Why do red-leaf trees turn red in autumn?

In autumn, green leaves turn red, which has both internal and external causes. There are two main substances that make leaves red: one is carotene, which is an orange-red pigment commonly found in chloroplasts, and the other is anthocyanin, which exists in the cytosol of vacuoles. When the cell sap is alkaline, anthocyanins are blue-purple, and when the cell sap is acidic, anthocyanins are red. After autumn, anthocyanins in cotinus coggygria leaves increased, and chlorophyll destruction disappeared due to the decrease of temperature, and green leaves turned into red leaves. The external factor of discoloration is climatic conditions: when the temperature drops rapidly to a certain extent and the temperature is much lower at night than during the day, the leaves have not fallen, but the chlorophyll has been mostly destroyed. At the same time, the increase of temperature difference between day and night also contributes to the formation of anthocyanins, so the leaves turn red quickly. If the temperature drops slowly, the temperature difference between day and night is not big, chlorophyll is not destroyed, and the tree has withered, it will not become beautiful red leaves. This is also the main reason why Cotinus coggygria in the city is often not red, but Cotinus coggygria in Xiangshan has become dazzling. Of course, the red leaves of Xiangshan are not unique. Many people know that maple leaves turn red in autumn. In fact, not all maple leaves will turn red in autumn, and not all maple leaves will turn red. There are not only Acer species, but also Liquidambar formosana, Bird cypress, Rhus verniciflua, Pistacia chinensis, Cotinus coggygria and so on. The red leaf tree species in Xiangshan, Beijing is mainly cotinus coggygria. The leaves of this kind of tree are almost round, with smooth edges, which are not very noticeable at ordinary times, but in more than 20 days before the leaves fall, they become bright red, everywhere and very beautiful.