I dont think it is just one element like heat. Its a combination of conditions. Its a combination of things; temperature, wind, humidity, sunlight intensity, insects, whatever.
For example, with wind and extremely high temperatures, water in the ground and mulching may not be enough. The plant may be losing so much water to the atmosphere that it is pumping too many minerals (and maybe the not really good minerals) to the leaves. Or maybe whatever water goes in the ground dissipates so fast since it is so dry they maybe the far root tips are not feeding? Im only guessing this may be whats happening, but something sure goes on.
We normally have a few days each summer where the high temperature is a bit over 100. A few days, not many. And we normally have three or four thunderstorms with RAIN each summer, just enough to keep a bit of moisture in the ground. The last two summers weve had days, even weeks, with temperatures over 110 and no rain at all for months at a stretch. In spite of spending way too much on water, Ive had some pretty bad production in the garden and even trees I planted three or four years ago and watered heavily still died. Since I watered and had the only green stuff in this valley, grasshoppers and rabbits were unreal. They all came here to chow down.
This may sound like a normal summer to you where you are, but it is kind of rough here with my normal techniques, crops, and timing.
Ill quit complaining. I know people in parts of Texas and other places have had it worse the last couple of years. (Its always bigger or worse in Texas. My sister-in-law is from Texas. Believe me I know how EVERYTHING is more extreme in Texas. )