HELP! Leaves turning white

I have some 15 day old seedlings and well the tips of the first true leaves are turning white and spreading up along the leaf. Some of my plants have justa tiny white tip and some other have bigger ones. Heres some pics. What can this be and how do I fix it? In soiless mix with no nutes and i have not given any nutes to these ladies yet either.

Pic 1
http://gallery.cannabis.com/data/500/123.jpg

Pic 2
http://gallery.cannabis.com/data/500/1234.jpg

temps are at 83 F all the time. Humidity goes any where from 23 percent to 50.