A: How do trees 'know' when to shed their leaves?

Hey there, the leaf fall is triggered by the phytohormone ethylene.

For ethylene production, the enzyme ACC synthase is needed. According to this publication here, UV-B triggers ACC synthase expression and thus ethylene synthesis. But in autumn or winter, there is not that much sunlight present anymore. Other stimulants are toxins, wounding, and pathogens. Therefore, I assume that the leaf fall in autumn is caused by a multiple setting where stress, high nutrient load, and other factors (indicators of mature plants or fruits), promote the synthesis of ethylene. The change of color is because the trees take all the "good" elements out of the leaves. These are for instance ions. In the center of chlorophyll, there is magnesium. Probably the tree knows that a high ethylene concentration is associated with leaf fall and therefore quickly evacuating the ions (and other good things) and thus changing the color of the leaves before they are gone. Regards Chapper

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