I keep Petite and Barteri under both direct sunlight and high intensity bulbs for 12 hour photo-periods and I have never had it wither. I also think 14-16 hours is way too long of a photo-period, unless you dose heavily and inject Co2.
What you have looks like a fungus.
Have a smell, does it smell rank?
Is there slime on the leaves and/or rhizome?