What causes tobacco leaves to have white tips

[dkh2]

What causes tobacco leaves to have white tips or a more greenish white tip ?
It wouldn't happen to mean it's time to harvest would it ?
I didn't notice this yesterday at all, it's just on the upper leaves of a few plants,
a couple Burley 21 and a couple Silk Leaf
and the very ends are more limper than the rest of the leaf.

 

[FmGrowit]

Probably just a little stress from lack of water. At this point in the game, I wouldn't worry about it.

When you harvest depends on what you're using the leaf for. Cigar wrappers are harvested early so they don't get thick veins. Most other types are harvested after about 75% of the leaf is yellow. Beyond that, the tobacco gets really strong.