Stomatal conductance

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 turgor pressure and osmotic potential of guard cells are directly related to the stomatal conductance.

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.

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.

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.

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.

[2] The second key element involved in light-dependent stomatal opening is photosynthesis in the chloroplast of the guard cell.

This increases the amount of solutes that are being produced by the chloroplast which are then released into the cytosol of the guard cell.

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.

Another study also showed that stomatal opening is dependent on guard cell photosynthesis.

This was carried out by isolating guard cells that were localized to the lower surface of the Adiantum leaves used in the study.

It was thus hypothesized that if guard cell chloroplasts are responsible for stomatal opening, it would be expected that light applied to the lower leaf surface would be much more effective at increasing stomatal conductance than light applied to the upper surface.

[3] Nocturnal stomatal conductance (gn) across both C3 and C4 plants remains a highly researched topic, as the biological function of this phenomenon is ambiguous.

Recent studies have compiled extensive literature/data sets that reveal relative growth rate is positively correlated with nocturnal stomatal conductance.

However, gn does not directly correlate with positive growth; in fact, the direct effects of nocturnal stomatal conductance lead to higher transpiration rate, which decreases turgor pressure and consequently growth.

Additionally, experiments have revealed that nocturnal stomatal conductance is regulated in an active manner, as there is a temporal change witnessed due to the presence of a circadian clock.

Lastly, there is not consistent evidence across various plant species that the main functions of gn are: to get rid of surplus CO2 (could limit growth), improve oxygen delivery, or aid in nutrient supply.

[4] Regulating stomatal conductance is critical to controlling to the amount of transpiration, or water loss from the plant.

Recent studies have investigated the relationship between stomatal conductance, cavitation, and water potential.

Cavitation events have been shown to decrease stomatal conductance while maintaining a stable water potential.

This limits transpiration and allows the plant to begin to repair the damaged, cavitated xylem.

[5] Similarly, some studies have explored the relationship between drought stress and stomatal conductance.

Recent studies have found that drought resistant plants regulate their transpiration rate via stomatal conductance.

The hormone ABA is triggered by drought conditions and can assist in closing the stomata.

However, closing the stomates can also lead to low photosynthetic rates because of limited CO2 uptake from the atmosphere.

[6] Stomatal conductance can be measured in several ways: Steady-state porometers: A steady state porometer measures stomatal conductance using a sensor head with a fixed diffusion path to the leaf.

A dynamic porometer measures how long it takes for the humidity to rise from one specified value to another in an enclosed chamber clamped to a leaf.

The Ball-Berry-Leuning model was formulated by Ball, Woodrow and Berry in 1987, and improved by Leuning in the early 90s.

Figure 1. Dual-beam experiment with epidermal peels of Commelina. Peels were irradiated with 0.2 mmol m −2 s −1 of red light; 0.01 mmol m −2 s −1 of blue light was added as indicated by the arrow . Dashed lines at the bottom of figure = aperture values at the end of 4h incubation in darkness, under normal and CO 2 -free air. The incubation media were aerated with CO 2 -free air throughout the experiment. (Schwartz and Zieger, 1984) [ 2 ]
Figure 2. Dependencies of PFD for stomatal conductance increase and photosynthetic CO2 fixation. Solid and dotted lines represent the increase in stomatal conductance and photosynthetic CO2 fixation, respectively. Red light was irradiated onto the upper (black circles) or lower side (white circles) of the leaf. (Doi and Shimazaki 2008). [ 3 ]