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Current time:0:00Total duration:7:56

Video transcript

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 law 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 when the plant actually flower but 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 lets us 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 one 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 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 that is dictating whether the plant is flowering and it turns out that for many short day plants often well actually most of the ones that have been studied that if you take if you have one of these short days but you interrupt the night with just a little brief moment of sunlight just a little 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 from some type of a light and so this tells us that the 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 and over here we can see that when you interrupt the day that doesn't make the difference that doesn't the plant doesn't are 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 this type of result when these experiments have been performed make us think okay it's at least for our many short day plants it seems like night length is what actually matters now for other types of plans and everything I talked about this isn't absolute it's not that all short-day plants operate one way or all a 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 they're the ones that are dependent on night length so night length and so a long day plant that's dependent on light length would be you can 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 photo period it's not just the day length that is dictating some type of biological process it's day length some other type of things so plus other things that might factor into it and this type of thing where you have day lengths or when you have some external q plus maybe some other cues maybe some of these are internal biological cues this is called an external coincidence model because the external factor is coinciding with maybe something that is happening in internal and an example of that is a plant where it produces mRNA every day every day is the as the day starts to end it starts to produce mRNA and this mRNA codes for protein this is the Arab topsis plant I can never pronounce things well but in the arab topsis plant it produces every day this is just part of its circadian cycle circadian cicle is just this daily cycle you can see that it produces it starts producing a lot of this constant gene mRNA and this constant mRNA produces the constants protein I'll just call that Co for short and the constant protein once it gets to a high enough threshold it is going to start 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 detail than this but this gives you the general sense but what happened you might say okay well if this is happening everyday if the you know during during you know during the day time I guess you could 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 konstanz protein if you start producing a bunch of this konstanz protein why does it this thing flower every day and the answer is is when it's when this constance protein gets to gets higher and higher but there isn't light it just naturally gets degraded it just naturally gets degraded so in the 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 if we have a longer day so if this day were to continue well that triggers proteins that actually protect these konstanz 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 Arab topsis plant will actually will actually flower so the whole point of this video is to appreciate that photo period photo period can affect whether a plant does something like flowering and not in flowering is it 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
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