This is a young leaf of the calamondin plant and it does look a little like what's called iron deficiency - green vein, yellowing young leaves. I have bought a bottle of iron sulphate but has been hesitant to use till the customer service of Richgro replies me on whether is it a natural mineral or chemically synthesized. Peter Bennett's book talks about using alum to rectify the iron deficiency but gosh, I wonder where I can get that from.
The new chillies of the cayenne pepper seem to be shrivelling and dropping instead of developing. I wonder if it is the cold or if it is iron again. The whole plant does look a little lighter green compared to before. I think something seems lacking especially with putting on so many chillies recently!
The young leaves of the cayenne pepper yellowing and falling.
Some of the older leaves of the thyme are yellowing too but I have a feeling it is due to the cold.
It is happening to some leaves of the rosemary as well. More research to be done.