Oh that sounds interesting.. do you have any more information on this?Odsox wrote:I agree (sort of).
It's probably not quite so important now as it is in early spring, but my experience is that it doesn't matter too much what the temperature at the top of the plant is, it's the temperature at the roots that controls the growth of tomatoes (and peppers).
You can grow and ripen tomatoes in the depths of winter with the foliage as low as 5c as long as the roots are at about 20c.
You'd think that the compost would warm up quite fast in a poly tunnel though right? Although equally I suppose it could drop quite fast too as the ambient temperature drops. Maybe I need to lag my potsOdsox wrote:So getting back to the original post, as compost/soil is an efficient insulator if you take a plant that is in a 3.5" pot and put it an 11" pot you have a 4" barrier of cold wet compost between the roots and the sunny outside world, potting from 3.5" to 6" means that the heat only has to penetrate about an inch of compost before it starts to warm the root ball.
Most of my potted up tomatoes... I ran out of compost for the last two
I assumed it was because the ones in pots are warmer during the day, being against a wall facing southish which retains heat long into the evening or because they like my home made compost with with added fertiliser. Oh I dunno, perhaps this is all too scientific for me! 
(thanks)