Photoperiodism Video Lecture | Science for ACT

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