Photoperiodism is a plant's developmental response to the seasonal change of the length of light and dark periods (day-length and night-length). It indicates the change of seasons and marks the onset of spring, summer, fall and winter (this refers to the north temperate zone and the south temperate zone).
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What is the point of the ability to perceive the differences in day- and night-length and therefore changes of seasons? Several developmental responses have been identified and scientifically described:
And furthermore: winter season is an adverse period in the life-cycle of a plant. Yet,with the help of photoperiodism, plants have strategies to survive cold, dry and otherwise harsh winter conditions:
Photoperiodism is genetically determined. Three groups of plants have been established, depending on their ability to recognize different photoperiods: long-day plants, short-day plants and day-neutral plants.
The flowering is in induced in long-day plants when the daylight length exceeds a critical time-span (Interestingly, the plant orients itself on the shorter dark period not on the longer daylight period). This happens usually in late spring and early summer. Then, the plants have their seeds ready to be distributed during summer time.
Long-day plants mainly occur in the north and south temperate zones and in most northern and southern regions where the winter is long and dark and the summer very short but with many daylight hours.
Short-day plants bloom when the days get shorter and daylight length decreases and when the nights grow longer. They need an extended period of darkness and they start their floral development when the day length has fallen below a critical time-span. They only grow vegetative in summer and then, flower in fall.
Short-day plants primarily occur in southern regions between the equator and the south temperate zone.
Day-neutral plants are plants that require no photoperiodism to induce flowering. The daylight length and the length of the dark night period do not play any role at all.
Other signals induce the floral development in these plants. These may be, for example, the overall developmental stage or vernalization (a period of lower temperature).
Day-neutral plants usually grow in the equator region and in adjacent southern regions, where the day and nighttime periods barely change.
How Do the Plants Perceive the Different Seasonal Dark and Light Lengths?
The light perception occurs in the leaves through photoreceptor proteins called phytochrome and cryptochrome. Phytochrome is activated by light absorption: light-red (665 nm) absorption actives and dark-red (735 nm) apsorption deactivates phytorchome. Cryptochrome is activated through either blue light (400 – 500 nm) or UV-A light (370 nm). The activated protein complex induces floral development in long-day plants and inhibits floral development in short-day plants.
The leaves then release a “floral hormone” called florigen once the floral development is induced. This plant hormone is then transported to the tip of the plant where the flower is grown.