Leaf abscission – so interesting!

Japanese maple on my running route.

Japanese maple on my running route.

So what is leaf abscission? It is when leaves fall from the tree. I thought it was pretty simple – fall arrives, weather gets cooler and leaves fall. But then Ben (age 5) opened “Big Green” the biology book (Campbell & Reece 6th ed.) last night to this page and I was floored. I had no idea it was so complex.

First – why do leaves fall anyway? To prevent the tree from desiccating (drying out) during winter when ground water is too cold to draw up the tree. That makes sense. Before the leaves abscise, food is stored in the stem for later use when the buds grow out in the spring.

Second – what causes the color change? New red pigments are made in autumn, and the existing orange and yellow carotenoids become visible when the green chlorophyll begins to break down. Cool!

Third – what happens when the leaf drops? An abscission layer forms when enzymes hydrolyze polysaccharides in the cell walls right near the stems. These cells don’t have reinforcing fiber in their cell walls so they become weak quickly. Gravity, wind and rain do the rest. Before the leaf falls, a layer of cork forms to prevent pathogens from entering the plant. This is like a plant scab on the stem.

Isn’t that really cool? So remember – leaves fall so the tree doesn’t dry out (water would evaporate through the leaves without a replacement supply from the roots). Red is a new color; yellow and orange become visible when the dark green chlorophyll (which allows the plant to make food from sunlight) breaks down. And the leaf finally falls when the abscission layer forms. A cork scab covers the little opening.

Have fun out there looking at stems!

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