After reading that watermelon rind contains 2-3x more citrulline than the flesh, I spent a summer blending watermelon rind into smoothies as a whole-food alternative to buying citrulline powder. I would take the rind from half a medium watermelon (roughly 500-700g of rind), blend it with banana and frozen berries, and drink it about an hour before my workout.
The estimated citrulline content from this amount of rind is approximately 1-1.5g, possibly up to 2g depending on the watermelon variety and ripeness. This is well below the 6-8g dose that studies use.
Did I notice anything? Barely. Perhaps a slightly better pump on arm days, but nothing I could confidently distinguish from day-to-day variation. The smoothie was delicious, though -- watermelon rind has a mild, cucumber-like flavor that blends well.
After three weeks, I bought a bag of L-Citrulline powder and took 6g alongside the smoothie. The difference was unmistakable. The powder produced a noticeably enhanced pump that the rind smoothie alone never delivered.
My takeaway: eating watermelon (including the rind) is great for general health and provides some citrulline, but the dose is simply too low to produce the performance benefits seen in clinical trials. You cannot practically eat enough watermelon to match a 6g supplement dose -- you would need to consume roughly 3-4 kg of rind daily. If you want the exercise performance benefits, buy the powder. If you just want a healthy smoothie ingredient, blend that rind.