THE LIVING WORLD
Unit Seven. Plant Life
Plants respond to different environmental stimuli in a variety of ways. As discussed earlier in the chapter, plants bend toward light as they grow in response to this environmental stimulus. A host of other plant responses, including flowering, dropping of leaves, and yellowing of leaves due to loss of chlorophyll, are also prompted by various environmental stimuli.
Essentially all eukaryotic organisms are affected by the cycle of night and day, and many features of plant growth and development are keyed to changes in the proportions of light and dark in the daily 24-hour cycle. Such responses constitute photoperiodism, a mechanism by which organisms measure seasonal changes in relative day and night length. One of the most obvious of these photoperiodic reactions concerns angiosperm flower production.
Day length changes with the seasons; the farther from the equator you are, the greater the variation. Plants’ flowering responses fall into three basic categories in relation to day length: long-day plants, short-day plants, and day-neutral plants. Long-day plants, like the iris in panel 1 of the Key Biological Process illustration below, initiate flowers in the summer, when nights become shorter than a certain length (and days become longer). Short-day plants, on the other hand, begin to form flowers when nights become longer than a critical length (and days become shorter); the goldenrod in panel 2 doesn’t flower in summer, but instead flowers in fall. Thus, many spring and early summer flowers are long-day plants, and many fall flowers are short-day plants. The “interrupted night” experiment in panel 3 makes it clear that it is actually the length of uninterrupted dark that is the flowering trigger. The flash of light during a long night triggers flowering in the iris and inhibits flowering in the goldenrod, even though the day is shorter.
In addition to long-day and short-day plants, a number of plants are described as day-neutral. Day-neutral plants produce flowers without regard to day length.
Flowering responses to daylight and darkness are controlled by several chemicals that interact in complex ways. Although the nature of some of these chemicals has been deduced, how the various chemicals work together to promote or inhibit flowering responses is still being debated.
Plants contain a pigment, phytochrome, that exists in two interconvertible forms, Pr (inactive) and Pfr (active). When Pfr is present, biological reactions like flowering are influenced. When Pr absorbs red light (660 nanometers — orangish red), it is instantly converted into Pfr. Conversely, when Pfr is left in the dark or absorbs far-red light (730 nm — deep red), it is instantly converted to Pr and the biological response ceases.
In short-day plants, the presence of Pfr leads to a biological reaction that suppresses flowering. The amount of Pfr steadily declines in darkness, the molecules converting to Pr. When the period of darkness is long enough, the suppression reaction ceases and the flowering response is triggered. However, a single flash of red light at a wavelength of about 660 nm converts most of the molecules of Pr to Pfr, and the flowering reaction is blocked.
Plants respond to their external environment largely by changes in growth rate. Plants’ ability to stop growing altogether when conditions are not favorable—to become dormant—is critical to their survival.
In temperate regions, dormancy is generally associated with winter, when low temperatures and the unavailability of water because of freezing make it impossible for plants to grow. During this season, the buds of deciduous trees and shrubs remain dormant, and the apical meristems remain well protected inside enfolding scales. Perennial herbs spend the winter underground as stout stems or roots packed with stored food. Many other kinds of plants, including most annuals, pass the winter as seeds.
Key Learning Outcome 34.10. Plant growth and reproduction are sensitive to photoperiod, using chemicals to link flowering to season.