Why sunflowers are extraordinary timekeepers

Sunflowers have an interesting pattern of development and flowering. They’re actually flowers inside flowers: the outer yellow petals surround a dense bed of inner, smaller flowers or florets where the pollen-bearing and female parts are located. Those inner florets develop in a sequential, spiral pattern from the centre out; but as soon as flowering begins, things change abruptly, and this occurs in concentric circles starting from the outermost florets and working inwards in a sequence of rings towards the centre. And there’s a very good reason why the flowers employ these different timing tactics, as Chris Smith hears from Stacey Harmer…

Stacey - If you look very early in development when you see this big sunflower disc. Every small floret in the middle of that disc is an individual flower that's gonna go on to make a seed. And if you look early in development, you see these beautiful spiral patterns across the disc. And that's something that people have been, you know, noted and been interested in for many, many years. But you look at them in the middle of flowering, you'll see that that spiral pattern has become a ring-like pattern. And that is because those individual florets are undergoing development in a day by day basis. And so on one day for example, you'll have an outer ring of dozens of florets changing the, the shape of the flowers as they're releasing pollen. And then the next day, the next most internal ring of florets undergoes that developmental transition. And so we just wanted to understand how could it be that you went from a spiral pattern of development to this ring-like pattern?

Chris - When you say a spiral pattern, I'm envisaging something a bit like a corkscrew. Is that what you mean?

Stacey - Almost true. It's spirals going in opposite directions. If you look at a pine cone, for example, if you look at the bottom of a pine cone and you can see the arrangement of the different scales going in two opposite directions. Yeah, I I can't think of a good non-biological analogy!

Chris - <Laugh>, so that's a hard question you can't answer! But is the question then how the flower has the transition from a spiral pattern of development that's clearly temporal to this concentric ring pattern that's also temporal, but, but there's a different spacial organization to it, how that occurs and why that occurs during the flowering process?

Stacey - You actually said that beautifully. That was exactly the question that we had and we decided that time lapse photography was our biggest tool. First we had plants in growth chambers so we could control the environment precisely and we had them in constant temperature. And then we had them in light dark cycles to mimic a long summer day, 16 hours of light, eight hours of darkness. And so we set up little cameras and just took photographs of the discs as they were undergoing development over time. And we could indeed see these beautiful rhythmic patterns. And, and the thing that was most remarkable about this is that if you looked within a ring as, as you described it, one of these concentric rings on the head, all of those florets were developing together. You could see the florets swelling up a little bit later. You could see the anthers emerging. Those are the, the organs that release pollen. And even though the florets had been specified early in development, days apart, they were developing at this point just completely in lockstep with each other so that they could release their pollen altogether.

Chris - So were they talking to each other to do that or is it pre-specified? And they're very good at counting, so they're keeping time and they're all individual entities, but they all know very well what time it is. So they know when it's their moment and that's why they're all in lockstep?

Stacey - Exactly right. But they, it's, it turns out we, we could do our, our imaging in different environmental conditions if we put them into constant darkness. We saw very much the same pattern going from the outside to the inside of the flower. And so that says that there's an internal timer as you suggested. However, if we put them into constant light conditions, then development was totally disrupted and we could, we did some experiments where we manipulated the timing that they saw light and we found that if they were exposed to light when they expected darkness, that caused a stop in the developmental process. But if they got the light during the daytime when they expected to see it, development continued just fine. So this kind of experiment allowed us to conclude that the development was controlled both by their internal circadian timers and also it was very sensitive to light signaling pathways.

Chris - Why do they do this?

Stacey - Sunflowers are members of the daisy family and that is one of the most successful plants that there are. And people believe that they're so successful because they have this composite flower, this false flower where you have many, many florets all undergoing development at the same time as you might expect, people have shown that the more florets are developing together, the more interested bees are in the flowers. And so we believe that having all of these flutes developing at the same time and in, in fact in a quite short span of time in the morning, makes them just irresistible to pollinating insects like bees.

Chris - Well I'm glad you brought up the question of pollen because I wanted to ask that on a sort of practical basis that are there male and female flowers then, or do the individual entities, the florets, are they some of them male, some female? How does this work?

Stacey - Right. This is another fascinating thing about these plants. The flowers are alternately male and then female. If you look at the a developing sunflower head, you know midway through the flowering process you'll see that there's a ring of florets on the outside that are female. They've released their, their stigma, which is what received pollen. And then if you go in a little bit, you'll see there's a ring of florets that are male that are releasing pollen. If you waited 24 hours and looked again, you would see that the florets that had been releasing pollen are now exerting their stigma. So they're now in a female stage of development. So an individual floret is male on day one and it's female on day two.

Chris - And do they manage to avoid self pollination so you don't end up with the floret pollinating the one next door to it by accident? Cause obviously you want a remote flower with potentially a, a few different genes doing your pollination, not you yourself.

Stacey - You're exactly right. Yes. And that's the, that's we think believe why this mechanism exists is to promote cross pollination that it's very clever. If you look at a a sunflower, you have those long ray pedals and bees will typically land on the pedals and then walk in towards the disc where the, the florets are. On the way, they walk over female flowers towards male flowers where they collect pollen and then they'll go off to a different plant, a different flower and repeat the process. And so that means that they take the pollen from plant number one and you know, track it over the female florets of plant number two on their way to collect more pollen.