White leaves

Hi and Good Morning

I have a poly tunnel in which I have started off all my fruit amd veg plants.  However, the first leaves after the cotyledons have turned white.  This has happened on the melons, ailsa craig tomatoes, broccoli and cauliflowers.  They all have good root systems. Is this due to scorching, as last month, even with the doors open, the temperature was reaching 35degressC, or have I some sort of pest?

Many thanks in anticipation of some advice