So in the last video, we have talked about
phototropism, which is a plant’s tendency to grow toward the light. There’s actually Auxin, a kind of plant hormones,
that makes phototropism happen by elongating plant cells. So say if a plant senses the light coming
from one direction, Auxin will accumulate in the dark side of the stem. And Auxin elongates those plant cells in the
dark side, and the stretched cells push the stem towards the light. So Auxin is just one of many kinds of plant
hormones that can affect plants’ behavior and development. Today we are going to have a look at five
major types of plant hormones including Auxin, Cytokinins, Gibberellins, Abscisic Acid and
finally Ethylene. So when we speak about Auxin, in this video,
and in most college Biology textbooks we are actually referring to a material called indoleacetic
or called IAA. IAA is generated in the apical bud of a plant
and is transported from top to bottom. It can also be found in embryos with seed
in fruits. So the two major functions of Auxin are: one,
it elongates plant cells; and two, it promotes plant cell’s division and differentiation. So how does Auxin make plant cells elongated? One hypothesis is that Auxin initiates elongation
by weakening cell walls. We have already seen how this elongation effect
makes phototropism happen. Auxin also gives plant longer stems and roots
and actually Auxin is the mechanism behind apical dominance, which is a phenomenon that
a plant’s main stem develops dominantly over its side stems. So now auxin effects on prompting cells division
give a plant stronger stem with wider diameter. And also it gives a plant more roots, which
means a better development of the root system. And actually, Auxin is the key fact for a
fruit’s development because Auxin exists richly in seeds of fruit and it helps the fruit to
grow bigger. Now as Auxin makes a plant looks taller, Cytokinins
make a plant look bushier by prompting its cell division. So a plant with rich Cytokinins in it tends
to get a more differentiated root system and also more axillary buds. Cytokinins can also keep plant tissue looks
young and fresh. So in commercial, people usually spray Cytokinins
on fresh cut flowers to help them maintain a fresh look. Now let’s assume you have two identical plants
kept in the same environment, with the same room temperature, whether the plant will look
tall and thin or bushy and short depends on the balance between Auxin and Cytokinins. So in commercial, people usually cut off a
plant’s apical bud where Auxin is generated. So the Auxin percentage in a plant will drop,
and the Cytokinins in it will be comparatively high. So you get a bushier plant, which is a more
favorable product in the commercial. Cytokinins are generated in a plant’s root
system and are conducted in a reverse direction with Auxin which is from bottom to top. So this is why we always see a triangle-shaped
plant, not a reverse triangle and nor a star-shaped plant. That’s because the plant is always growing
horizontally in the bottom part and vertically in the apical part. And now let’s talk about Gibberellins. Gibberellins can promote a plant’s seed germination,
bud development stem elongation, root development and it’s in a plant’s apical bud, young leaves,
and root system. So yeah you can say it’s like overall nutrition
to a plant and it’s everywhere. And in commercial, Gibberellins can be very
useful in treating dwarf plants. And it can also produce bigger fruits by elongating
those fruit stems so each of those fruits has a bigger room to grow. And who does not like bigger fruits? But when we talk about plant hormones, it’s
not like the more the merrier. Actually rice seedling with too many Gibberellins
in it can grow spindly and tall and it will topple even before it can produce any grain. So far we’ve been talking about growth hormones. But there’s actually a kind of inhibiting
hormones called Abscisic Acid. Abscisic Acid counteracts with other growth
hormones by inhibiting the plant’s growing process. So Abscisic Acid is like a sleeping pill for
plants. It can put plant seeds in sleep, it can also
put a plant in sleep by closing stomata when there’s a lack of water in the environment. But “Why?”, you may ask, “does a plant need
to be put in the sleep?”. Well, a short version is that timing is very
important for a plant’s development. Say a seed does not want to germinate in a
harsh wintertime and a tree does not want to grow more leaves during a drought. But again, a plant’s behavior is not determined
by a type of hormone alone. For example, if a seed will germinate or not
is actually depending on the counteraction of Gibberellins and Abscisic Acid. And that seed will only germinate when the
percentage of Gibberellins in it overwhelms Abscisic Acid. And now finally let’s talk about Ethylene. Counteracting with Auxin which promotes growing
process, Ethylene triggers plant’s aging process like fruit ripening and falling leaves. I’m gonna put an A and B here. First, let’s talk about fruit ripening. The process includes breaking down of cell
walls which softens fruits and also a conversion from starch and acid to sugar which sweetens
the fruit. But the fact that Ethylene is a form of gas
under room temperature means that it will flow around. So if you have a bunch of apples and one of
them is ripe, the Ethylene it releases will ripen the rest of them one after another. And that’s why people say that “A rotten apple
can ruin a whole batch of them”. And because of the same reason, it’s always
a good idea to hang up the bananas you bought from the market. The point is to prevent Ethylene from accumulation,
instead of to fool the bananas by making them think that they’re still growing on the tree
so they won’t rot. And now let’s think about falling leaves. Falling leaves are just beautiful but why? I mean why is it necessary for the leaves
to fall? Well, the answer is that leaves are falling
to prevent a plant from drying out in winter when the roots cannot take enough water from
the frozen ground. The falling of the leaves decreases transpiration
from leaf’s surface and thus protects the plant. Well, I’m gonna say once more if the leaf
stays on the tree or it falls depends on the counteraction between Auxin and Ethylene,
in which Auxin prompts the plant’s growth and prevents the leaf from falling, and Ethylene
forms an abscission layer and stimulates the synthesis of enzymes that digests the abscission
layer. Um, let me explain. Now I have a stem here in brown and the blue
one is the protection layer which is generated even before the leaf falls. And after the leaf falls, the protective layer
will form a leaf scar which prevents the plant from infection. And the red layer is the abscission layer
that Ethylene forms – and the enzyme is eating the abscission layer away. So when the abscission layer is thin enough,
and plus the weight of the leaf itself and the blowing wind, the leaf will just fall. So when winter comes, the drop in temperature
triggers the increase of Ethylene while Auxin in leaves drops when the leaves age. And Ethylene becomes comparatively high in
leaves, making the leaves fall. Okay, so let’s draw a conclusion from today’s
discussion. There are many kinds of plant hormones and
we have seen five types of them. There’s Auxin, Cytokinins, Gibberelism, Abscisic
Acid, and Ethylene, among which Auxin elongates plant cells, results in apical dominance,
promotes fruit growth and makes phototropism happen. Cytokinins prompt cell’s division, makes plants
bushy, and keep plants young and fresh. Gibberellins trigger seed germination and
give plants long stems. But too many Gibberellins will make your plant
turn yellow and tall and fruitless. Abscisic Acid puts seeds and plants in sleep. And finally, Ethylene controls the aging process
including fruit ripening and falling leaves. And we have also learned that it’s not a single
type of hormone alone but their combined effect that determines a plant’s behavior and development
to better adjust itself to the changing environment. That’s for today. Hope you enjoy.