Aqua Man
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So during photosynthesis (lights on) the plants transpire because the stomata are open and the plants pulls water from the soil to bring in nutrients and cool itself as it release vapor from the stomata (located bottoms side of the leaves) this is how a plant drinks and light is needed for this process. So vapor pressure plays a part in regulating the flow of water through the plant and the plant will open and close its stomata to either release more or less vapor depending on many factors. Our goal is to keep them as wide open as possible during the day to get the best photosynthetic rates and thus growth.
At night the plant closes the stomata as it senses lack of light to photosynthesize. So the VPD does not play a role because the plant is no longer tanspiring but kicks into full respiration instead of transpiration. Now a tiny bit of both happen during light and dark periods.
There is so much more to this but thats the gest of it.
TLDR VPD only needs to be applied during lights on and RH of 10-15% lower at night is favorable.
At night the plant closes the stomata as it senses lack of light to photosynthesize. So the VPD does not play a role because the plant is no longer tanspiring but kicks into full respiration instead of transpiration. Now a tiny bit of both happen during light and dark periods.
There is so much more to this but thats the gest of it.
TLDR VPD only needs to be applied during lights on and RH of 10-15% lower at night is favorable.
