It's certainly not heat stress. Nute burn, well the symptoms look similar. I only use organics (BioBizz) and have given so far about 2-3ml/1.5l of Grow and Bloom every watering. That doesn't seem excessive to me?
No one has yet suggested phosphorus deficiency?
The
edges all around the leaves or half of the leaves can be brownish and work its way inwards a bit causing the part of the
leaves to curl up in the air a bit. Sometimes the stems can be red, along with red petioles that can happen when having a Phosphorus deficiency.
Taken from
http://www.icmag.com/ic/showthread.php?t=11688. Symptoms look quite alike to me. Stems and petioles are red/purple but that can be just typical for the plant tho.
Opinions?