Leaves that turn red or yellow in autumn have lost what?

Leaves that turn red or yellow in autumn have lost chlorophyll, the green pigment responsible for photosynthesis. As chlorophyll breaks down, other pigments such as carotenoids (which produce yellow and orange colors) and anthocyanins (which create red hues) become more visible. This change is triggered by shorter daylight hours and cooler temperatures, signaling the trees to prepare for winter.