Vapor Pressure Deficit in Plants

In the delicate balance of plant survival, vapor pressure deficit plays a leading role. And as cheesy as that sounds, it has a lot to do with how the external environment affects a plant’s inner world.

Vapor Pressure Deficit (VPD) is the difference between the current moisture in the air and when it is fully saturated. But in plants, this translates to the moisture within the leaves versus the outside air. When VPD is higher, it means the outside is quite dry, which can stress plants, reducing growth and yield.

Below, I discuss about VPD in general and the effects it has on plants and their growth:

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What is Vapor Pressure Deficit?

VPD refers to the difference between how dry the air is compared to when it’s fully loaded with moisture. Compared to relative humidity, it’s a better measurement to use to find out how much water plants will lose to the air, but more on that later. So, what exactly is VPD about?

When the air has all the moisture it can hold, VPD is zero. But as the air gets drier, VPD goes up. This dryness affects plants, causing most to close their stomata and conserve water, therefore slowing down gaseous exchange and growth. But it is worth noting that some plants, especially those living in deserts, have adaptive mechanisms built in to handle high VPD conditions and keep their stomata open to continually grow. Other plants also do a combination of both by keeping their stomata partially open.

With that said, how do we measure vapor pressure deficit? We use the following VPD formula:

1. Find out the air temperature (T) and relative humidity (RH) of the room where your plant is in. Having a hygrometer will tell you what these values are right off the bat.
2. Calculate the room’s SVP (Saturation Vapor Pressure) using the following formula:

SVP = 0.6108 x e^(17.27 x T)/(237.3 + T)

Where e = 2.718 and T = temperature in °C.

3. Calculate the room’s AVP (Actual Vapor Pressure) using the relative humidity with the following formula:

AVP = RH/100 x SVP

Where RH = relative humidity in %.

4. Find VPD using the following formula:

VPD = SVP – AVP

Where VPD is expressed in kPa (kilopascals).

For the Maths wiz out there, this may seem like a fun exercise. But for the majority of us, this might incite some bad memories and anxiety. So, I’ve created a simplified VPD calculator for you to download and use. It also comes with a VPD chart for easier reference.

But if all that is too troublesome, you can always get an all-in-one tool like this Smart Hygrometer Thermometer from Amazon to do all the measuring and calculations for you. This affordable product has an impressive 13,091 ratings with 4.2 stars. An alternative one you can get is the AC Infinity Controller 69 PRO, also from Amazon, which Nick uses all the time. Just a heads up though, it’s a bit more on the pricey side, with about only 309 ratings and 4.6 stars.

There have also been experiments where scientists have created a wearable sensor for the plant. It does real-time on-leaf monitoring of relative humidity, temperature, and VPD of plants in both controlled environments and under field conditions. However, given that it is still in the development stages, it’s not available for public use just yet.

Now, going back to Vapor Pressure Deficit vs Relative Humidity, aren’t they basically the same thing?

Well, yes and no. Here’s the difference between the two:

On a larger scale, studies have found that VPD plays a significant role in plant responses to climate change. With increasing global temperatures, VPD is expected to rise, affecting plant transpiration rates, water use, and general plant health. In short, when VPD goes up, the overall growth of ecosystems goes down. This, by extent, affects how the CO₂ level in the atmosphere changes year by year. 

Not only that, because higher VPD levels lead to drier conditions, this also increases the risk of wildfires. So if you ever wonder why there’s been a lot of drought and fires happening, know that the VPD levels are probably climbing high each year.

VPD & Plants

Vapor Pressure Deficit & Transpiration

VPD in plants is essentially the driving force that regulates the rate of water transport from the roots to the rest of plants, i.e. transpiration. It depends on the water vapor concentration inside of the leaf and the external environment. When the VPD is low, transpiration rates are also low so there is not much loss of water. But when VPD is high, it means the outside air is dry and three things can happen in a plant:

1. The plant closes its stomata to conserve water but also limits gaseous exchange which affects photosynthesis. This is what will happen to most plants in general, especially the ones that are fussy with their water intake like Peace Lilies and Ferns.
2. The plant keeps its stomata open, losing water but using its deep roots or specialized water storage to keep the plant cool and continuously absorb nutrients. Desert or drought-tolerant plants often resort to this method naturally due to their origins of growing in arid environments, like Snake Plants, Cacti & Succulents.
3. The plant partially closes its stomata, allowing some gaseous exchange to occur but reducing water loss. This usually happens for plants with various cultivars or have a knack for surviving in different types of conditions, like Pothos.

