Stomatal conductance, usually measured in mmol m−2 s−1 by a porometer, estimates the rate of gas exchange (i.e., carbon dioxide uptake) and transpiration (i.e., water loss as water vapor) through the leaf stomata as determined by the degree of stomatal aperture (and therefore the physical resistances to the movement of gases between the air and the interior of the leaf).
The stomatal conductance, or its inverse, stomatal resistance, is under the direct biological control of the leaf through its guard cells, which surround the stomatal pore. The turgor pressure and osmotic potential of guard cells are directly related to the stomatal conductance.
Stomatal conductance is a function of stomatal density, stomatal aperture, and stomatal size. Stomatal conductance is integral to leaf level calculations of transpiration. Multiple studies have shown a direct correlation between the use of herbicides and changes in physiological and biochemical growth processes in plants, particularly non-target plants, resulting in a reduction in stomatal conductance and turgor pressure in leaves.
Relation to stomatal opening
For mechanism, see: Stomatal opening and closingStomatal conductance is a function of the density, size and degree of opening of the stomata; with more open stomata allowing greater conductance, and consequently indicating that photosynthesis and transpiration rates are potentially higher. Therefore, stomatal opening and closing has a direct relationship to stomatal conductance.
Light-dependent stomatal opening
Light-dependent stomatal opening occurs in many species and under many different conditions. Light is a major stimulus involved in stomatal conductance, and has two key elements that are involved in the process: 1) the stomatal response to blue light, and 2) photosynthesis in the chloroplast of the guard cell. In C3 and C4 plants, the stomata open when there is an increase in light, and they close when there is a decrease in light. In CAM plants, however, the stomata open when there is a decrease in light.For more details about CAM plant stomatal conductance, see: CAM Plants
Stomatal response to blue light
Stomatal opening occurs as a response to blue light. Blue light activates the blue light receptor on the guard cell membrane which induces the pumping of protons out of the guard cell. This efflux of protons creates an electrochemical gradient that causes free floating potassium (K+) and other ions to enter the guard cells via a channel. This increase in solutes within the guard cells leads to a decrease in the osmotic potential of the cells, resulting in a decrease in water potential. Then, because water flows from a system with higher water potential to a system with lower water potential, water floods into the guard cells, causing the guard cells to become enlarged and therefore causes the stomata to open.
Studies showed that stomata responded greatly to blue light, even when in a red-light background (see Figure 1). In one study, the experiment began once stomatal opening had reached its saturation in red-light. Then, when blue light was added, stomatal opening increased even further, showing that a different photoreceptor system, stimulated by blue light, mediates the additional increases in opening.
Photosynthesis in the chloroplast
The second key element involved in light-dependent stomatal opening is photosynthesis in the chloroplast of the guard cell. In response to carbon dioxide (CO2) entering the chloroplasts, photosynthesis occurs. This increases the amount of solutes that are being produced by the chloroplast which are then released into the cytosol of the guard cell. This causes a decrease in osmotic potential, causing a decrease in the water potential inside the guard cells. Again, this decrease in water potential causes water to enter into the guard cells. The guard cells subsequently swell up with water and the stomata is opened.
Recent studies have looked at the stomatal conductance of fast growing tree species to identify the water use of various species. Through their research it was concluded that the predawn water potential of the leaf remained consistent throughout the months while the midday water potential of the leaf showed a variation due to the seasons. For example, canopy stomatal conductance had a higher water potential in July than in October. The studies conducted for this experiment determined that the stomatal conductance allowed for a constant water use per unit leaf area.