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- [Instructor] So one question that biologists have long asked is how do plants know what to do at different times of the year? And one mechanism by which they know, kind of, you could say, what time of year it is, is through photoperiodism, photo for light, and then period you could think of length, the length of the light, or another way of thinking about it, based on the day length. And an often cited example of photoperiodism is that many plants figure out when to flower based on the length of the day. Certain plants flower when the days are long, those are naturally called long day plants, and there are certain flowers, actually flower when the day is short, and those are logically called short day plants. So the day length seems to have something to do, somehow regulates when the plants actually flower, but how does the plant actually do that? How does it respond to the length of the day? Well, different experiments have been performed, and for example, short day plants, let me scroll down here to look at an experiment dealing with short day plants, so it turns out that short day plants, when you look at this, actually let's just look at the data before I explain what's going on here. So this is just the hours of the day, this is midnight, and then we get back to midnight, I guess that's one way to think about it, and we can see here that this is a short day plant because when the day is short, when it's only eight hours and the night is long, this plant flowers. So this one flowers, and when it's the other way around, when the day is long and the night is short, well, it doesn't flower, and so this is a short day plant. And so an interesting question is, is it the length of the day that is dictating whether the plant is flowering, or is it the length of the night that is dictating whether the plant is flowering? And it turns out that for many short day plants, often, actually most of the ones that have been studied, that if you have one of these short days but you interrupt the night with just a little brief moment of sunlight, just a few minutes of sunlight right over here, the plant will not flower. And so one conclusion that you can take from this is it's actually not so much the length of the day because if it was just the length of the day it would make sense that it would still flower here. But it's actually the length of an uninterrupted night. How long does the night last without interruption from some type of a light? And so this tells us that these short day plants, it's actually more dependent on the night length. Night length, so you could think of a short day plant as a long night plant, but as we know, and over here we can see that when you interrupt the day, that doesn't make the difference, the plant doesn't all of a sudden start flowering when it says, oh, I got a little bit of darkness in my day, my day has been broken up somewhere. So this type of result, when these experiments have been performed, make us think, okay, at least for many short day plants it seems like night length is what actually matters. Now, for other types of plants, and in everything I talk about, this isn't absolute, it's not that all short day plants operate one way or all long day plants operate another way, but this is to give you a sense of the various mechanisms we find in the world around us. So for example, many long day plants actually do operate on the day, actually, let me write this down. So long day plants, you can think of them in two groups, there are the ones that are dependent on night length, so night length, and so, a long day plant that's dependent on night length, you could call it a short night plant. Short night, and these are called, these are called dark dominant. Dark dominant. But you have other long day plants, plants that might flower when the days are longer, say as we are entering the summer, that actually are dependent on day length. And sometimes it's not just the photoperiod, it's not just the day length that is dictating some type of biological process, it's day length plus some other type of thing. So, plus other things that might factor into it. And this type of thing where you have day length, or when you have some external cue, plus maybe some other cues, maybe some of these are internal biological cues, this is called an external coincidence model, because the external factors coinciding with maybe something that is happening internal. And an example of that is a plant where it produces mRNA every day, every day as the day starts to end, it starts to produce mRNA, and this mRNA codes for protein, this is the Arabidopsis plant, I can never pronounce things well, but in the Arabidopsis plant it produces every day, this is just part of its circadian cycle. Circadian cycle is just this daily cycle, you can see that it produces, it starts producing a lot of this CONSTANS gene mRNA and this CONSTANS mRNA produces the CONSTANS protein, I'lljust call that CO for short, and the CONSTANS protein, once it gets to a high enough threshold, the plant is going to start flowering. And I'm oversimplifying the mechanism, as all biological mechanisms we see, when you dig down, it's a lot more detailed than this, but this gives you the general sense. But what happened, you might say, okay, well if this is happening every day, if during the daytime I guess you can say, the mRNA levels are low, but then as we get further and further into the day, the mRNA levels go up and we start producing this CONSTANS protein, if you start producing a bunch of this CONSTANS protein, why doesn't this thing flower every day? And the answer is when this CONSTANS protein gets higher and higher but there isn't light, it just naturally gets degraded. It just naturally gets degraded. So in the situation that I'm drawing right over here, if this is a shorter day, well, this plant won't flower. But if we have a longer day, if we have a longer day, let me make the day a little bit longer now, so if we have a longer day, so if this day were to continue, well, that triggers proteins that actually protect these CONSTANS protein and keeps them from being degraded. And so in this longer day situation, so in this longer day situation, make it clear that the day has gotten longer here, these things won't be degraded, you can think of it as the light is triggering things that are protecting these proteins, and so in this longer day scenario, these things, this Arabidopsis plant will actually flower. So the whole point of this video is to appreciate that photoperiod can affect whether a plant does something like flowering or not, and flowering isn't the only biological process that might be dictated by day length, and day length isn't the only way of cueing to a plant what time of year it is or whether it should regulate. And sometimes it's a combination of things, sometimes it's a circadian rhythm combined with day length, and we've also seen sometimes it's not the day length but it actually might be the night length that matters most.