These are symptoms of scorching, which are caused by a lack of sap reaching the ends/tips of the leaves.  This can be caused by several things: drought stress, heat stress, too much water (which kills some of the roots and therefore prevents the plant from taking up water), and over-fertilizing.

Are there drainage holes in that planter?  If not, then the plants are probably drowning. 

Broccoli and similar plants are cool-season crops, and a sudden spike in heat could over-stress the plants.  You might also want to review your fertilizer schedule, to be sure you have not been overdoing it.

I would probably recommend harvesting the heads that are there, because you probably don't have too much longer before they start to bloom and become of poor quality.