Early senescence on Runtz?

I think you are right. And I would just gradually let them eat those nutes from the leaves. You can help them out, although they are nicely defoliated, the underside does contain many leaves that are more or less in the shade. These are eating up the nutes that could go to the higher buds and leaves. Defoliate the leaves in the darker spots once more and you will stretch this bit more w/o having to feed moe, while making sure buds get most of the available nutes. Try with one plant and look at difference in growth if unsure. Good luck!