[barrkel]

How would selective breeding affect the relative concentration / dilution of nutrients due to increased CO2?

It seems to be that the CO2 issue directly affects the inputs, making one input far more abundant than it used to be. To the degree that plants were constrained by CO2 as input, it logically leads to dilution of nutrients - unless you specifically bred plants that grew slower and produced lower yields, which seems unlikely.

(The actual mechanism, around plant's self-regulating water management affecting how much it retains vs needs to take in via the ground, is unlikely to be bred for one way or another.)