PLANT OF THE WEEK
Dr. T. Ombrello - UCC Biology Department
While most people dislike the annual autumn chore of raking up the multitude of leaves that fall from our trees and shrubs, just about everyone agrees that the plants treat us to a spectacular show with a wide range of colors for a few weeks each year. We tend to take the bright colored fall foliage for granted in our area. One only has to move to the South where autumns are warmer or to Europe where it is cloudier to see how bland the falling leaves can be. The American colonists found that the bright fall colors were not just based on the species of native trees. Sugar Maples, when transplanted to Europe during the 18th century, produced a poor autumn color in their new home. Today it is known that our fall leaf coloration is due to having the right species (with the potential for producing varied bright colors) and the right climate.
While the explanations of how and why our leaves turn colors in the fall are far from complete, we do have good insight into what is going on. In order to understand fall leaf coloration, we have to start with the summer, when most plant leaves are green. The leaves are green because they contain an abundance of the green pigment chlorophyll. Interestingly enough, colors we see with our eyes are the colors that are reflected from a surface. So, a green leaf appears green because it is reflecting primarily green and absorbing other colors of light. In any case, the chlorophyll plays a major role in the process of photosynthesis, by which a plant makes its food (sugars). During the summer, the plant maintains a high level of chlorophyll, and the food made via photosynthesis not only fuels growth, but is transported to and stored in stems and roots.
In late summer, the shorter days and cooler nights effectively stop the production of new chlorophyll and promote its breakdown. The breakdown products of the chlorophyll are transported from the leaves to the stems and roots where they are stored until the next growing season. Other pigments in the leaves, however, do not degrade readily and remain intact. The carotenes, with their orange colors and the xanthophylls with their yellow colors are no longer masked by the green chlorophyll and become readily apparent. There are many different types and combinations of carotenes and xanthophylls in plant leaves, hence, the wide variety of colors and shades.
Before the chlorophyll is completely degraded, the bright autumn sunshine stimulates increased sugar production through photosynthesis. The sugar, however, is not readily transported from the leaves to the stems and roots because of cool night temperatures. Under these conditions, the leaves transform the sugar into anthocyanins, a large and varied group of pigments displaying colors ranging from red and pink through blue and purple. Once again, different species under different conditions produce varied amounts and types of anthocyanins, so we see many colors.
The pigments responsible for the fall colors of leaves are around us every day in other ways. Carotenes, for instance, are named for the first plant tissue they were isolated from – carrot roots. Carotenes are also found in pumpkins and squash. Xanthophylls make corn yellow, while anthocyanins make beets red and eggplants purple/blue-black. Many fruits change in color while ripening in the same manner as leaves turn colors in the fall.
Another interesting aspect of autumn leaves is the way in which they cleanly break from the stems as they fall. No jagged or ripped edges are found. Nature has designed a fascinating way to allow plants to shed their senescing leaves. A healthy, active leaf during the growing season produces a plant hormone called auxin. The auxin moves from the blade, down the leaf stalk into the stem on a constant basis as long as the leaf is healthy. If anything should inhibit the normal operation of the leaf, such as the changing weather in the fall, auxin production slows or ceases. In response to the lack of auxin, a layer of cells at the junction of the leaf stalk and the stem (called the abscission zone) becomes structurally weaker and allows for even the slightest breeze to cleanly snap the leaves off.
Why nature has “designed” such a system of elaborate fall colors for our native trees and shrubs is the subject of active research. Perhaps it is tied to some adaptive strategy for survival that mankind has not yet recognized. Several researchers believe the pigments protect the delicate leaf tissues from excessive light (serving as a sun screen) as they prepare for death and eventual abscission. Others believe that the bright colors “advertise” to insect pests that the tree has strong defenses against them, and that they should stay away. Regardless of the reasons, try to think about all the trouble nature has gone through to put all those brightly colored leaves on your lawn as you labor to rake them up each autumn
|Red Maple tree