August 25, 2022 at 5:34 pm | Updated August 26, 2022 at 5:12 pm | 8 min read
Though a common plant function, much remains unknown about stomatal conductance, especially at larger ecosystem scales while its associated processes like photosynthesis and, to a lesser extent, transpiration, are largely well studied. This research gap is surprising considering that sophisticated tools exist to rapidly and accurately measure stomatal conductance in the field. Modern infrared gas analyzers (IRGAs), for instance, used to analyze photosynthesis, are also equipped to measure stomatal conductance.
Definition of Stomatal Conductance
Figure 1: Structure of stomatal complex with open and closed stoma.
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Stomatal conductance (gs) is the diffusion of gas, such as carbon dioxide, water vapor, and oxygen, through the stomata of a plant. It also functions as the measure of stomatal opening in response to environmental conditions.
Stomatal conductance occurs specifically through the stomata when they are open; The reverse is known as stomatal resistance.
Stomata are found mainly in the epidermis of leaves, and other parts of the above-ground shoot in vascular plants.
The stoma is an aperture formed by two guard cells. Connected to the guard cells are subsidiary cells. When the guard cells are flaccid, they close, and when they are turgid, they open, forming the stomata. See Figure 1.
The shape of guard cells varies based on species. Based on the shape of guard cells and the number and arrangement of subsidiary cells, there are many kinds of stomata, some of which are shown in Figure 2.
In most plants, stomata are present on both sides of the leaves. In some, they are found only on one side. The density of stomata can also differ based on species. However, environmental factors can also play a role in determining the number of stomata.
Figure 2: “Stomatal traits vary between species. The eudicots (A) Arabidopsis thaliana and (B) Phaseolus vulgaris display kidney-shaped guard cells (colored in green). The grasses (C) Oryza sativa and (D) Triticum aestivum show dumbbell-shaped guard cells (solid green) and specialized subsidiary cells (light green gradient). Clear differences in stomatal size and stomatal density can be observed,” Bertolino et al. 2019. (Image credits: https://www.frontiersin.org/article/10.3389/fpls.2019.00225 )
Internal, plant-level factors consistently affect the need and application of stomatal conductance.
Internal factors controlling stomatal conductance are:
1. Signals from the guard cell and stomatal density
2. Changes in leaf water potential
3. The concentration of the plant hormone abscisic acid (ABA) in the xylem sap
4. Need to take in CO2 for photosynthesis
5. Association with arbuscular mycorrhizal fungi (AMF), which form symbiotic associations with 80% of plant species, can change stomatal conductance. The exact mechanism and mode of action are yet unclear.
Figure 3: Influence of soil moisture on stomatal conductance, Carminati, and Javaux, (2020). (Image credits: https://doi.org/10.1016/j.tplants.2020.04.003)
Environmental factors can also affect stomatal conductance.
External environmental factors influencing stomatal behavior are as follows:
1. Light is the primary external factor that determines stomatal conductance, as stomata are activated and open during daylight.
2. Humidity influences stomatal conductance. Low humidity reduces stomatal conductance to preserve water. Stomata will open in high humidity even if the leaf water content is less.
3. Soil water and nutrient status will also impact stomatal conductance, which will decrease when soil moisture is less. See Figure 3.
4. Air temperature rises will increase stomatal conductance, independent of plant water status and photosynthesis. While this helps plants cool through evaporation and increases photosynthesis, plants lose more water.
5. Elevated atmospheric CO2 concentration decreases stomatal conductance, which can end up reducing photosynthesis.
6. Salinity stress also reduces stomatal conductance–this is significant as nearly 7% of the global land is saline.
The internal and external factors that control stomatal conductance will also interact, so stomatal response can be complex.
Functions of Stomatal Conductance
The influencing factors described above will also affect the vital plant functions that stomatal conductance is involved in. For instance, stomatal conductance is necessary for gas exchange in plants. Stomata are usually open in the daylight to allow CO2 in for photosynthesis and oxygen for respiration.
When stomata are open, water escapes in the form of vapor into the atmosphere, as transpiration. Therefore, stomatal resistance is necessary to limit plant water loss by closing stomata.
Increases in temperature and water status of air and soil, which control stomata, will all influence the amount of carbon fixed by photosynthesis as well as the amount of water lost by transpiration. Ultimately the survival and productivity of plants are affected.
