It is well known that green plants use the light energy from the sun during photosynthesis to produce sugar and other organic compounds, which they use as food. What is less obvious is that there are two different types of light response by plants to sunlight: photoperiodism and phototropism. These processes have different mechanisms, but both are responses to light that allow a plant to orient itself in its environment. Photoperiodism refers to the response of a plant’s growth cycle (phenology) to changes in day length or photoperiod, whereas phototropism refers to the response of a plant’s growth direction (vertical root growth or phototropism) to changes in light intensity or phototropic lighting conditions. In this article, we will look at the details of both processes, their similarities, and their differences.
What is Photoperiodism?
Photoperiodism refers to the response of a plant’s growth cycle (phenology) to changes in day length or photoperiod. It is an ability in plants to respond to the length of day or night concerning growth and flowering. For example, some plants complete their entire life cycle, from germination to flowering, in less than one year. These are called annuals. Others take two or more years to complete their life cycle. These are called perennials. Most annuals and some perennials are photoperiodic and have an internal timer that causes them to start growing when the days are long enough, even if there is no rain or the soil is not yet warm enough for germination. Similarly, when the days become too short, a plant stops growing, even if the soil is warm enough to remain active and continue growth. So, photoperiodism is a plant’s ability to “know” how much light there is, even when that light is not shining directly on the plant. A plant’s response to changes in day length is plotted as a photo-response curve. The curve shows the critical day length above which a plant promotes flowering and the length of the day at which a plant becomes insensitive to changes in day length. Several plant responses to changes in day length are notable, including the change in the plant’s growth pattern, the change in the order of when certain functions occur in the plant, and the change in the number of certain chemicals that the plant produces.
How does a plant sense day length?
Plants can sense the length of the day by measuring changes in the level of a plant hormone called gibberellin (GA). GA is more abundant in the growing parts of the plant, the shoot tips, than in the roots. It is the relative increase in GA levels in the shoot tips after a certain length of day that promotes flowering in plants. A plant in the dark does not make any GA. When the growing part of a plant senses a certain length of the day, it starts to make GA. This GA is sensed by other parts of the plant, and the plant’s growth is altered.
When does a plant respond to changes in day length?
For most plants, flowering is promoted by increasing day length. Thus, the flower-making process is triggered by the length of the day, which is usually measured in hours of light. A plant, however, does not begin flowering when it gets 18 hours of light per day. The number of hours of light per day must increase above a certain value, but the exact value depends on the plant species. The flowering response to increasing day length is triggered by a combination of factors, including day length, temperature, and relative amount of light and dark during the day. The critical day length above which a plant promotes flowering is related to the temperature. For example, a plant species may promote flowering when light is present for 14 hours at 20° Celsius, but the same plant species may require only 10 hours of light at 10° Celsius. The relative amount of light and dark during the day can also affect when flowering occurs. If a plant is exposed to long periods of light and short periods of darkness, flowering may be delayed.
If a plant’s response to increasing day length is plotted on a graph, the graph is called a photo-response curve. A photo-response curve shows two important things. First, it shows the critical day length above which a plant promotes flowering. The second thing that a photo-response curve show is the length of the day at which a plant becomes insensitive to changes in day length. In other words, when the plant “forgets’ that the days are getting longer and stops promoting flowering. The length of the day at which a plant forgets about changes in day length is important for farmers and gardeners who use photoperiodism to control flowering. For example, a farmer might want to harvest peas when the days are 14 hours long. To do this, pea seedlings are grown in dark containers and exposed to increasing day lengths. When the day length is 14 hours, the plants are exposed to light and harvested soon thereafter.
Plant Responses to Day Length Changes
There are several plant responses to changes in day length. The most notable is the change in the plant’s growth pattern. In the spring, plants grow more slowly and flowers are produced. In the fall, plants grow more quickly and flowers are not produced. Plants also respond to changes in day length by changing the order of when certain functions occur in the plant. For example, in the spring, when plants are growing slowly, photosynthesis occurs first. However, in the fall, when plants are growing quickly, respiration occurs first. Finally, plants respond to changes in day length by changing the number of certain chemicals they produce. For example, when plants sense that the days are getting shorter, they produce less chlorophyll. This causes the leaves of the plant to change from green to yellow or red. This helps the plant conserve energy during the shorter days of the fall.