Lets talk about VPD...it's worth the converstion.
Plenty of info available about this already and I've done a bit of research but I haven't seen much activity on any recent threads about it but I found it to be an extremely interesting subject so I figured I'd revamp the conversation.
As RH goes down and temps go up VPD increases...the air's ability to absorb water and cause evaporation increases. Too high of a VPD and plants respond to the potential of excess water loss by closing their stomata and curling up their leaves. Which in in effect does significantly slow the rate of transpiration but.....Closed stomata also decreases the plants ability to absorb a sufficient amount of CO2, which is needed for plant growth and curled leaves reduce the amount of surface area exposed to light, both negatively affecting the rate of photosynthesis that occurs within the plant. Essentially air with a high VPD causes the plants to respond to a stressor in a way that only causes further problems....bad news it is.
As RH goes up and temps go down VPD decreases...the air's ability to absorb water and cause evaporation decreases. Too low a VPD and the plants ability to transpire is greatly reduced inhibiting its ability to absorb nutrients up from its roots. When a plant is not able to transpire, and is coupled with a saturated root zone brought about by decreased absorption of water through it's roots, pressure can build up to the point that water is forced out through the leaves in a process called guttation. This in addition to the likely event, in this particular case, that dew has formed as the excess water vapor in the air condenses into liquid, creates an ideal environment suitable for all kinds of nasty little microbial life to invest your plants. Not to mention that despite the fact that a plant responds to elevated levels of humidity by opening its stomata, in turn, increasing CO2 absorption, the plant lacks adequate levels of certain vital nutrients needed to utilize this excess CO2 to it's advantage due to it's low rate of transpiration.
Finding the proper VPD (balance between temp and RH) allows for sufficient transpiration to occur within the plant, thus allowing for adequate nutrient absorption through the roots, and also allowing for sufficient CO2 absorption from the air via the plants stomata. Ideally temps and RH at higher values that are balanced to create a beneficial VPD are most effective in producing optimal levels of these two elements of plant growth.
The greater the temperature the greater the airs ability to hold water and cause evaporation. The greater the RH in the air the wider the plants stomata will open. The reason being, the more water vapor (humidity) in the air the more pressure the air exerts on the plant, so the plant must "push back" and in essence try harder to transpire. Which it does by opening it's stomata wider, in turn increasing it's ability to absorb CO2. So, if humidity is raised and a plant responds by widening the opening to it's stomata and temps are raised to allow for a greater level off water vapor (AKA evaporated water AKA transpiration) to be held by the air......A plant will still, despite the increased pressure being placed upon it (high RH), be able to properly transpire thus absorbing a sufficient amount of nutrients as it pulls up through it's roots water to replace what it has lost...and absorb increased amounts of CO2 from the air by it's widened stomata openings that the plant will be ready to utilize as it has an adequate supply of nutrients at it's disposal.
As long as it is balanced right and a few other key variables are properly controlled....
High temps + High RH = healthy, thriving plants
...just look at the jungle. Very high temps, very high humidity...ABUNDANT and THRIVING plant life.
Most of my research has about VPD has come from this website and these ideas are not by any stretch of the imagination my own, only my understanding of the concept as presented to me through the knowledge of others much more versed in the subject than I.
Please feel free to share any info you feel is beneficial to further one's knowledge about the subject from simple to complex as long as it is applicable it is welcome.