and carbon dioxide levels
In order to carry on photosynthesis, green plants need a supply of carbon
dioxide and a means of disposing of oxygen. In order to carry
on cellular respiration, plant cells need oxygen and a means of
disposing of carbon dioxide (just as animal cells do).
Unlike animals, plants have no specialized
organs for gas exchange (with the few inevitable exceptions!). The are
several reasons they can get along without them:
- Each part of the plant takes care of its
own gas exchange needs. Although plants have an elaborate liquid
transport system, it does not participate in gas transport.
- Roots, stems, and leaves respire at
rates much lower than are characteristic of animals. Only during
photosynthesis are large volumes of gases exchanged and each leaf is
well adapted to take care of its own needs.
- The distance that gases must diffuse in
even a large plant is not great. Each living cell in the plant is
located close to the surface. While obvious for leaves, it is also
true for stems. The only living cells in the stem are organized
in thin layers just beneath the bark. The cells in the interior are
dead and serve only to provide mechanical support.
- Most of the living cells in a plant have
at least part of their surface exposed to air. The loose packing of
parenchyma cells in leaves, stems, and roots provides an
interconnecting system of air spaces. Gases diffuse through air
several thousand times faster than through water. Once oxygen and
carbon dioxide reach the network of intercellular air spaces (arrows),
they diffuse rapidly through them.
The exchange of oxygen and carbon dioxide
in the leaf (as well as the loss of water vapor in transpiration) occurs
through pores called stomata (singular = stoma).
Normally stomata open when the light strikes
the leaf in the morning and close during the night.
The immediate cause is a change in the
turgor of the guard cells. The inner wall of each guard cell is
thick and elastic. When turgor develops within the two guard cells
flanking each stoma, the thin outer walls bulge out and force the inner
walls into a crescent shape. This opens the stoma. When the guard cells
lose turgor, the elastic inner walls regain their original shape and the
||Osmotic Pressure, lb/in2
The table shows the osmotic pressure
measured at different times of day in typical guard cells. The osmotic
pressure within the other cells of the lower epidermis remained constant
at 150 lb/in2. When the osmotic pressure of the guard cells
became greater than that of the surrounding cells, the stomata opened. In
the evening, when the osmotic pressure of the guard cells dropped to
nearly that of the surrounding cells, the stomata closed.
The increase in osmotic pressure in the guard
cells is caused by an uptake of potassium ions (K+). The
concentration of K+ in open guard cells far exceeds that in the
surrounding cells. This is how it accumulates:
- Blue light is absorbed by phototropin
- a proton pump (an H+-ATPase)
in the plasma membrane of the guard cell.
- ATP, generated by the light reactions of
photosynthesis, drives the pump.
- As protons (H+) are pumped
out of the cell, its interior becomes increasingly negative.
- This attracts additional potassium ions
into the cell, raising its osmotic pressure.
Although open stomata are essential for
photosynthesis, they also expose the plant to the risk of losing water
through transpiration. Some 90% of the water taken up by a plant is lost
Abscisic acid (ABA) is the hormone that
triggers closing of the stomata when soil water is insufficient to keep up
with transpiration (which often occurs around mid-day).
The density of stomata on a leaf varies with
such factors as:
- ABA binds to receptors at the surface of
the plasma membrane of the guard cells.
- The receptors activate several
interconecting pathways which converge to produce
- a rise in pH in the cytosol
- transfer of Ca2+ from the
vacuole to the cytosol
- The increased Ca2+ in the
cytosol blocks the uptake of K+ into the guard cell while
- the increased pH stimulates the loss of
Cl- and organic ions (e.g., malate2-) from the
- The loss of these solutes in the cytosol
reduces the osmotic pressure of the cell and thus turgor.
- The stomata close.
- the temperature, humidity,
and light intensity around the plant;
- and also, as it turns out, the
concentration of carbon dioxide in the air around the leaves.
The relationship is inverse; that is, as CO2 goes
up, the number of stomata goes down, and vice versa. Some evidence:
- Plants grown in an artificial
atmosphere with a high level of CO2 have fewer stomata
- Herbarium specimens reveal that the
number of stomata in a given species has been declining over the
last 200 years - the time of the industrial revolution and rising
levels of CO2 in the atmosphere.
These data can be quantified by determining
the stomatal index: the ratio of the number of stomata in a given
area divided by the total number of stomata and other epidermal cells in
that same area.
How does the plant determine how many
stomata to produce?
It turns out that the mature leaves on the
plant detect the conditions around them and send a signal (its nature
still unknown) that adjusts the number of stomata that will form on the
Two experiments (reported by Lake et al.,
in Nature, 411:154, 10 May 2001):
Because CO2 levels and stomatal
index are inversely related, could fossil leaves tell us about past levels
of CO2 in the atmosphere? Yes. As reported by Gregory Retallack
in Nature, 411:287, 17 May 2001), his study of the fossil
leaves of the ginkgo and its relatives shows:
- When the mature leaves of the plant (Arabidopsis)
are encased in glass tubes filled with high levels (720 ppm) of CO2,
the developing leaves have fewer stomata than normal even though they
are growing in normal air (360 ppm).
- Conversely, when the mature leaves are
given normal air (360 ppm CO2) while the shoot is exposed
to high CO2 (720 ppm), the new leaves develop with the
normal stomatal index.
These studies also lend support to the
importance of carbon dioxide as a greenhouse gas playing an important role
in global warming.
- their stomatal indices were high
Both these periods are known from
geological evidence to have been times of
- late in the Permian period (275 -
290 million years ago) and again
- in the Pleistocene epoch (1 - 8
million years ago).
- low levels of atmospheric
carbon dioxide and
- ice ages (with glaciers).
- Conversely, stomatal indices were low
during the Cretaceous period, a time of high CO2 levels and
Woody stems and mature roots are sheathed
in layers of dead cork cells impregnated with suberin - a waxy,
waterproof (and airproof) substance. So cork is as impervious to oxygen
and carbon dioxide as it is to water.
However, the cork of both mature roots and
woody stems is perforated by nonsuberized pores called lenticels.
These enable oxygen to reach the intercellular spaces of the interior
tissues and carbon dioxide to be released to the atmosphere.
The photo shows the lenticels in the bark
of a young stem.
In many annual plants, the stems are green
and almost as important for photosynthesis as the leaves. These stems use
stomata rather than lenticels for gas exchange.