from what you have posted i would go w/ root temps getting too hot??
but really this showed up after a few hours of hot temps?
because i am seeing iron and nitrogen issues but both can be related or for seperate reasons is why i ask?
did you fert close to the times of high temps assuming can't get a look at the roots ??
can i see a closer pic of the roots and "bends" you speak of??
you have a small area so eventhough your temps seem close to ok a higher temp and alot of medium being able to be hit by higher light exposure will change the outcome is the reason i ask?