• Vapour-pressure deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated.

• VPD is important in greenhouse regulation.

  • When the VPD is very low, a film of water may form on a plant leaf, making the leaf far more susceptible to rot.
  • As the VPD increases, the plant needs to draw more water from its roots. In the case of cuttings, the plant may dry out and die.

• The ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa.

https://xiaozhuo.shinyapps.io/Project_9-4/

1-Input the parameters: i.e. RH = 55%, AT = 22\(^oC\), LT = 19\(^oC\)

2-Read the VPD and its evaluation:

3-Observe the distribution of VPD evaluation in the plot:

4-Read the leaf temperature intervals at current RH and AT:

[1] "Optimal 17.1 ~ 22.1" "High  > 22.7"        "Low   < 16.3"