leaves turn upside down

[Paracoculi]

I have some Salvia growing in a little greenhouse and I noticed the big lower leaves sometimes turn completely upside down.
The plants look very healthy and the environmental conditions seem to be about right, so what could be the cause?

I'll try to post some pictures soon.

 

[Ilian]

this is pretty common
a lot of growers report it and it actually seems to happen to the healthiest plants..
i wouldnt worry about this

 

[Paracoculi]

Thanks! That's good to know. Usually the leaves turn back to their original position after a while, it only happens to the biggest plant.