this isn't 100% science 'er anything

but I've always heard that leaf curl UP is usually from heat...or possibly from a mag defieciency....or lastly...some strains simply "pray" and point to the sky. (indicas, mostly, from my experience)

Leaf curl UNDER is usually over-ferting....or..GASP....MITES.

there are other causes but these are the ones I've seen the most

ww