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  • Phytochrome
    • The photo-receptors responsible for absorbing light is known as PHYTO-CHROMES (in leaves)
    • Phytochrome exist in two forms;
      • Phytochrome 660(Pr) ---interchangeable)---> Phytochrome 730(Pfr)
        • Sunlight contains more of the wavelength 660nm (more red light)
    • During the day Pr will accumulate and it will also be converted into Pfr
      • During the night Pfr will be converted back into Pr
    • Plants measure day length/length  of darkness by the amount of phytochrome exiting in each form
    • The presence of the phytochrome stimulates the hormone that causes a plant to flower.
      • Hormone = florigen
        • Produced in the leaves and is transported via the PHLOEM to the buds ---> causes it to flower.
    • There are three different types/groups of plants that are affected by hours of light and darkness
      • Day neutral plant = they are not affected by the number of hour of light. E.g tomatoes, cotton, cucumber
      • Long day plants = flowering occurs when the plant is exposed to dark periods (number hours of dark) shorter than a critical length (light hours).
        • These flower when light hours is greater than dark hours e.g. Cabbage, petunia
      • Short day plants = flowering occurs when the plants are exposed to dark periods longer than a critical length (light hours). Flower when the number of hours light is less than the hours of dark e.g. ponisetta


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