Everyone should read this

[rbarata]

It's informative and useful to avoid orchid loss

[Ray]

Hoo boy. You do realize that many orchids don't function the way most plants do, right?

While that article has some valid take-always for us, it focuses on plants which have C3 respiration, while most orchids are C4 or CAM plants that handle transpiration differently.

One of the most glaring discrepancies for me is humidity. In very high humidity, the transpiration rate is suppressed, which does the same for growth. Phalaenopsis, however, have evolved and live under nearly 100% RH, yet do fine.

[rbarata]

...while most orchids are C4 or CAM plants that handle transpiration differently.

Ray, do you know a good article that explains those types of transpiration. I've found this when I was trying to find something I've read some time ago that says orchids with thick leaves take more benefits from being watered by night time while thin leaves find it more beneficial to be watered by day time.
Maybe the differences you've mentioned can explain that.

[neophyte]

this is a slightly old thread, but there is a good khan academy page that explains C4 and CAM transpiration.

here is the link: C3, C4, and CAM plants (article) |
Khan Academy

all you need to know is that the Calvin Cycle is the part of photosynthesis that actually produces sugar and does not require light to function (although most plants, ie C3 and C4 plants, do the Calvin cycle during the day anyway). The Calvin cycle uses an enzyme called rubisco, which binds to CO2 and incorporates it into sugars. The issue is that it can also bind with O2, and when the enzyme incorporates O2 into sugar, it creates the wrong type of molecule and it takes the plant lots of energy to undo that, which is wasteful. C4 and CAM prevent that.

the general idea is that C4 and CAM make CO2 uptake more efficient without opening the stomata (pores on the bottom of leaves that facilitate uptake/release of water, CO2, O2, etc) for too long, which would cause the plant to lose too much water.