Stomatal Conductance Calculator
Enter transpiration rate and the leaf-to-air water vapor mole-fraction difference to compute stomatal conductance gs = E ÷ Δw, for quantifying stomatal aperture and plant water/gas-exchange status.
Input Data
Results
At a glance:Stomatal conductance to water vapor (gs) quantifies how readily water vapor passes through leaf stomata, a key index of a plant's control over water loss and CO₂ uptake. It follows an Ohm's-law-like diffusion relationship: transpiration rate E = gs × Δw, where Δw is the mole-fraction difference between the near-saturated water vapor inside the substomatal cavity and that in the outside air (approximately the vapor concentration gradient driving transpiration). Rearranging gives gs = E ÷ Δw. With E in mmol H₂O m⁻² s⁻¹ and Δw in mmol mol⁻¹ (millimoles of water per mole of air), gs is in mol H₂O m⁻² s⁻¹ (numerically E ÷ Δw). A larger gs means wider-open stomata and easier water/CO₂ exchange, usually with higher transpiration and higher potential photosynthesis; conversely, under drought, soil water deficit, high vapor-pressure deficit (VPD), darkness, or abscisic acid (ABA) signaling, guard cells lose turgor and stomata partially or fully close, lowering gs to reduce water loss at the cost of also limiting CO₂ entry and photosynthesis. Thus gs is a core physiological parameter for monitoring water stress, diagnosing photosynthetic limitation (distinguishing stomatal vs non-stomatal limitation), and comparing varietal stomatal behavior. Strictly, transpiration is governed in series by stomatal and boundary-layer conductance with a small correction for viscous water-vapor flow; porometers and gas-exchange systems output gs accordingly. This calculator uses the simplest gs = E ÷ Δw form for quick inversion from transpiration and vapor difference; ensure E and Δw correspond to the same leaf and instant, with consistent units.
Formula
Diffusion relation: transpiration E = gs × leaf-to-air vapor diff. Δw.
Stomatal conductance: gs = E ÷ Δw.
Units: E(mmol m⁻² s⁻¹) ÷ Δw(mmol mol⁻¹) → gs(mol m⁻² s⁻¹).
$$E = g_s \cdot \Delta w$$$$g_s = \frac{E}{\Delta w}$$How to Use
- Measure leaf transpiration rate E with a porometer or gas-exchange system.
- Obtain the leaf-to-air water vapor mole-fraction difference Δw (often output directly, or from leaf-temperature saturation vapor and ambient humidity).
- Enter E and Δw (matching units); the tool returns gs; larger gs means wider-open stomata.
General interpretation of gs (relative; varies by species and environment)
| Condition | gs tendency | Note |
|---|---|---|
| Good light, ample water | Higher | Open stomata, high transpiration and potential photosynthesis |
| Drought / soil water deficit | Declines | ABA closes stomata to conserve water |
| High VPD (hot dry air) | Declines | Partially close to avoid excess water loss |
| Night / low light (most C3, C4) | Very low | Stomata mostly closed, little exchange |
gs is instantaneous and environment-sensitive; compare varieties or treatments under similar light, temperature, and VPD.
Case Studies
gs from transpiration and vapor difference
A leaf: transpiration E = 4 mmol H₂O m⁻² s⁻¹, leaf-to-air vapor diff. Δw = 20 mmol mol⁻¹.
gs = 4 ÷ 20 = 0.2 mol H₂O m⁻² s⁻¹ (i.e. 200 mmol m⁻² s⁻¹).
A common magnitude for well-watered, open-stomata conditions; compare with stress treatments for stomatal behavior.
gs drops under drought
Well-watered leaf: E = 4, Δw = 20 → gs = 0.2 mol m⁻² s⁻¹.
Same plant after days of drought: E drops to 1.2, Δw still ~20 → gs = 1.2 ÷ 20 = 0.06 mol m⁻² s⁻¹.
gs falls to under one-third, showing stomata largely closed to conserve water; photosynthesis is also likely stomatally limited — a sensitive water-stress indicator.
FAQ
What is Δw (leaf-to-air vapor difference) and how to get it?
Δw is the vapor concentration gradient driving transpiration, equal to the water-vapor mole fraction inside the substomatal cavity (near leaf-temperature saturation) minus that of the outside air. Porometers and gas-exchange systems usually compute and output Δw (or as VPD) from leaf temperature and chamber humidity. By hand you need leaf-temperature saturation vapor pressure, ambient vapor pressure, and atmospheric pressure, converted to mole fractions and subtracted. Drier air and higher leaf temperature raise Δw.
How is gs related to photosynthesis and CO₂?
Stomata are both the water-vapor exit and the CO₂ entrance. High gs lets CO₂ enter mesophyll easily, aiding photosynthesis, but loses more water; low gs conserves water but limits CO₂ supply. Whether 'stomatal limitation' applies is judged by intercellular CO₂ (Ci): if photosynthesis drops and Ci also drops, it is mostly stomatal limitation; if Ci does not drop or even rises, it is non-stomatal (biochemical) limitation. gs is the pivot linking water and carbon assimilation.
Why are there both mol/m²/s and mmol/m²/s for gs?
They differ only by 1000× (1 mol = 1000 mmol). Literature commonly uses mol H₂O m⁻² s⁻¹ for the broader range, with daily values mostly between 0.05 and 0.5; some use mmol m⁻² s⁻¹ (i.e. 50–500). This calculator, with E in mmol m⁻² s⁻¹ and Δw in mmol mol⁻¹, outputs gs in mol m⁻² s⁻¹. Always confirm units match before comparing data.
Why does midday gs often fall ('midday depression')?
At noon full sunlight, air is hot and dry (high VPD), transpiration demand surges, and plants partially close stomata to avoid excess water loss, causing gs and photosynthesis to dip around midday and recover in the afternoon. This is a protective stomatal response to high VPD and water balance, especially marked in hot-dry and water-limited conditions.
What to watch when measuring gs?
gs is instantly environment-sensitive; measure after the leaf reaches steady state under stable light and chamber conditions, recording light, leaf temperature, VPD, and CO₂; pick representative, spot-free mature leaves and note which surface bears most stomata. Compare varieties or treatments measured simultaneously under similar environments to avoid weather or time-of-day bias. Boundary-layer conductance also matters; instruments usually include its correction.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.