Hi, seeing as there are no yellow leaves on the adjacent sunflower, I would confidently say its a complete lack of water, when its hot leafy plants transpire like crazy...that water need to be replaced. Perhaps in future a good layer of mulch or straw on the top of soil would retain moisture better and if you could somehow keep those fabric pots out of the drying wind as well because they are worse than terracotta for rapid evaporation. Perhaps a capillary wick system would suit your method..that way you can be confident the plants can access all the water they like and have the wherewithal to withstand a bit of heat. Good Luck!
