Video Transcript
What is the primary role of the
hormone ABA, abscisic acid, in plants? (A) To stimulate cell growth in the
stem. (B) To stimulate stomatal
closure. (C) To promote plant growth toward
a light source. Or (D) to initiate seed
germination.
Let’s start by having a look at
some key terms. Hormones are chemical messengers
that travel throughout an organism to help it respond to stimuli. Remember, a stimulus is a change in
an organism’s internal or external environment that can trigger an effect. Abscisic acid, which is often
shortened to ABA, is a plant hormone involved in the regulation of plant growth and
stress responses. Let’s explore a couple of the main
functions of ABA to work out the correct answer to this question.
ABA functions to maintain dormancy
in seeds while environmental conditions are not favorable. It does this by controlling the
uptake of water in the seed’s embryo tissues. ABA also plays a crucial role in
preventing excess water loss from stomata. Stomata, one of which is called a
singular stoma, are small pores mostly found on the underside of leaves, designed to
allow gas exchange. Each stoma is surrounded by guard
cells, which can open the stomata to allow this gas exchange to occur or close the
stomata to prevent excess water loss. This stomatal closure is vital as
water vapor can be easily lost through the stomata, but the plant needs to retain as
much water as possible for photosynthesis.
Guard cells open and close because
of their turgidity. When water availability is high,
the guard cells are filled with water, which makes them turgid, causing the stomata
to remain open and resulting in some water loss. When the guard cells are not full
of water, the stomata close. The closing of stomata can be
stimulated by ABA. When water availability is low, ABA
binds to receptors present in guard cells, which causes water to leave the guard
cells, so they become less turgid, closing the stomata. This results in less water being
lost through the stomata, so more water is conserved by the plant.
This means that the correct answer
to this question is (B). The primary role of ABA in plants
is to stimulate stomatal closure.