Moreover, plants emit over 30,000 volatile organic compounds. Their emission is controlled by the stomata but also depends on compound volatility and concentration in the leaf water and lipids. External factors of light and temperature that control stomatal conductance also influence the emission of these volatile compounds.
Thus, stomatal conductance is essential for plants and for the global carbon and water cycle. Because the process regulates CO2 and water vapor, it also plays an essential role in climate. However, this aspect received little attention until climate change became a noted force to reckon with. Therefore, in-depth information on the pattern of stomatal conductance in various ecosystems, geographies, and climates, is not yet readily available.
Nocturnal Stomatal Conductance
Leaf stomata are closed at night as no photosynthesis is triggered without daylight. However, nocturnal conductance (gn) does occur through cuticles. Some nocturnal conductance also happens through stomata.
Though it is not well understood, there are many hypotheses to explain why nocturnal stomatal conductance exists, such as:
- Removal of excess CO2 formed during respiration
- Transportation of oxygen mixed in xylem sap to leaves in tall trees
- Transportation of nutrients
- Leakage due to improper closure of stomata
- Predawn controlled opening of stomata allowing for an early start in photosynthesis, which is helpful in cases or phases of rapid growth
As of this writing, however, there is no broad agreement on any of these hypotheses.
Measuring Stomatal Conductance
Stomatal conductance is measured as the rate of entry of carbon dioxide or exit of water vapor through stomata.
Conventional methods of measuring this process relied on porometers, which can be dynamic or steady-state.
- Dynamic porometers use a humidity sensor in the closed chamber where the leaf is placed to measure water vapor changes.
- Steady-state porometers measure water vapor levels before and after the entry of air into a chamber with the leaf.
Modern methods include the use of infrared gas analyzers that are capable of giving a much clearer picture by measuring all vital gas exchanges–stomatal conductance, photosynthesis, and transpiration–simultaneously. For example, the CI-340 Handheld Photosynthesis System, manufactured by CID-Bioscience Inc. is a highly-portable device, with leaf chambers in ten sizes to suit different leaves. It also has modules that can control CO2, H2O, temperature, and light intensity to study the effect of these factors on the process.
Applications of Stomatal Conductance
Figure 4. Measuring stomatal conductance in coffee under different temperatures and plant water status, Craparo et al., 2017. (Image credits: https://doi.org/10.1016/j.scitotenv.2017.07.158.)
Climate change-associated rises in temperature and drought, along with global water shortages, have increased interest in studying and utilizing stomatal conductance data. Some of the common applications are discussed below.
1. Water use efficiency and drought: Measuring stomatal conductance is important to evaluate water use efficiency by plants. This has important implications in crop research where scientists try to find means of drought-proofing future varieties of grains, legumes, oilseeds, and fresh produce.
2. Water use efficiency and carbon increase: To optimize forest management, stomatal conductance is being studied on an ecosystem scale, to see the effect of rising levels of carbon dioxide, on water use efficiency. Through stomatal conductance, a tradeoff is expected between photosynthesis and water use. While studies show photosynthesis is increasing, studies do not agree on the increase in water use efficiency achieved on the scale of trees and ecosystems.
3. Indicator of stress: Leaf stomatal conductance is being used to measure plant water stress, due to salinity, heat, and drought, in plant breeding and forest research.
4. Estimate pollutant levels: Plants absorb many atmospheric compounds, such as tropospheric ozone, that are harmful to plants and ecosystems. The ozone can reduce biodiversity, change species composition, and water and nutrient cycles. The role that stomatal conductance plays in different soil and air conditions in regulating the entry of these compounds is being considered at continent scales.
5. Adaptation to the environment: Since stomatal conductance is connected to vital physiological processes like photosynthesis and water balance, it is used to study a plant’s response to the environment and its capacity to adapt, survive, and grow. See Figure 4.
Research Has Just Begun
Much of the in-depth research on stomatal conductance at ecosystem scales is less than ten years old. Hence, the information on quantification is by no means complete. To achieve a holistic understanding of this information, plant-level measurements from all corners of the world are necessary before they can be scaled up. Despite current theories, there is also no clear consensus on the internal control of stomatal conductance.
Given the importance of stomatal conductance to not only plant health but the health of our ecosystems globally, the opportunities for future research in this area remain vast. Using modern tools, innovative researchers are sure to provide further insight into this vital process in coming years.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
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