>>120694>>120737I was talking about the yellow leaves with the contrasting green veins, mainly showing in older growth. It's a classic sign of a magnesium or phosphorus deficiency, depending on the plant genus. These nutrients are rather mobile within the plant, so it recuperates them from older growth when there is a shortage, causing the yellowing. Google some pictures with 'magnesium shortage peppers' or something like that, and you'll see what I mean.
It's normal for this to occur to some degree when a plant starts blooming and fruiting; it takes a lot from a plant to do so and it will sacrifice foliage for a higher chance of reproductive success (bigger and more fruits). Your jalapeno plant seems to be afflicted quite extensively though.
Pic kinda related; some of my tomato plants last year, suffering from the same shortage while blooming. To a lesser degree though.