I think it may be more than just the broken taproot. What soil mix are you using? I have mine outside in Northern California, the low humidity doesn't seem to be an issue for pitangatuba. The grow light may be the issue, I have no experience with those. I'd hold off on fertilizing until the plant is stable. Too much fertilizer and it won't be able to take up water. Not sure if this one's a goner, but adding a plastic bag over the pot for humidity can help after a rough transplant. Also shade. Pitangatubas are monoecious, so I guess April is a "they"?