Originally printed in the Nov. 13, 2002 edition of the Illinois Valley News
Plants need to prepare for winter and there are many methods by which this is accomplished. Many of our local native plants, such as oaks and maples are triggered into this process by changing amounts of daylight. As the nights get longer and the days are shortened, no matter what the temperature, the plants begin closing up their chemical and growth routine. These plants are called photoperiodic. Many domestic plants such hydrangea, apples and grapes are controlled by temperature and require frosty nights before they begin to shut down.
This year the fall has been unusual and it has created some problems for plants.
September and October nights were warmer than usual, and with no rain, some plants went on as though summer would last forever. The photoperiodic plants did begin to close up shop however. The chlorophyll was not replenished and fall colors began as usual. Light sensitive chemicals called auxins are plant growth hormones which are involved in this procedure. Part of the process is for an abscission layer to form where the leaf connects to the stem. This is a layer which slowly seals over the opening to the stem and then the leaf will fall by itself.
Just before Halloween we had a period of severe cold when the nighttime temperatures dropped way below freezing and then warm temperatures returned during the day. The auxins which control abscission were not fast enough and many leaves were frozen on the stem. Ice crystals forming inside the leaf cells killed the leaves just like frozen pipes will break. These dead leaves will not fall off as normal but will gradually break off with wind and rain. Some may persist until spring growth appears. Very likely, there has been some damage to the stems as well , but it will not be noticed until growth begins again.
Auxins also play a role in another plant process called phototropism. This is when plants curve while growing toward a light source. House plants and outside plants as well, will grow toward light and will often have longer and weaker stems that will bend easily. Auxins in the stem tip cause tip cells to grow in length. This growth region in the stem tip is called the apical meristem. If sunlight is evenly spread around the stem during a 24 hour period the stem will grow straight up. If more light comes on one side only, the auxins on that side will be diminished and the auxins on the shady side will be increased and cause the shady side to grow longer. The net result is a bending of the stem away from the shade and toward the light. This is exaggerated when a plant such as lawn grass is covered by a board. The auxins will cause the stems to grow rapidly, with weak stems and no uniform direction. The yellow color is because no chlorophyll is produced if no light is present. If the plants do not reach sunlight they will die from the extra expenditure of energy.
There are other growth hormones in plants other than auxins. They will cause the seeds to germinate, flowers to form, and fruit to ripen. Temperature and lengths of daylight can be the mechanisms which start them into activity as well. These chemicals are all produced by different plant cells which are controlled by the genes of the plant.