To obtain Vapour Pressure Deficit (VPD), subtract the actual vapour pressure of the air from the potential saturated vapour pressure (VPsat – VPair). Stomatal conductance (mmol m−2 s−1) is measured by the rate of CO2 entering, or water vapor exiting, through stomata. This means that when VPD is high, the vapour pressure of air is low, and stomata will close to conserve moisture, decreasing conductance. So your inverted, rather than normal, curves are correct. The Mwendia paper attached states that High VPD associated with drought caused stomatal closure, or low G values, in keeping with your results (see Fig. 3A in the paper).

High VPD usually does inhibit transpiration (low G values) and therefore sap flow. However, in the Zeppel paper attached, as VPD increased in the morning, sap velocity increased, and as VPD decreased in the afternoon, sap velocity declined (also Fig. 3), in keeping with your results. The authors said this hysteretic response is because of the increased evaporative demand of the atmosphere and variation in stomatal sensitivity :)

I agree with Andrew Paul McKenzie Pegman response

These are adaptative behaviour of plants. In case of high evaporative demand (VPD) induced by drought for example, plants reduce exchanges with external environment by closing stomata to avoid loosing too much water. However, the increase of sap flow could be explained by high root activity to uptake water and nutriment in order to compensate for a lack.

Maybe not very relevant to the question, but might be an interesting read. In the case of increased VPD during the day, plants tend to grow at night! An interesting study by Tarvainen et al. 2021 Article Handling the heat ‐ photosynthetic thermal stress in tropical trees

The transpiration rate is described by Fick's first law of diffusion, which says that the flux density of a substance (water vapour here) is proportional to the force that produces the movement of the substance multiplied by the diffusion coefficient of that substance into the media (here water vapour in air). The driving force for the water vapour movement is the difference in partial pressure of the water vapour at the sites of water evaporation, within the leaf, and that outside the leaf. This is the definition of the water vapour pressure deficit (VPD), however the units of VPD, which are usually kPa, are not the ones of a force (e.g. Newtons), but are of pressure. Hence instead of considering the diffusion coefficient expressed in m2 s-1, we consider the water vapour conductance which incorporates the distance and the area through the water vapour moves (e.g. stomata, but also the boundary layer adjacent to the leaf) and is expressed in m s-1 or more usually in mol m-2 s-1 or as required here in mol m-2 s-1 Pa-1.

All together we can say that the transpiration rate (E = gtwv * VPD), where gtwv is the total conductance for water vapour across the leaf, which includes the stomatal conductance and the boundary layer conductance in series. From this later equation you may see that if the increase in VPD is higher than the reduction in stomatal conductance the transpiration rate of a single leaf will increase even if the stomata are more closed. In this regard it may help to think that closed stomata also increase leaf temperature, which enhances leaf VPD. You can get all the equations and much better explanations from the book:

Nobel PS. 2009. Physicochemical and Environmental Plant Physiology 4th edn. Burlington, MA, USA: Elsevier Academic Press.

Also you can enjoy playing with transpiration, VPD, gsw and gblw using a simple leaf energy balance model in our latest accepted article (see the Excel file in the supporting information from the publisher):

Pan, L., George-Jaeggli, B., Borrell, A., Jordan, D., Koller, F., Al-Salman, Y. et al. (2021) Coordination of stomata and vein patterns with leaf width underpins water-use efficiency in a C4 crop. Plant, Cell & Environment, 1– 19. https://doi.org/10.1111/pce.14225

If we assume that water just moves from roots to the atmosphere without affecting any other water reservoir within the plant, what for trees may be too many assumptions because capacitance play a role here, by mass balance sap flux should correspond with the total transpiration of the canopy. However every single leaf should have different VPD, gsw and gblw, which can be further modelled using single or complicated canopy layer models.

Regards,

Javier

It is possible that as VPD increases, water transpires more through the cuticle rather than the stomata, leading to increased transpiration (and thus sap flow) while decreased stomatal conductance.

Reference: Aparecido LMT, Woo S, Suazo C, Hultine KR, Blonder B. High water use in desert plants exposed to extreme heat. Ecology Letters, 2020, 23(8): 1189-1200.

When the stomata of plants first felt the change of VPD, they began to close gradually. However, the transpiration pull did not disappear completely in a short period of time, and the existence of transpiration pull led to the increase of transpiration flow in plants to resist drought. In my opinion, stomatal closure is the next response of plants that cannot maintain physiological functions even after increasing transpiration flow. The increase of transpiration flow and stomatal closure coincide in time, so the two phenomena are observed simultaneously.