Photoperiod is the ratio of day length to night length over a 24 hour period. Botanists classify plants into four groups based on plants response to photoperiod:

  • day-neutral plants, day length has no effect on flowering, which occurs at maturity

  • short-day plants, flower only if daylight is shorter than some critical length, typically late Summer/Autumn

  • long-day plants, flower only if daylight is longer than some critical length (usually 9-16hrs)

  • intermediate-day plants, flower only when exposed to days of intermediate length; vegetative growth if exposed to too long or too short day length

Flowering can only occur if the plant has become reproductively mature after its juvenile stage. Experiments found that a bluish protein pigment controlled photoperiodism.  Phytochrome is also involved in fruit ripening, seed germination, flowering and greening.In experiments it was noted that:

  1. Flowering is inhibited by interrupting the dark period with red light

  2. An interruption of the dark period with red light can be reversed if followed immediately with far-red light(shortest wavelengths of infrared light)

This pigment was called phytochrome, which can exist in two forms, Pr and Pfr. There is also cryptochrome which absorbs blue light. Most recently botanists have discovered there are as many as 4 or 5 different phytochromes in one plant (type A,B,C, and E in angiosperms). Type A is most common.

Pr <-----> Pfr

Phytochrome is synthesised in the dark as Pr, which is the inactive form of the pigment. Pr absorbs red light (or white/visible light) and is converted to Pfr, the active form of phytochrome in cells. In turn, active Pfr absorbs far-red light (730 nanometers) and is converted to Pr. In nature, Pr is synthesised slowly at night and converted rapidly to Pfr during the day (in white light). This conversion in the dark is a critical factor in determining the ratio of Pfr to Pr (day length to night length or time of year), as they average level will indicate to the plant how long the day is, and therfore what time of year it is.

Photoperiod can effect development, particularly the ratio of males to females (males-short day, females-long day).  Phytochrome also has important part to play in seed germination. Red light stimulates germination and far-red inhibits germination of some species, typically small seeds. Seeds have to be 1-2mm of soil surface, this helps maintain a seedbank in the soil as they remain inactive in soil until close to surface.

Phytochrome also influences the early growth of seedling. When a seed germinates underground, or in darkness, the seedling has abnormally elongated stems, small roots and leaves, a pale colour, and it appears spindly. This condition is called etiolation, and is most easily detected by yellowing of normally green tissue. The rapid elongation is to ensure the plant reaches light before exhausting their food reserves.

Phytochrome and photoperiod also affect several other responses, including shoot gravitropism, stomatal formation, leaf abscission, spore germination, chlorophyll synthesis, chloroplast development, leaf expansion, and nastic movements. In each of these phytochrome acts as a light receptor.

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