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Course: Biology library > Unit 34
Lesson 1: Plant responses to lightPhotoperiodism
How plants can regulate processes like flowering based on day length (photoperiod).
Want to join the conversation?
- how can a plant conduct respiration and photosynthesis at the same time?(1 vote)
- Plant cells have sites dedicated to both respiration and photosynthesis. The mitochondria are for respiration, while the chloroplast are for photosynthesis. Both processes produce the materials needed for the other to function.
Does this help?(4 votes)
- What is the meaning of MRNA(1 vote)
- it is the messenger RNA which copies the DNA sequence and leaves the nucleus through nucleoporin and attach to the ribosome where the amino acids attach to it according to the sequence forming a protein(3 votes)
- Can you explain about the forms of Phytochrome, as it is related with the Photoperiodism
A huge Thanks in advance(1 vote)- Phytochrome is a homodimer: two identical protein molecules each conjugated to a light-absorbing molecule.
Phytochromes exist in two interconvertible forms
PR because it absorbs red (R; 660 nm) light
PFR because it absorbs far-red (FR; 730 nm) light.
Absorption of red light by PR converts it into PFR.
Absorption of far-red light by PFR converts it into PR.
In the dark, PFR spontaneously converts back to PR
https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Biology_(Kimball)/Unit_16%3A_The_Anatomy_and_Physiology_of_Plants/16.4%3A_Plant_Development_-_Fundamentals/16.4E%3A_Photoperiodism_and_Phytochrome(2 votes)
- Could it be that some plants prefer moonlight over sunlight?(1 vote)
- So basically it varies on the species of plant. When it is a long day plant, it will flower when the length of the light period is longer than the dark period. During the prime flowering time, different phytochrome pigments will be converted into different phytochrome pigments because there is a greater light intensity for this to happen. Furthermore, when it is a long day plant, there will certain activators that will allow for certain proteins like the FT protein to be transcribed and translated (by mRNA and other enzymes). These proteins that have been activated will bind to certain genes that will allow for flowering to occur? Other factors that affect it are temperature and CO2, O2 concentration?(1 vote)
Video transcript
- [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.