Video Transcript
Which of the following is not a
limiting factor for transpiration? (A) Light intensity, (B) CO2
concentration, (C) wind, (D) humidity, or (E) temperature.
This question asks us about
transpiration, so let’s start by reviewing what this key term means. Transpiration is the loss of water
through evaporation into the atmosphere. And 90 percent of all transpiration
occurs from small pores found on the surface of the leaves called stomata. These stomata are really useful as
they allow the exchange of gases, such as carbon dioxide that is required for
photosynthesis, between the leaf and the atmosphere.
Photosynthesis is the process by
which many plants are able to combine carbon dioxide and water, in the presence of
light energy, into glucose and oxygen, which can be used to release energy in
cellular respiration. As plants require water for
photosynthesis to make their own food, they usually aim to minimize the amount of
transpiration that occurs. As we know, photosynthesis requires
light energy, which is usually only available to plants during the daytime. So this is when stomata will be
open to take in carbon dioxide from the atmosphere.
When light intensity is too low for
photosynthesis to occur, for example, at night, the stomata tend to close as there’s
no point in them being open to take in carbon dioxide if it can’t be used to make
glucose. Stomatal closure during times of
low light intensity means that the loss of water through transpiration will
decrease, so light intensity is a limiting factor for transpiration, as when light
intensity is low, the rate of transpiration decreases. The question is asking us to find
an answer choice that is not a limiting factor for transpiration, so option (A),
light intensity, cannot be the correct answer to this question.
Another limiting factor affecting
the rate of transpiration is temperature. As you probably know, the hotter
the temperature, the faster liquid water evaporates into water vapor, because the
water molecules gain more kinetic energy, causing them to move faster and spread out
into a gas. If water is evaporating from the
stomata faster, the rate of transpiration is also increasing. This means that option (E),
temperature, is also incorrect.
Humidity is another factor that
affects the rate of transpiration, as if the atmosphere surrounding the leaf is very
humid, it contains a lot of water molecules, which means that the concentration
gradient between the leaf and the atmosphere is not very steep. Therefore, the higher the humidity,
the lower the rate of transpiration, as water molecules will have less of a tendency
to naturally diffuse from the leaf into the atmosphere if there is a higher
concentration of water molecules in the atmosphere than in the leaf. As a result, option (D), humidity,
can also not be our correct answer.
The final major factor affecting
the rate of transpiration is wind speed. A high wind speed causes the water
molecules directly surrounding the leaf to disperse and be swept away. This increases the concentration
gradient for water molecules between the leaf and the atmosphere, which means the
rate of transpiration will increase at higher wind speeds. Therefore, option (C), wind, is
also a limiting factor affecting the rate of transpiration and cannot be the correct
answer.
The remaining option, carbon
dioxide concentration, is an important factor affecting photosynthesis in plants,
but it does not directly affect transpiration rate. So the answer choice that is not a
limiting factor for transpiration is (B), CO2 concentration.
