bottom leaves curling?

Im not sure exactly what it is but heres my problem. My plants are about 5 ft tall or so rite now, but once they reached around 3 1/2 - 4 ft i noticed the lower node leaves began to wilt. The tops of the plants are fine and there on the 3rd week of flower. I know that the bottom leaves fall off when the plant gets bigger so it can focus energy on the top. But these leaves arent just falling off here an there, everytime i water i pull a few off each plant. Im using a 600watt HPS an i have 4 plants. I also put a 150watt HPS at the bottom of the plants so the bottom half can get light as well. Im not sure what is causing this problem but if anyone knows, some help would be greatly appreciated!

if they are green your ok they curling cause they not getting enough light most likely

I was thinking that to. There all green so thats a good thing. They dont get much light on the bottom cause there so bushy and that little 150 isnt helping much. Thanks!