The bottom leaves will normally die and fall off as the plant grows. It looks like they're 'stretching' a bit...a sign of insufficient lighting (it would help to know which strain(s) you're growing). Make sure the flouros are no higher that 4" above the plant tops. The 4ft dual bulb (40W each) that you get from Lowes/HD for $8 work very well...and you can raise/lower the fixture as needed. Make sure not to use any ferts for the first few weeks. Best of luck.