Why do the leaves of some plants turn red in autumn?

Because chlorophyll in plant leaves is unstable and easy to decompose in cold weather, lutein, carotene and red pigment are relatively stable, so leaves in autumn are generally yellow and some are red.

Anthocyanin is a big family, and there are about 300 brothers and sisters in nature. The colorful flowers, fruits and vegetables we see every day are all related to its accumulation. Anthocyanins in cells mainly exist in vacuoles and are colorless, but when dissolved in water, they will produce different colors under different pH conditions. In late autumn, due to the temperature drop, the ability of transporting sugar and water in leaves is weakened, and the concentration of glucose is increasing. The increase of soluble sugar makes cells acidic, and anthocyanins turn red under acidic conditions, so leaves turn red.