I hear that a lot, but am not a believer. It usually isn't the leaves not getting/using the phosphorous, (they're still green) it's the stems and petioles, which is a major structural and functional distinction.

I use 1/4 tsp of phDown per gallon of water to correct my ph. phDown is phosphoric acid. This gives me a huge bump in phosphorous every time I water, and every time I fertilize. But, I still get those red racing stripes and purplish petioles too. Could be from cooler nights, warmer days, too much potassium, (or not enough?) light spectrum issues, lights too close (dark red (purple) is a hue some plants use as a 'sunblock') micronutrient differences or deficiencies, roots too dry, roots too moist or combinations of these and more. From what I've seen, nobody knows exactly. Lots of hypothesis, but nothing proven through biology, nor backed-up through testing by others.

No big deal, to me. Especially if that's the worst thing you can find 'different' about your crop is the purple. :thumbsup: