Why yellowing under led's

Without knowing anything about the temps in your greenhouse, or anything about the LED wavelengths, it's kind of hard to guess at. If the space is cool, I'm guessing that you're not keeping the soil moist enough, because transpiration isn't happening as much at cool tems, and there is either a lockout of nutes, or the pH is high, because there isn't enough water in the soil, making the plant read it as "hot " soil. Without knowing temps, light distance, etc.., it's only guessing.