Phytochromes, Photomorphogenesis and Tropisms

Phytochrome exists in two interconvertible forms:P_r + red light (660 nm) → P_fr
P_fr + far-red light (730 nm) → P_r
P_fr → P_r slowly in darknessSunlight contains more red than far-red light, so in daylight P_fr accumulates. At night P_fr slowly converts back to P_r. P_fr is the physiologically active form: it causes de-etiolation, promotes germination and affects flowering. In long day plants P_fr promotes flowering; in short day plants it inhibits flowering, so they need long nights to let the P_fr level fall.A tropism is a growth response in which the direction of growth depends on the direction of the stimulus. Shoots show positive phototropism, growing towards light. Directional light is detected mainly by blue-light receptors (phototropins) in the shoot tip, and the response is brought about by the plant growth substance auxin (IAA).When light comes from one side, IAA is transported to the shaded side of the shoot. There it stimulates cell elongation: IAA causes hydrogen ions to be pumped into the cell wall, the lower pH loosens the bonds between cellulose microfibrils, and the cells take up water and elongate. The shaded side grows faster, so the shoot bends towards the light.Phytochrome and phototropism work together. Phytochrome tells the plant about the quality and duration of light, while phototropism points its growth towards the light source.