paling bottom up? did it start in tip of leaf and progress from there? nitrogen deficiency.
yes, plants can shed leaves that are not getting enough light. but if it's more than 1 or 2 randome leaves, it's more likely a nute deficiency -- a mobile nute since it's starting at the bottom.
if canopy is strong you can be patient, some lower leaves loss is no big deal. if it continues to climb, up your proportion of N in your fertilzer while maintining levels of other nutes as best you can. 5-10%? how big of a reaction depends on rate of decline. Yellow leaves can get lush again, but anything with necrotic damage will not heal.