Photoperiodism | Plant Biology | Khan Academy

So one question that biologists have long asked is: how do plants know what to do at different times of the year? 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 as length; the length of the light, or another way of thinking about it is based on the day length.

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. There are certain flowers that 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 with how it 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. 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 one way to think about it is that we can see here that this is a short-day plant because when the day is short, when it's only 8 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—it doesn't flower. This is a short-day plant.

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? 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.

One conclusion that you can take from this is that 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 a night last without interruption from some type of light? This tells us that these short-day plants are actually more dependent on the night length. So, you could think of a short-day plant as a long-night plant.

As we know, 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." This type of result, when these experiments have been performed, makes 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. 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: night length. A long-day plant that's dependent on night length could be called a short-night plant. These are called dark-dominant. But you have other long-day plants that might flower when the days are longer, say as we are entering the summer, that actually are dependent on day length.

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. This type of situation where you have day length—where you have some external cue plus maybe some internal biological cues—this is called an external coincidence model.

An example of that is a plant that produces mRNA every day. Every day, as the day starts to end, it starts to produce mRNA, and this mRNA codes for a protein. This is the Arabidopsis plant—I can never pronounce things well, but in the Arabidopsis, it produces the constant protein. I'll just call that Co for short.

The constant protein, once it gets to a high enough threshold, is going to start— the plant is going to start flowering. I'm oversimplifying the mechanism. As with all biological mechanisms we see, when you dig down, it's a lot more detailed than this, but this gives you the general sense.

What happens, 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 constant protein—if you start producing a bunch of this constant protein, why doesn't this thing flower every day?

The answer is that when this constant 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, if this day were to continue, well, that triggers proteins that actually protect these constant proteins and keep them from being degraded. So, in this longer-day situation—so, in this longer-day situation—let me 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. So, in this longer-day scenario, these things—this Arabidopsis—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 queuing to a plant what time of year it is or whether it should regulate.

Sometimes, it's a combination of things. Sometimes, it's a circadian rhythm combined with day length. We've also seen sometimes it's not the day length, but it actually might be the night length that matters most.