The Crucial Role of Stomata in Plant Transpiration and Photosynthesis
When did Stomata Develop?
Stomata developed almost 400million years ago in the Silurian – Devonian period when plants left the seas and ‘invaded’ the land. In order to survive, the plants had to develop features that would prevent excessive water loss whilst allowing access to CO2 for photosynthesis.
Stomata and vascular tissue evolved almost simultaneously and these three adaptations to the terrestrial environment were KEY to the inhabitancy and development of large terrestrial plant species.
Stomata General Information
Stomata are pores formed by a pair of cells, the guard cells which can open and close to control the exchange between a plant and the environment. These pores are the entry points for CO2, for photosynthesis and an exit for water vapour from the transpiration stream.
Stomata’s major function is to allow sufficient CO2 to enter the leaf thus optimising photosynthesis, while conserving as much water as possible.
Under some environmental conditions, evaporative cooling of the leaf by water loss via transpiration may be a factor in lowering leaf temperature.
Oxygen exchange between a plant and its environment is not greatly affect by stomata.
Control of Stomatal Movement and Their Role In Transpiration and Photosynthesis
In plants 99% of water taken in by the roots is released into the air as water vapour. FACT! In a single day 200 to 400 litres of water can be lost by a single deciduous tree growing in a temperature summer!
Water loss via water vapour is termed transpiration; this may involve any above ground part of the plant body. The leaves of the plant are the principal organs of transpiration and the stomata are the conduit for the water loss.
Excessive transpiration (output exceeds input) stops/slows the growth of many plants and kills many plants by dehydration. Stomata have special adaptations that will be mentioned shortly to minimise water loss while promoting the acquisition of CO2.
The cuticle serves as an effective barrier to water loss
The waxy cuticle on a leaf is an effective barrier to water movement. It is estimated that only about 5% of water loss from leaves is via the cuticle. The waxy cuticle restricts diffusion through the leaf so that water vapour and other gases must enter and exit via leaf stomata.
As long as stomata are fully closed and the temperature is stable then the air contained in the leaf will ‘normally’ be saturated with water vapour.
The waxy cuticle in most plants prevents gases exchange although this depends on the thickness and composition of the cuticle. Since the level of diffusion of gases through the leaf is so low the opening and closing of stomata controls the exchange of water vapour and other gases across the leaf surface.
How do Stomata Open and Close? – Stomatal features
Changes in the shape of the guard cells bring about the opening and closing of the stomata. Although stomata occur on all aerial parts of the primary plant body, stomata are most abundant on leaves.
Stomata are present on both sides of leaves but are more frequent on the lower (abaxial) surface of the leaf. In grasses stomata are usually present in equal numbers on both sides due to the positioning of the leaf towards the sun.
The stomata lead to a honeycomb of air spaces which constitute 15-40% of the total leaf volume. This space in the leaf contains air saturated with water that has evaporated from the damp surfaces of the mesophyll cells.
The closing of stomata not only prevents loss of water vapour but also prevents entry of CO2 into the leaf.
The stomata of dicots consist of two kidney-shaped guard cells, whereas grass guard cells tend to be more elongated. Guard cells contain very few chloroplasts while their neighbouring epidermal cells contain many chloroplasts.
Stomatal openings occur when solutes are accumulated in the guard cells, which causes osmotic movement of water into the guard cells. This builds up in turgor pressure in excess of that in the surrounding epidermal cells causes the stomata to open.
Stomatal closing is brought by the reverse of the process above; with a decline in guard cell solutes. Water will move out of the guard cells thus causing a turgor pressure change (decreases) and the stomata will close. Active solute transport is therefore essential to maintain or lose turgor pressure in the osmotic movement of water (opening and closing the stomatal cells).
Stomatal Movement Involves a Specific Hormone Response Pathway
A number of endogenous and environmental signals influence stomatal pore size such as CO2, water, light and circadian rhythms. Abscisic acid (ABA) is on endogenous signal that is important in the control of stomatal movement.
The important solutes that contribute to the osmotic potential of guard cells are Cl-, K+ ions, which are actively pumped into the cells and malate2- (anion) a negatively charged carbon compound that is synthesised by the guard cells.
The opening of anion channels results in the rapid movement of anions, primarily Cl-, malate 2- from the cytosol to the cell wall. This depolarisation of the plasma membrane triggers the opening of K+ channels.
The result is the movement of K+ ions from the cytosol to the cell wall. This rapid movement of Cl-, malate2- and K+ results in a less negative osmotic potential of the cytosol and a more negative osmotic potential of the wall.
Water then moves down its water potential gradient from the cytosol to the cell wall, reducing the turgor of the guard cells and causing closure of the stomatal pore.
When Abscisic acid (ABA) signal is removed, the guard cells slowly transport the potassium and chloride ions back into the cell. A more negative osmotic potential is re-established within the guard cells, water flows into the cells by osmosis. This water flowing into the guard cells increases the turgor pressure of the stomata thus causing it to open.
A Radial Orientation of Cellulose Microfibrils is Required For Pore Opening
The structure of the guard cells plays a crucial role in stomatal movements. The structure allows radial orientation of the cellulose microfibrils in the guard cells. This radial micellation allows the guard cells to lengthen while preventing them from expanding laterally.
The second constraint is found at the ends of the guard cells, where they are attached to one another. This common wall remains almost constant in length during opening and closing of the stoma.
Environmental factors or signals also affect stomatal movements
A number of environmental factors affect stomatal movement such as CO2, light and temperature. In most species an increase in CO2 causes stomata to close. This varies greatly from species to species. In the majority of plant species, the stomata opens in the light and closes in the dark; this is explained by the fixation of CO2.
Blue light has been known to stimulate stomatal opening independently of CO2 levels. The blue light response is involved in stomatal opening in the early morning and in stomatal responses to sunflects and spots of light. The stomata opening can range in duration from a few seconds to minutes in blue light and normal light.
Within normal ranges (10o to 25oC), changes in temperature has little effect on stomatal behaviour, but high temperature over 30o can lead to stomatal closure. An increase in temperature results in an increase in respiration.
Stomata do not only respond to environmental factors but also exhibit daily rhythms (circadian rhythms).
There you go! The role of plant stomata in transpiration and photosynthesis.
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