This looks like it could be a nitrogen deficiency. If the plants are large and flowering, some of the lower leaves in particular may start senescence (a sort of programmed leaf death and removal process), which often starts with yellowing. You could try giving the plants a little bit more fertilizer or compost, but my guess is that this slight deficiency will not affect the overall flowering/seed filling of your plants. It does not look like a virus because it is very slight and does not seem to have any other symptoms (stunting, leaf distortion) that are typically associated with acute viruses of crops.