Photoperiodism. Control over plant flowering is in your hands. The signal that prompts the plant to bloom at the right time of year is called photoperiodism. It is defined as the change in the proportion of light and dark in a 24 hour period. There are three ranges of photoperiodism which induce the plant to flower if a critical point is exceeded.
These are :
Short Day and Long Night
Long Day and Short Night
(uses other signals to start the process).
Examples of plants in the first group are Chrysanthemums, snowdrops, primroses and of course the Christmas favourite, the Poinsettia. In this group are all the plants that burst into bloom in autumn, winter and spring.
Plants in the second group, that flower when the days lengthen and the nights shorten, include Spinach, Lettuce, and summer blooming plants like Dahlias.
Temperature. Another critical factor in the blooming process is temperature. It appears that temperature changes affect the process, either slowing it or speeding it up depending on species.
Each species of plant will need light and dark in the correct ratio, at a specific temperature and for a specific duration, if it is to bloom. For example, Henbane needs 11.5 hours of light at 28.3 degrees Celsius, however if the temperature is lowered to 15.6 degrees then only 8.5 hours of light are needed for the switch to be turned on.
It has been found that there is a definite critical point at which the switch is activated. Henbane, for example, will not flower after 10 hours of light but after another 20 minutes the switch is activated and the Henbane flowers.
Leaves. The only part of the plant that carries the flowering signal, which triggers the plant hormones to alter their function, is the leaves. The message receivers are so sensitive that if only a very small part of one leaf is left on the plant the whole process will still be triggered. If the plant is totally stripped of all its leaves then it will not bloom because it cannot receive the signal.
Pigmentation. Plants have very specific colour pigments and two of these are responsible for the “flower now” message. These two photochemically reactive, colour pigments are called Phytochrome Red (Pr) and Phytochrome Far Red (Pfr).
Phytochrome Red (Pr) absorbs Red light at 660-760 nm, and Phytochrome Far Red (Pfr) absorbs Far Red light at 760-800 nm. Between them, they act as the on/off switch for blooming.
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