[oldcow]

I have one broccoli plant in my raised box that was doing especially well: sturdy, with plenty of beautiful, dark, healthy leaves. My pride and joy... It was fine yesterday; this morning one of the big leaves is completely floppy. No change in colour, no sign of insect damage, all the other leaves look perfect (for the moment, I bet the other ones will follow suit). The plant has always been under insect mesh, and the soil in the beds is a brand new mix, so I don't think there's any previous disease there. The plant was grown from seed by me, so I don't think I "imported" any disease.
Any clue about what could be going wrong?

[Manlea1]

I would guess its ad a bit of a chill,been dropping quite low the last couple of nights 

[oldcow]

Wow, I thought broccoli were hardy... It was cold here, but no frost... And the other smaller brassicas are not showing any sign of distress (yet...). I guess if the problem is the cold there's nothing I can do to help it recover, right? I'll just have to wait and see. 

[realfood]

In fact, there are two different groups of Broccoli plants which are:-

Sprouting Broccoli or Brassica oleracea Cymosa group.

This type of Broccoli is sown in the Spring and usually eventually harvested in late Winter and early Spring. In other words, it is a group of hardy, over-wintering varieties of Broccoli to eat in late Winter and Spring. But just to complicate matters, there are now sprouting broccoli cultivars that are ready for eating in the Autumn.

Calabrese or Italian Sprouting Broccoli, or Brassica oleracea Italica group.

This type of Broccoli is sown in the Spring  and harvested in the Autumn and early Winter, as it is not frost hardy. Some catalogues also list the Romanesco varieties (look rather like green cauliflowers), in this group. The edible spears are usually shades of green.