Some tree leaves are noted for their spectacular fall color display. The changing fall colors are caused primarily by a photoperiodic response but also by a temperature response. As nights become longer in the fall, leaves stop producing chlorophyll. As the chlorophyll chemically degrades, it is not replaced. Other leaf pigments, the carotenoids, become visible and the green/orange splotches become more visible as the green chlorophyll turns orange. Carotenoids include the orange carotenes and the yellow xanthophylls. Anthocyanins produce the deep red and purplish-red colors in the fall display.
Vernalization is the low-temperature stimulation of flowering. Vernalization is important for fall-sown grain crops, such as winter wheat, barley, and rye. For example, wheat seeds are sown in the fall and survive the winter as small seedlings. Exposure to cold weather causes the plants to flower in the early spring, and an early crop is produced. If the same wheat is sown in the spring, it will take about two months longer to produce a crop. Thus, cold temperatures are not absolutely required for most crops, but they do expedite flowering. Farmers often use vernalization to grow and harvest their crops before a summer drought sets in and stunts growth.
A biennial plant is a plant that lives for two years, usually producing flowers and seeds during the second year. Biennial plants, such as carrots, beets, celery, and foxglove, survive their first winter as short plants. In the spring their flowering stem elongates rapidly, a process called bolting. Most biennials must receive cold weather to vernalize before they flower during the second year. They will then die after flowering. Treating a biennial with gibberellin is sometimes a substitute for cold temperatures in vernalization, and will stimulate the plant to grow.
Plants monitor changes in day length with a bluish, light-sensitive protein pigment called phytochrome. Phytochrome exists in two forms, based on the wavelength of the light that it absorbs. It is generally produced in meristematic tissues in very minute amounts. The two stable forms can be converted to each other by absorbing light. Pred (Pr) that absorbs red light and Pfar-red (Pfr) that absorbs far-red light. In the daylight, more Pr is converted to Pfr (the active form) than vice versa. Pfr will convert back to Pr over several hours in the dark where it would be stable indefinitely. The conversion in light is almost instantaneous. The phytochrome mechanism is what transforms the crook in the hypocotyls of the emerging seedling into a straight stalk. Stem elongation appears to be inhibited by Pfr. However, if light levels are low, the shaded stems of a tree, for example, more far-red light will reach them and cause the conversion to Pr which lowers inhibition and allows the stems to grow longer and out from under the shade.
The interconversion abilities of phytochrome:
Plants can be divided into three groups, depending on their response tot he photoperiod, which again acts a season indicator.
One group, called day-neutral plants (DNPs) are not affected by day length. Examples of DNPs for flowering include tomatoes, dandelions, roses, corn, cotton, and beans.
Short-day plants (SDPs) flower in the spring or fall, when the day length is short. For example ragweed flowers when the days are shorter than 14 hours and poinsettias flower when the days are shorter than 12 hours. Chrysanthemums, goldenrods, and soybeans are SDPs for flowering.
Long-day plants (LDPs) flower when the days are long, usually in summer. For example, wheat flowers only when the days are longer than 10 hours. Radishes, asters, petunias, and beets are LDPs for flowering.
It has been discovered that the important factor in flowering is the amount of darkness, or night length, that a plant receives. Each plant species has its own specific requirements for darkness, called the critical night length. Although it is now understood that night length, and not day length, regulates flowering, the terms short-day plant and long-day plant are still used. A short-day plant flowers when the days are short and the nights are long. Conversely, a long-day plant flowers when the days are long and the nights are short compared to the requirements of another plant.
A plant’s response to changes in the length of days and nights is called photoperiodism. Photoperiodism affects many plant processes, including the formation of storage organs and bud dormancy. However, the most studied photoperiodic process is flowering. Some plants require a particular night length to flower. In other species, a particular night length merely makes a plant flower sooner than it otherwise would.
In nontropical areas, plant responses are strongly influenced by seasonal changes. For example, many trees shed their leaves in the fall, and most plants flower only at certain times of the year. Plants are able to sense seasonal changes. Although temperature changes are involved in some cases and to certain degrees, plants mark the seasons primarily by sensing changes in night length.
Many growth regulators are widely used on ornamental plants. These substances do not fit into any of the five classes of hormones. For example, utility companies all over the country often apply growth retardants, chemicals that prevent plant growth, to trees in order to prevent them from interfering with overhead utility lines. It is less expensive to apply these chemicals than to prune the trees, not to mention safer for the utility workers. Also, azalea growers sometimes apply a chemical to the terminal buds rather than hand-pruning them. Scientists are still searching for a hormone to slow the growth of lawn grass so that it doesn’t have to be mowed so often.