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 case and to certain degrees, plants mark the seasons primarily by sensing changes in night length. Photoperiodism: 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. Critical Night Length: 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. Responding to Day Length and Night Length 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 of 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. Phytochrome Regulation in Plants: 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) which absorbs red light and Pfar-red (Pfr) which 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 abilities of phytochrome: Vernalization: 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. .