«Top Secret» by John Reynolds Gardiner

Nine-year-old Allen Brewster has a fantastic idea that might even win the silver trophy at the science fair: human photosynthesis. He has several obstacles to overcome, but Allen persists in his mission by using the scientific method and finally comes up with a concoction that turns him into a human plant. He turns green, craves sunlight and water instead of food, and sprouts baby roots if he stands in mud too long. But then Allen is deemed a threat to national security, and his project is suddenly top secret.