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Vapor Pressure Deficit (VPD) - A TerraSoil Overview

TerraSoil

03 Aug 2024

Understanding and Using VPD to Achieve Better Growth


What is Vapor Pressure Deficit?

Vapor Pressure Deficit (VPD) is a critical concept in plant physiology and agriculture, representing the difference between the amount of moisture in the air and the amount of moisture the air can hold when it is saturated.

VPD is expressed in units of pressure, such as kilopascals (kPa) or millibars (mb).

Mathematically, VPD can be defined as:

VPD=ea−es

Where:

  • es is the saturation vapor pressure at a given temperature.

  • ea is the actual vapor pressure of the air.


Importance of VPD in Agriculture

VPD is a crucial parameter for understanding plant transpiration and water use efficiency. It influences several physiological processes, including stomatal conductance, nutrient uptake, and photosynthesis. Optimal VPD levels are essential for maximizing crop yield and quality.

  1. Transpiration and Stomatal Conductance: VPD affects the rate of transpiration and the opening of stomata. High VPD can lead to excessive water loss, while low VPD can result in inadequate transpiration.

  2. Water Use Efficiency: Maintaining an optimal VPD is essential for efficient water use, reducing water stress, and enhancing drought resistance.

  3. Plant Growth and Development: VPD influences nutrient uptake and photosynthesis, impacting overall plant growth and development.


Influence of VPD on Plant Growth

The influence of VPD on plant growth can be summarized as follows:

  1. High VPD: This condition typically leads to increased transpiration rates, which can cause water stress, stomatal closure, reduced photosynthesis, and potentially stunted growth if the plant cannot uptake water quickly enough from the soil to match the loss through transpiration.

  2. Low VPD: In this scenario, transpiration rates are reduced, potentially leading to insufficient nutrient transport and less evaporative cooling, which can increase leaf temperature and reduce photosynthesis.


Statistical Insights

  • Photosynthesis and Biomass: Studies have shown that VPD levels between 0.8 and 1.2 kPa are often optimal for photosynthesis and biomass accumulation in many crop species.

  • Water Use Efficiency: At a VPD of about 1.0 kPa, many plants achieve optimal water use efficiency.


VPD Chart

Below is a VPD chart illustrating the relationship between temperature, relative humidity, and VPD values. The below chart highlights the typical VPD requirements for common herbaceous plants.



Different VPD Requirements for Plants

Different plants have varying VPD requirements depending on their native environments and physiological adaptations.

  • Tropical Plants: Typically prefer lower VPD (0.5 - 1.0 kPa).

  • Desert Plants: Can tolerate higher VPD (1.0 - 2.0 kPa).

  • Examples:

    • Tomatoes: Optimal VPD is around 0.8 - 1.2 kPa.

    • Lettuce: Prefers a VPD of 0.7 - 0.9 kPa.

    • Cannabis: Often performs best at a VPD of 0.8 - 1.5 kPa.


Effects of Changing Humidity and Temperature

  • Humidity:

    • Increased Humidity: Lowers VPD, reducing transpiration and potentially causing overheating in leaves.

    • Decreased Humidity: Raises VPD, increasing transpiration and risk of dehydration.

  • Temperature:

    • Increased Temperature: Raises saturation vapor pressure, increasing VPD and transpiration rates.

    • Decreased Temperature: Lowers saturation vapor pressure, decreasing VPD and transpiration rates.


Controlling VPD

To control VPD effectively in agricultural settings:

  1. Humidity Control: Use humidifiers or dehumidifiers to adjust air moisture levels.

  2. Temperature Management: Implement heating or cooling systems to maintain optimal temperature ranges.

  3. Ventilation: Ensure proper air circulation to prevent extreme VPD fluctuations.

  4. Irrigation Management: Adapt watering schedules to meet plant water needs under varying VPD conditions.


References

  1. Jones, H. G. (1992). Plants and microclimate: a quantitative approach to environmental plant physiology. Cambridge University Press.

  2. Taiz, L., & Zeiger, E. (2010). Plant Physiology. Sinauer Associates.

  3. Nobel, P. S. (2009). Physicochemical and environmental plant physiology. Academic Press.

  4. Monteith, J. L., & Unsworth, M. (2013). Principles of Environmental Physics: Plants, Animals, and the Atmosphere. Academic Press.

  5. Tardieu, F., & Davies, W. J. (1993). Integration of hydraulic and chemical signalling in the control of stomatal conductance and water status of droughted plants. Plant, Cell & Environment, 16(4), 341-349.

  6. Whitehead, D. (1998). Regulation of stomatal conductance and transpiration in forest canopies. Tree Physiology, 18(11), 633-644.

  7. Sperry, J. S., & Hacke, U. G. (2002). Desert shrub water relations with respect to soil characteristics and plant functional type. Functional Ecology, 16(3), 367-378.

  8. Steduto, P., & Albrizio, R. (2005). Resource use efficiency of field-grown sunflower, sorghum, wheat, and chickpea: I. Radiation use efficiency. Agricultural and Forest Meteorology, 130(3-4), 254-268.

  9. Comstock, J. P. (2002). Hydraulic and chemical signalling in the control of stomatal conductance and transpiration. Journal of Experimental Botany, 53(367), 195-200.

  10. Sinclair, T. R., & Ludlow, M. M. (1985). Who taught plants thermodynamics? The unfulfilled potential of plant water potential. Functional Plant Biology, 12(3), 213-217.

 

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