University of Mississippi!

[Marine2143]

https://www.researchgate.net/publicatio ... conditions

[unkle_psycho]

Important subject, and it certainly gives some direction, even if its studying leaves in a test tube. I do wonder how much the constant RH biases the study towards the temperatures that fit the VPD..

[Marine2143]

Aloha & Mississippi is like Hawai'i with high humidity most of the time. VPD is more difficult to control here with the high cost of running de-humidifiers.

[unkle_psycho]

A lot of people present this study as mapping out optimum temperatures, but really it is observing photosynthesis and water vapor exchange under three variables: Temperature, light intensity, and CO2.

I think there is a chance to be mislead if one does not take into account that the vapor exchange under 30 is outside acceptable horticultural standards

For me I see a huge difference at 30c increasing RH from 50% towards 70%. The study shows some gain moving from 25c to 30c, I think the gains would be much bigger if RH was higher (if environment was dialed in). If the 30c setup was dialed in, the study might have concluded that the optimum temperature is 30c, rather then 25-30c.

[unkle_psycho]

Thinking more about this subject I had the idea of seeking some research comparing vpd to photosynthesis, to see if the reduction in amount of photosynthesis would be similar to the reduction this study shows at 30c.

I'll need to search more and digest the info, but right of the bat this Australian data suggests quite a decline. In the first of the four boxes you can see how there is a slight increase in photosynthesis as the pressure goes below 1.0, there is a crazy decline when it rises above 1.0.

https://www.researchgate.net/figure/Rel ... 1_51369666

This decline looks very similar to the decline in the cannabis study, and it happens in very similar areas of VPD. This kinda supports come claims I've seen in earlier discussions about the same study, where some people were getting absolutely fabulous growth around 39c, outside in a super humid environment.

Perhaps I should see if Chandra is the type of fellow to answer emails... zee forums vant to know

[DaveJonesFan]

I just grabbed a humidifier to add to my room. I've been noticing something weird since switching to LED's, in that I can't let my soil moisture go as dry as I could with HID, or at least not in the same way. It seems like the opposite of what is supposed to be with less IR coming down, but I think I may have such high ppfd that transpiration rates are higher and I notice bad growth very quickly without soil moisture. Maybe the roots need a higher turgid pressure maintained in this environment and can't cope with closing stoma as easily as before, as with HID they would stay green and thriving even going near bone dry pot weights, as long as they didn't 'droop' from low pressure. It's just not the case for me with these high power LED's, upper leaves in the lower humidity region close to the light go to shit long before any 'droop'.

With HID, I was striving to keep air temps around plants at 72F, as any higher and close hanging lights and IR would sort of crisp the top buds too much and change the flavor of the oils, now letting temps rise to 80-85F with the LED's, The top buds aren't having that crisp up effect but the leaves around the top buds transpire something crazy and look shit with any drop in soil moisture and humidity.

My humidity has always been tied to my soil moisture and with HID the top inch or two of soil could go fairly bone dry regularly while maintaining some moisture underneath and some water weight to the feel of the pot, and humidity levels in the tent. Now my pots start feeling light more all the way through without the top most soil getting so dry, more of an even amount of moisture pulled homogeneously from the soil under high transpiration. Humidity levels plummet long before the whole pot feels very light weight though. I seem to be having better luck with the drip type watering which for me is keeping it relatively moist with frequent small dose hand waterings.

Hopefully some extra humidity in room will help even things out.

I also need to keep working on the lowering of nute levels with these higher water needs because I'm definetely getting salt buildup as well with more transpiration which then exacerbates these issues.

[unkle_psycho]

I just grabbed a humidifier to add to my room. I've been noticing something weird since switching to LED's, in that I can't let my soil moisture go as dry as I could with HID, or at least not in the same way. It seems like the opposite of what is supposed to be with less IR coming down, but I think I may have such high ppfd that transpiration rates are higher and I notice bad growth very quickly without soil moisture. Maybe the roots need a higher turgid pressure maintained in this environment and can't cope with closing stoma as easily as before, as with HID they would stay green and thriving even going near bone dry pot weights, as long as they didn't 'droop' from low pressure. It's just not the case for me with these high power LED's, upper leaves in the lower humidity region close to the light go to shit long before any 'droop'.

With HID, I was striving to keep air temps around plants at 72F, as any higher and close hanging lights and IR would sort of crisp the top buds too much and change the flavor of the oils, now letting temps rise to 80-85F with the LED's, The top buds aren't having that crisp up effect but the leaves around the top buds transpire something crazy and look shit with any drop in soil moisture and humidity.

My humidity has always been tied to my soil moisture and with HID the top inch or two of soil could go fairly bone dry regularly while maintaining some moisture underneath and some water weight to the feel of the pot, and humidity levels in the tent. Now my pots start feeling light more all the way through without the top most soil getting so dry, more of an even amount of moisture pulled homogeneously from the soil under high transpiration. Humidity levels plummet long before the whole pot feels very light weight though. I seem to be having better luck with the drip type watering which for me is keeping it relatively moist with frequent small dose hand waterings.

Hopefully some extra humidity in room will help even things out.

I also need to keep working on the lowering of nute levels with these higher water needs because I'm definetely getting salt buildup as well with more transpiration which then exacerbates these issues.

PPFD should affect the water use, but could the change be related to some qualitative aspects of heat? When I generate heat with a HID my soil temperatures stay cooler, the heat is radiating from above. When I use an electrical heater it is down with the pots, so I imagine it should have quite an effect on the ratio of heat between the bottom and top of the plant?