Q - Why do guard cells swell and close the stomata? A - To regulate water transpiration, manage high temperatures and regulate CO2 concentration in the leaves. It’s a balance between gas exchange and water loss.

You’re running hot - 85F. My only concern at that temperature, if the plant cannot compensate higher rates of water transpiration, soil too dry, the stoma will shut.

renobambino said:

Hey people I was wondering if anyone could help me with this. So I know people generally aim for 1.2 to 1.6 VPD in mid to late flower. What about if you're running C02 and striving for 86F? with a humidity of 65% and a leaf temp of 82F the VPD is 1.5 which isn't to bad. I was wondering would it even matter about VPD in flowering as long as you've got your watering dialed in? I really want the stomata optimally open to take advantage of the c02, can someone explain?

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VPD is crucial to consider when you're using supplemental CO2, especially in the flowering stage. With CO2 enrichment, running at a higher temperature, like the 86°F you mentioned, is beneficial because it can increase the rate of photosynthesis. However, it’s important to balance this with the right humidity levels to create an optimal VPD.

At a leaf temperature of 82°F and an ambient humidity of 65%, you're right; you're in the ballpark of 1.5 kPa for VPD, which is pretty good. Maintaining a VPD around this range should indeed keep the stomata optimally open, allowing them to transpire and take in CO2 effectively.