How is the release of harmful substances in plants?

Plants are living organisms, i.e. during metabolism and they form decay products that need to be removed. They are removed during leaf fall: in deciduous plants of temperate climates, leaf fall is massive, that is, all leaves fall off in autumn (if the leaves remain, they will continue to evaporate water and the plant will die). And in conifers, needle leaves have a small area and are covered with a waxy coating, which reduces evaporation. But you need to remove harmful substances, so old leaves constantly fall off.