Yellow sweetpea leaves

[carosanto]

Hi

I've got some overwintered sweetpeas, which were growing nicely, and bushing out just fine.  The leaves are going a bright yellow.  I've been growing sweetpeas for years and feel I should know the answer to this problem, but all I can find on the internet is 'fluctuations in temperature'.  Well we have gone from v warm, to b freezing.  Is this the answer, and will they survive?

Thanks for any help.

Caro

[carosanto]

[manicscousers]

Ours are 3' tall and some of the leaves were yellowing so I've fed them  ;D