[Q] Intensity/Humidity

[Iammygod]

You may know VPD determines the transpiration rate.
With brighter light leaves get more light (and it increases leaves temperature) so they need more CO2 to perform photosynthesis. To absorb more CO2 stomata need to stay open. But as leaves temperature increases by brighter light, leaves vapor pressure is also increased and the result is ligher VPD if RH stays the same. The higher the VPD the more transpiration will occur. It makes leave and stomata to dry and dry stomata will close in order to save water. With colsed stomata plant is not able to absorb CO2 and photosynthesis will decrease. So we have to decrease VPD somehow. and it can be done with HIGHER RH not LOWER.

As a general, The higher the air temperature and the brighter the light (higher VPs), the higher the RH is needed(higher VPa) in order to decrease VPD, (=VPs - VPa), to stay in correct range.


This is my point of view and it doesn't match with the above pic. Correct me if im woring please.

 

[Iammygod]

Let me explain the second paragraph and say why i think this one is wrong too in my POV.

With extremely bright light, we need higher level of CO2 than free air, otherwise CO2 will become limiting factor of photosynthesis. Extreme bright light raise the leaves temperature much more. to stay in the correct range of VPD we need much higher vapor pressure in the air otherwise stomata will dry and close soon. Overall, we need higher vapor pressure in the air with extreme bright light. no cold air can hold high vapor pressure and that's why we need warmer air in extreme bright conditions. RH is friction of saturated vapor pressure, you know.

A Plant under extreme bright light needs more CO2 level, higher air temperature and higher RH in order to stay in correct VPD range. with low temperature or RH, VPD goes out of the range. Without CO2 in this condition you not only are wasting electricity bot also you are going to see unhealthy plant and slow growth.

 

[Iammygod]

I need some explanations please. Any help?

 

[kingfish]

Forget about CO2 in a small grow. Spend your money on seeds or something that can actually benefit your plants. CO2 in a small home grow setup is a waste of money and time.