What is VPDUpdated a day ago
1. What is VPD
Vapor Pressure Deficit (VPD), commonly measured in kilopascals (kPa), refers to the difference between the saturated vapor pressure and the actual vapor pressure at a given temperature. It is an important indicator of the atmosphere’s evaporative capacity and reflects the degree of air dryness. VPD is one of the most critical climatic variables in ecological models that simulate water and carbon fluxes and states. It is also an important meteorological variable in fire warning models and models predicting the spread of pests, diseases, and epidemics (Zhang et al., 2014).
VPD acts as the driving force behind plant transpiration. An increase in VPD promotes the opening of stomata, facilitating water absorption from the roots and carbon dioxide uptake from the air for photosynthesis. However, when VPD exceeds a certain threshold, plants reduce stomatal opening to prevent excessive water loss, thereby inhibiting growth (Yan et al., 2016). Under low VPD conditions, the moisture on leaf surfaces becomes saturated, which can easily lead to plant diseases and pest outbreaks. Additionally, the internal water flow within the plant slows down, hindering the transport of substances and ultimately obstructing crop growth (Yin et al., 2021).
The optimal VPD range for the growth of most crops is 0.5–1.5 kPa (Li et al., 2022). When VPD increases from 1 kPa to 1.8 kPa, the growth of most crops declines significantly due to reduced stomatal conductance. When VPD drops below 0.43 kPa, fungal pathogens and mineral deficiencies appear, and when it falls below 0.2 kPa, the risk of disease infection is highest (Xu et al., 1991; Shamshiri et al., 2016).
An increase in VPD significantly impacts forest mortality rates (Yuan et al., 2019). Years with extremely high VPD can impose severe drought stress on vegetation, leading to disease outbreaks, wildfires, and increased mortality risk (Seager et al., 2015). VPD has been used to observe interannual variations in burned areas across many parts of the western United States (Chiodi et al., 2021). When other environmental factors remain constant, an increase in nighttime VPD can accelerate the drying rate of flammable wood at night, or in the presence of dew or high soil moisture, it can hinder the absorption of moisture by flammable materials (Potter, 2012; Seager et al., 2015). Forest fires are more likely to be triggered by lightning strikes during periods of high VPD. In northern Alaska, when the VPD over the five days prior is above average, wildfires are more likely to develop into large or even mega fires (Sedano & Randerson, 2014).
The African citrus psyllid (Trioza erytreae) is the vector insect for Candidatus, a bacterium responsible for citrus yellow shoot, which poses a severe threat to citrus crops. The prevalence of African citrus psyllid is negatively correlated with the number of days when VPD exceeds 34.5 mbar (1 mbar = 0.10197 kPa). A VPD above 34.5 mbar results in a 70% mortality rate in African citrus psyllid eggs and nymphs (Paiva et al., 2020). The mold mite (Tyrophagus putrescentiae), a pest affecting food storage safety, experiences a 90% mortality rate after 58 hours of exposure to environments where VPD exceeds 12 mbar. As VPD increases, the time to mite death shortens (Eaton & Kells, 2008). Maintaining a relatively high VPD during crop rotation intervals and in grain storage rooms can reduce pest and disease impacts.
Some researchers have used cumulative VPD as an irrigation threshold, which significantly reduces water usage without affecting crop growth or quality (Keary et al., 2019; Tran et al., 2018; Zhang et al., 2022).
VPD Calculation Formula:
SVP=[610.78×107.5Ta/(237.3+Ta)]/1000
VPD=(100-RH)×SVP
Ta refers to air temperature.
RH refers to relative humidity.
2. Greenhouse VPD Control
VPD is primarily influenced by temperature and humidity. By regulating temperature and humidity, the VPD inside a greenhouse can be adjusted. Temperature is positively correlated with VPD—when temperature increases, VPD rises. Humidity is negatively correlated with VPD—when humidity increases, VPD decreases.
References:
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