dwarven as sorghums go.  And marked with a red A is the white tassel bag from the micro plant pictured above.  It is REALLY short.
Above you see C and D.  These refer to two of the key elements of sorghum plant height: C is the height between the "flag leaf" (the very top leaf, just below the grain head), and the second leaf.  In all these short plants, the main leaves are quite close together, and then there's some greater distance between the second leaf and the flag leaf.  In regular plants, those distances are all about the same.  This suggests that the distance between leaves (called the "internode length" because each leaf is a node) is not controlled by the same gene as the distance between the flag leaf and the second leaf.  Additionally, at D, you can see that in these two plants, there's a significant difference in length between the flag leaf and the start of the grain head (which is right at the bottom of the bags).  In a large-scale planting, you want the distance between flag leaf and grain to be as great as possible, and uniform across all the plants in the field.  Otherwise you're likely to end up with a lot of leaf and stem trash in your harvest.  So, as a breeder, I will be selecting for long D lengths and shorter C lengths.  Assuming I can find such a thing.
Then, over on the right, there's plant B.  Sorghum can look a lot like corn when it's allowed to grow tall, but it really acts more like wheat or barley.  These are traditional small grasses.  The seed germinates and produces leaves and a stem.  Then, once there are three or four leaves, the plant stops making new leaves and instead creates a new little stem off to one side, which will produce its own leaves and eventually its own grain head.  These little side stems are called "tillers" and almost all grasses make them, including corn and sorghum.  In corn, the tillers are a nuisance, often sterile or with one of those combined tassel/ear things I've posted pictures of.  Tillers tend to be a nuisance in sorghum, too, not because they don't produce viable seed--they do--but because they are generally shorter and much later maturing than the main stem, so the seeds aren't ready when harvest time comes along and even if they were, the combine won't cut them because they'll be so much shorter than the main grain head.
In wheat, barley, and rice, however, the plants add new tillers throughout the growing season, then, responding to a change in day length or temperature (or both), all the tillers will produce a grain spike at the same time.  So when you see a field of wheat, say, and you go out and look and count 30 or 40 separate spikes, those could be from as few as one or two plants.  This means you can get a field full of grain with fewer seeds.
Now, a single grain spike of wheat or two-row barley might have 24 or 30 seeds max.  A single spike of sorghum can have over 3,000 (although in a typical field it's likely to be substantially less).  But you need one seed for each spike.  What if we could get sorghum to grow like wheat--make a bunch of tillers at the beginning of the season, and send up all the grain heads at the same time?  Potentially, this could mean more grain comes out of the field with less seed being planted.  Since seed is a substantial cost to farmers, this would make sorghum much more economical to grow.
Well, that's what plant B in the picture above looks like.  It produced five tillers (for six total stems) before it started flowering, and it looks like all six grain heads will be flowering within about a week of each other.   (In the picture, I have spikes 1-5 labelled.  The sixth one is actually hiding directly behind number 1.)  I didn't expect to see anything like this and actually almost tore the plant out last month because it looked so...broken.  I'm glad I didn't.  Each of the six grain heads is smaller than the one head on most of the other plants, but taken as a group I suspect they might be much bigger.  Given that this plant is growing in the same soil and within two feet of several other plants that look normal, I really don't (want to) think this is an environmental effect (although, again, viruses can do very strange and unexpected things to plants).  So this is now sorghum project number two: a tillering sorghum (Smitty's Tillering Sorghum sounds pretty bad, so for now I'm calling it Hydra).  If this turns out to be a trait that can be passed down, and again the plant isn't for some other reason horrible, this could be a very interesting side project.