But in most scenarios, a high VPD will cause plants’ growth to slow down due to reduced transpiration via completely closed or partially open stomata. 

So if you’re wondering what’s a good VPD range for your plants, then I have your answers in the following table:

Important note: This is a general VPD range guideline for indoor plants. The optimal VPD values for other plants may vary depending on their cultivar, species and other factors. Other than that, be mindful of the plant’s overall basic care and not just its VPD levels for its best growth.

There’s also something called ‘Leaf Vapor Pressure Deficit’, where T is the temperature of the leaf’s surface taken using an infrared thermometer. Afterwards, you use the same VPD formula and calculations to find out the Leaf VPD of a plant. But unless you are growing plants in a laboratory, have plenty of time and love working with the finest of details, then you can skip this part and just focus on the room’s VPD to ensure your plant is thriving.

Frequently Asked Questions:

What decreases vapor pressure?

A decrease in temperature causes vapor pressure to decrease as well. Another way to decrease it is by adding a non-volatile solute, i.e. a substance that doesn’t evaporate or has a vapor pressure, to a solvent. By covering the surface, the solute particles stop the solvent from turning into vapor. Examples of this can be in the form of adding oil or even sugar to water, changing the contents enough to prevent the water from evaporating.

What happens when vapor pressure decreases?

A liquid’s tendency to evaporate reduces when vapor pressure decreases. This can be due to lower temperatures or a decrease in the concentration of the substance in the vapor phase. As a result, the liquid is less likely to evaporate and may condense more readily. In addition, its boiling point also increases because more energy is required to evaporate the liquid.

References:

1. Amitrano, C., Rouphael, Y., De Pascale, S., & De Micco, V. (2021). Modulating Vapor Pressure Deficit in the Plant Micro-Environment May Enhance the Bioactive Value of Lettuce. Horticulturae, 7(2), 32. https://doi.org/10.3390/horticulturae7020032
2. Du, Q., Jiao, X., Song, X., Zhang, J., Bai, P., Ding, J., & Li, J. (2020). The Response of Water Dynamics to Long-Term High Vapor Pressure Deficit Is Mediated by Anatomical Adaptations in Plants. Frontiers in plant science, 11, 758. https://doi.org/10.3389/fpls.2020.00758
3. Grossiord, C., Buckley, T.N., Cernusak, L.A., Novick, K.A., Poulter, B., Siegwolf, R.T.W., Sperry, J.S. and McDowell, N.G. (2020), Plant responses to rising vapor pressure deficit. New Phytol, 226: 1550-1566. https://doi.org/10.1111/nph.16485
4. Maria Sanchez , Thomas R. Sinclair & Deepti Pradhan (2020): Transpiration response to vapor pressure deficit and soil drying among quinoa genotypes (Chenopodiumquinoa Willd.), Journal of Crop Improvement. https://doi.org/10.1080/15427528.2020.1817221
5. Runkle, E. (2021). Water Vapor-Pressure Deficit. Retrieved 25 August 2023, from https://www.canr.msu.edu/floriculture/uploads/files/Water%20VPD.pdf
6. Runkle, E. and Wollaeger, H. (n.d.), VPD vs. Relative Humidity. Retrieved 25 August 2023, from https://www.canr.msu.edu/uploads/resources/pdfs/vpd-vs-rh.pdf
7. Song, X., Miao, L., Jiao, X., Ibrahim, M., & Li, J. (2022). Regulating Vapor Pressure Deficit and Soil Moisture Improves Tomato and Cucumber Plant Growth and Water Productivity in the Greenhouse. Horticulturae, 8(2), 147. https://doi.org/10.3390/horticulturae8020147
8. Yin, S., Ibrahim, H., Schnable, P. S., Castellano, M. J., Dong, L., A Field-Deployable, Wearable Leaf Sensor for Continuous Monitoring of Vapor-Pressure Deficit. Adv. Mater. Technol. 2021, 6, 2001246. https://doi.org/10.1002/admt.202001246