Wednesday, November 2, 2016

Why trees drop their leaves

I once lived in a small town in the upper midwest. How small was it? It was so small there was only a single Dairy Queen, a crumbling, 1950s cement block building with only window service. It had a couple of weathered picnic benches where teen agers gathered during the summer months. It closed for the winter early in November and didn't reopen until the weather warmed up in March.

It closed because the building wasn't insulated and the heating costs would have been enormous. Plus, when there is snow on the ground, no one stops at a restaurant that lacks heated indoor seating. The decision to close for the winter was economic – operating expenses would increase while revenues decrease. The owner would lose money by staying open all winter.

Leaves are living structures and, like all living things, there is a constant turnover of the material they are made of. Being exposed to bright sunlight, while necessary for them to make
carbohydrates, damages their components, and the damaged parts are repaired or replaced, using the food produced by the leaves. In the summer the repair work pays off because the leaf produces more food than it consumes in repair. But as fall approaches this margin between cost and benefit shrinks. There is less sunlight to capture because daylight hours are fewer and the sunlight is less intense. The temperature is also lower, causing all the chemical reactions of photosynthesis to slow down. In addition, water becomes less available. 

In order to manufacture carbohydrates by photosynthesis a leaf needs carbon dioxide and water. The carbon dioxide it gets from the air through tiny holes in the leaf surface. The water comes from the soil, via the roots. But those holes that bring in the carbon dioxide also allow water to evaporate from the leaf. The lost water must be replaced by water taken in by the roots. When winter arrives the upper layers of the soil may be frozen, making water uptake impossible. This would put the plant in danger of losing too much water by evaporation from the leaves. It could limit water loss by closing the holes in its leaves, but then it could not get carbon dioxide from the air and the making of food would grind to a halt. So, like the Dairy Queen in my old town, the economic decision is to close down for the winter. For the Dairy Queen that meant turning off the utilities for the winter. For a tree it means turning off the water and the nutrient supply to the leaf – accomplished by letting the leaf die.

Leaves contain a lot of useful material that would be lost if they just fell off the tree. As winter approaches the leaf senesces (the formal term for leaf aging) – its contents, like chlorophyll and other materials that contain nitrogen, are broken down and recovered by the tree. The removal of the green-colored chlorophyll causes the leaf to change color. At the same time a special layer of cells is forming in the leaf stem (the petiole) at the point where it attaches to the twig. These special cells weaken their connection to each other, forming what is called an abscission layer. (Abscission means to "to cut from" -- it is the place where the leaf will be separated from the tree.) Eventually the leaf cannot be supported and the entire structure falls from the plant. 

The formation of the abscission  layer is controlled by the interaction of two plant hormones, ethylene and auxin. 

Ethylene is a gas and is best known for causing fruit to ripen. The changes that take place in fruits as they ripen are like the changes in the abscission layer – the cells lose their connection to one another and the fruit loses its firmness, becoming soft and mushy. You may have taken advantage of this by placing a ripe banana in a bag with an unripe fruit, like a pear. A ripe banana produces a lot of ethylene that is trapped inside the closed bag causing all the fruits in the bag to ripen faster. Ethylene stimulates the same kind of changes in the cells of the abscission layer, causing them to loosen their connections to each other.

Auxin is a plant hormone with many effects on plant growth and development. It is perhaps best known for its role in apical dominance – auxin produced by the growing tip of a tree inhibits the growth of buds further down the tree. When the tip of the tree is cut off the inhibition is removed and the next bud starts to develop, becoming the new growing tip. 

Auxin has another important property. It inhibits the action of ethylene.

When the leaf is young and fresh it produces enough auxin to inhibit the effect of ethylene. But as the season progresses and the leaf begins to senesce the amount of auxin it produces decreases. The effect of ethylene is no longer inhibited and the cells of the abscission layer begin to secrete enzymes that dissolve their cell walls, weakening their connection to adjacent cells. Eventually the connections become so weak that the weight of the leaf causes it to separate from the branch at the point of weakness. Then the leaf falls. 

Shedding their leaves in the fall enables trees to avoid water loss during a time of year when water in liquid form is less available. It also allows them to avoid investing in increasingly unprofitable repair of leaf tissues. It is cheaper to dispense with the carbohydrate factory in the fall and then rebuild it again in the spring.