[AlaninCarlisle]

I've never really understood what causes the leaves on my greenhouse toms to look stressed and curl up, although this doesn't seem to affect the fruit. Now the mystery deepens as I'm also growing some of the same varieties in my polytunnel at the same time and their leaves look perfect despite the tunnel being hotter than the greenhouse.

Can anyone offer an explanation?

[New shoot]

Is the tunnel covered with opaque plastic Alan? 

Could be the sun magnified by greenhouse glass is causing the leaf stress, whereas the tunnel provides bright diffused light 

[mumofstig]

It happens when there is a big difference between day and night-time temperatures.

So is the polytunnel better at retaining the day's heat than greenhouse 

[AlaninCarlisle]

Mumofstig, I guess the volume of the polytunnel is about ten times that of the greenhouse and its ground area is soil rather than the cobbled area of the greenhouse so it'll be much slower to heat up and cool down each day

[Aunt Sally]

You have your answer