User:Natv513/Stomatal conductance

Stomatal conductance, usually

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:

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.

CAM plants, however, the stomata open when there is a decrease in light.

For more details about CAM plant stomatal conductance, see: CAM Plants

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. [2]

Photosynthesis in the chloroplast

The second key element involved in light-dependent stomatal opening is

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.

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. And indeed, when red light was applied to the lower surface, stomatal conductance increased at a light intensity of <5 μmol m⁻² s⁻¹ and continued to increase with increasing light intensity, reaching a maximum at about 20 μmol m⁻² s⁻¹.^[3]

Nocturnal stomatal opening

Nocturnal stomatal conductance (g_n) across both

Studies have shown that nocturnal conductance is not the result of stomatal leakiness. As there is extensive genetic variation across a variety of C3 and C4 plants, g_n has most likely been selected for during evolution of said plants. 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. Finally, it has been witnessed that g_n declines during drought, demonstrating an active response to drought^[4]. These reasons disprove the theory that stomatal leakiness causing nocturnal stomatal conductance.

Lastly, there is not consistent evidence across various plant species that the main functions of g_n are: to get rid of surplus CO₂ (could limit growth), improve oxygen delivery, or aid in nutrient supply^[4].

Stomatal Transpiration

Regulating stomatal conductance is critical to controlling to the amount of transpiration, or water loss from the plant. Since over 95% of water loss comes directly from the stomatal pore, changes in stomatal resistance are critical to regulating water loss. Stomatal conductance also assists in the regulation of CO₂ uptake from the atmosphere. Regulation of stomatal transcription is especially important when transcription rates are high. High transcription rates can lead to cavitation events, or when the tension in the xylem increases to the point where air vessels begin to fill the xylem. This is harmful to the plant because these air bubbles can block the flow of water up the xylem to the aerial parts of 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. In other words, cavitation events cause stomata to close to different extents. 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. This minimizes water loss and allows the plant to survive under low water conditions. However, closing the stomates can also lead to low photosynthetic rates because of limited CO₂ uptake from the atmosphere ^[6].

Methods for measuring

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. It measures the vapor concentration at two different locations in the diffusion path. It computes vapor flux from the vapor concentration measurements and the known conductance of the diffusion path using the following equation:

${\displaystyle {\frac {C_{vL}-C_{v1}}{R_{vs}+R_{1}}}={\frac {C_{v1}-C_{v2}}{R_{2}}}}$

Where ${\displaystyle C_{vL}}$ is the vapor concentration at the leaf, ${\displaystyle C_{v1}}$ and ${\displaystyle C_{v2}}$ are the concentrations at the two sensor locations, ${\displaystyle R_{vs}}$ is the stomatal resistance, and ${\displaystyle R_{1}}$ and ${\displaystyle R_{2}}$ are the resistances at the two sensors. If the temperatures of the two sensors are the same, concentration can be replaced with relative humidity, giving

${\displaystyle R_{vs}={\frac {1-h_{1}}{h_{2}-h_{1}}}R_{2}-R_{1}}$

Stomatal conductance is the reciprocal of resistance, therefore

${\displaystyle g_{vs}={\frac {1}{R_{vs}}}}$ .

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 resistance ${\displaystyle R}$ is then determined from the following equation:

${\displaystyle \Delta t={\frac {\left(R+A\right)l\Delta h}{1-h}}}$

where ∆${\displaystyle t}$ is the time required for the cup humidity to change by ∆${\displaystyle h}$, ${\displaystyle h}$ is the cup humidity, ${\displaystyle l}$ is the cup "length", and ${\displaystyle A}$ is an offset constant.

Null balance porometers maintain a constant humidity in an enclosed chamber by regulating the flow of dry air through the chamber and find stomatal resistance from the following equation:

${\displaystyle R_{vs}={\frac {A}{f}}\left({\frac {1}{h}}-1\right)-R_{va}}$

where ${\displaystyle R_{vs}}$ is the stomatal resistance, ${\displaystyle R_{va}}$ is the boundary layer resistance, ${\displaystyle A}$ is the leaf area, ${\displaystyle f}$ is the flow rate of dry air, and ${\displaystyle h}$ is the chamber humidity.

The resistance values found by these equations are typically converted to conductance values.

Models

A number of models of stomatal conductance exist.

Ball-Berry-Leuning model

The Ball-Berry-Leuning model was formulated by Ball, Woodrow and Berry in 1987, and improved by Leuning in the early 90s.^[7] The model formulates stomatal conductance, ${\displaystyle g}$ as

${\displaystyle g=g_{0}+{\frac {a_{1}A_{n}}{(c_{s}-\Gamma )(1+{\frac {D_{s}}{D_{0}}})}}}$

where ${\displaystyle g}$ is the stomatal conductance for CO₂ diffusion, ${\displaystyle g_{0}}$ is the value of ${\displaystyle g}$ at the light compensation point, ${\displaystyle A_{n}}$ is CO₂ assimilation rate of the leaf, ${\displaystyle D_{s}}$ is the vapour pressure deficit, ${\displaystyle c_{s}}$ is the leaf-surface CO₂ concentration, ${\displaystyle \Gamma }$ is the CO₂ compensation point. ${\displaystyle a_{1}}$ and ${\displaystyle D_{0}}$ are empirical coefficients.

References