Plant might be in a race with the season....As I figure it, your should be looking at what.....a October 20???? harvest?

The internal gradual yellowing is almost always a normal sign. Its the ones that yellow in spots, or tips, or veins that worry you.

The gradual yellow is fine and most likely a result of the suns angle producing far more red waves than the plant has ever seen. Such a tall plant will require the sacrifice of many fan leaves.

Either that or slugs have eaten your tap root and she's fucked