New Type Of Yellow Leaf

Hi,first groover, outdoor Sensi Star and Hollands Hope. The lowest leaves are getting yellow from the veins suddenly last weekend. No change in nutrients, watering, the plants were in perfect shape, the temperature dropped a little but still over 20 C degrees.
The mystery: I have plants in a pot and plants in a hidden riverbank. The problem started at the same time at both place with different soils. The riverbank plants are without watering and extra nutrients in a good soil, the pot based plants are watered every day (hot place with high evaporation) and NPK on every 5th watering in aerated soil.
I don't have any idea what's happened.
Anyone have a clue?

hey man, lol sorry bout this but I dont really know either. I rekon they could just be falling off cause your plant is growing bigger and taller. But some expert advice on this would be great wouldnt it.....
Anyway have a good day
Peace

Strange one this is coz i've got exactly the same thing happening with mine, i'm growing 'green spirit' under 600w sodiums, the upper of all plants look good, nice green but some of the lower leaves have gone or started to go yellow. My seeds were feminised and the nutes i'm using is canna vega A&B.

It looks it was a beginning of a quick growing period, the plants are healthy, no more yellow leaf.