grapefruit rind

Question

Why are some grapefruit rinds thick and some thin? What causes that? Is it true that the fruit will be tart on tree with thorns?

Answer 1

Very simply, a thick peel on any kind of citrus fruit is caused by a nutrient imbalance. You can read more about this here: https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-fruit-thick-rind.htm

And, no, the presence of thorns is not an indicator of a fruit's tartness or sweetness.