How plants use the pattern of changes in day/night length to solve the paradox that day length is equal twice a year.
Plants possess the ability to balance the daily changes in light and temperature, which is of great advantage to them, being sessile organisms that are closely associated with their environment. The circadian clock makes an important contribution to photosynthesis, in this that the clock is believed to increase plant growth and sustain. As days become shorter and cooler, plants are able to change their gene expressions that prepares for the end of the growing season and for the onset of winter. At the most basic level, circadian rhythms are the recurring expression of genes in individual cells, which is controlled by a central clock that responds to light and temperature inputs.
Photo periodism, another important phenomenon, is defined as the biological response of an organism to changes in the pattern of light/dark length in a 24-hour daily cycle. Plants use this phenomenon to determine the seasons and to synchronize their seasonal events such as flowering. Photoperiodic flowering plants are classified as long-day plants or short-day plants, and the regulatory mechanism is actually determined by the hours of darkness, and not by the length of the day. In addition to flowering, photo periodism in plants also influences the growth of roots or stems, or the loss of leaves during various seasons.
A long-day plant requires lesser hours than the critical number of hours of darkness in each cycle of 24-hours of a day for the induction of flowering. These plants typically flower in the northern hemisphere during early summer or late spring Examples of long-day obligate plants are: Henbane (Hyoscyamus) and Carnation (Dianthus)
These plants flower when the night is longer than the critical length. Flowering is not induced in these plans under long days. They necessitate a continuous period of darkness before floral induction can be initiated. Examples of obligate short-day plants are
Coffee and Chrysanthemum
Behavior of plants during equinoxes:
The vernal equinox symbols the beginning of spring in the Northern Hemisphere and the time at which the sun is seen in the sky for exactly 12 hours all over the world. After the vernal equinox, the sun goes higher in the sky and, as a result, the day length increases until the summer solstice marks the beginning of summer which is also the longest day of the year. Because the vernal equinox directs changes in light and temperature, it also causes extreme changes in plant behavior. These behavior changes that help plants to solve the paradox of equal day length twice a year are as follows:
Seeds of different plants require different germination conditions. Majority of plants require warm soil temperatures and water, both of which become accessible as the days lengthen following the vernal equinox. Seeds scattered in the previous year come to life in the next spring, absorbing the spring rains that trigger the enzymes to begin the growth process and the first spring plants start rising from the soil.
Flowering in many plants are seen sometime after the vernal equinox. Certain plants always flower around the same time in the spring, because of abundant light as the sun passes higher in the sky. Plants govern the times of flowering on the basis of the balance of a compound called phytochrome that is present in two forms in their cells, one of which activates in the dark and the other in the light. Phytochrome triggers flowering during the lengthening of day length after equinox
Availability of more light and higher temperatures also set off increased plant growth. Because photosynthesis depends on enzymes that don't function optimally at low temperatures, the higher temperature following the vernal equinox causes increased energy production, that is used up for plant growth
Spring wildflowers begin to bloom soon after the vernal equinox, but disappear shortly afterwards. They are known as spring ephemerals, and they quickly gather the early spring light to produce seeds, but they die back again as tree flora develops and symbols the shift of spring into summer.