My plants in my polytunnel (especially my tomatoes) have crispy yellow leaves!!

[Swifey40]

Why is that? It's my first time having a polytunnel so I'm probably doing it all wrong. Can someone help please. Tia.

Could it be too hot and dry. Our greenhouse has vents that open on very hot days, so that air circulates around the plants.

We leave our poly tunnel doors open during a warm or sunny day to let the air circulate.
Sorry if this seems obvious, but are they definitely yellow leaves or the yellow flowers coming through?

No, definitely the leaves, I've grown tomatoes before, just never inside.
I try and open one or both ends every day, at about 9:30am and then shut it at about 7:30pm. We are in Somerset, and on clay.