I am growing blackberries! The cultivar is triple crown thornless. This is their 3rd year, so they have both primocanes & floricanes. The primocanes wilt in the hot OK sun, but they seem to be growing okay when the sun goes down. The floricanes don't seem to wilt. I know that they have adequate water. I've seen this behavior before in other plants. But this behavior was so remarkable that I wonder if you are aware that some plants use wilting as a survival mechanism in intense sunlight.
Certified GKH Gardening Expert
Afternoon leaf wilt;
this is a very common defense mechanism among plants. If the air lacks humidity during the day, this can also cause this. As long as the leaves recover, and don't start falling off, then there should be no harm in this behavior.