Light burn happens when leaves sit too close to your grow light -- whether that's a high-powered LED or a big HPS -- and the intense PAR causes photo-oxidative damage to the tissue. The first thing you'll usually notice is the top leaves 'praying' upward or tacoing before any color change appears, and then the tissue closest to the light starts bleaching yellow-to-white while the rest of the leaf stays green. That's the key differentiator from heat stress: heat stress causes broad curling with browning edges across the canopy but no bleaching, whereas light burn is strictly proximity-based -- only the leaves physically closest to the bulb get hit. It's also different from iron deficiency, which targets new growth regardless of light distance. One easy diagnostic trick: light-burned leaves feel firm and resist plucking, while nitrogen-deficient yellow leaves wilt and practically fall off on their own.
