1st week flowering , vottom leaves turning yellow

Hows it going . Its my 7th week overall , 6 for Veg and this is my first week flowering , i noticed that a couple of the big leaves at the very bottom of the plant are turning yellow , but the tops leaves of the plant are nice and healthy , a 400 watt HPS, i am woundering if anybody has any info on this topic?

its from not enough light reaching the bottom of the plant...its ok. If the top of the canopy is healthy...