Not hot enough; or not enough sunlight; or not the right soil pH (should be 5.5-6.5); or not enough fertiliser; or too much fertiliser; if in a pot, could be potbound.

Basil leaves aren't always symmetrical. They frequently curl up a bit, even in perfect conditions.

I wouldn't worry about it unduly.

Some basil types have very rough looking leaves and are usually referred to as a ruffle type basil. There are many different sized leaves, and some kinds can be huge as big as your open hand.

The problem is my Opal Basil,
I have it for one day in the store bought pot, then transplanted it the very
next day. I watered it from the soil down, But the next day i watered
it i watered it like as if it were raining, Now my Bushy half green half
purple basil has been drooping for days. But thats not all My once Dark
Purple leaves are now turning light purple to almost green.
What do i do?
YasminShi

I Also planted it in a planter with my Oregano and Sweet Basil, Thought thata might have something to do with it, Can anyone help me!?
YasminShi

Patience is a virtue and transplants are going to test your patience.. Don't overwater a new transplant, as you are then encouraging damping off that can also kill it very quickly. Leaf color should return once they get plenty of sun and are used to their new surroundings.

hy....im in arkansas and its hot and humid....i have about 30 purple basil plants and they are losing their color.....they are fading from purple to green.....the taste is fine, the plants are full and flush....they are planted in 4" pots and im wondering if it could be a root bound problem...but if it were not for the discoloration i wouldn't think there was any thing wrong.....im using a rich but broken down compost from a friends horse farm but its just rich topsoil.....any ideas on which way to look? thanks

hy....im in arkansas and its hot and humid....i have about 30 purple basil plants and they are losing their color.....they are fading from purple to green.....the taste is fine, the plants are full and flush....they are planted in 4" pots and im wondering if it could be a root bound problem...but if it were not for the discoloration i wouldn't think there was any thing wrong.....im using a rich but broken down compost from a friends horse farm but its just rich topsoil.....any ideas on which way to look? thanks