Are they in pots? If so, the suns heat may be cooking the roots.

Could also be overnute.
Around my front porch (high desert, southwest USA) are some privet bushes. When I over-fert them, or mess with their ph too drastically, the growing tips burn pretty bad. Root stress and heat.

Heat stress alone causes major wilting of entire plant, not just the cola tips. (unless too close to a HID lamp)