Photomorphogenesis is the control of plant growth and development by light, independent of photosynthesis. Light affects the shape (morphology) of the plant, not just how much food it makes.The effect is clearest when seedlings grown in the dark are compared with seedlings grown in the light. A seedling grown in darkness is etiolated:• a long, thin stem with long internodes
• small, unexpanded leaves
• a pale yellow colour, because no chlorophyll is made
• a hooked tip that protects the shoot tip as it pushes through the soil
• weak roots and little supporting tissueEtiolation is an adaptation. A seedling germinating underground puts its limited food reserves into growing upwards quickly to reach the light.When the seedling reaches the light it undergoes de-etiolation. Stem elongation slows, the hook straightens, the leaves expand and chlorophyll is made, so the leaves turn green and photosynthesis can begin.
These changes are triggered by very small amounts of light, far less than is needed for photosynthesis, which shows that they are controlled by a light receptor. The main receptor is phytochrome, which absorbs red and far-red light. Red light is the most effective at causing de-etiolation, and its effect can be reversed by far-red light.Plants shaded by other plants receive light rich in far-red, because leaves absorb red light. They respond by growing taller with longer internodes, the shade avoidance response.
• small, unexpanded leaves
• a pale yellow colour, because no chlorophyll is made
• a hooked tip that protects the shoot tip as it pushes through the soil
• weak roots and little supporting tissueEtiolation is an adaptation. A seedling germinating underground puts its limited food reserves into growing upwards quickly to reach the light.When the seedling reaches the light it undergoes de-etiolation. Stem elongation slows, the hook straightens, the leaves expand and chlorophyll is made, so the leaves turn green and photosynthesis can begin.
These changes are triggered by very small amounts of light, far less than is needed for photosynthesis, which shows that they are controlled by a light receptor. The main receptor is phytochrome, which absorbs red and far-red light. Red light is the most effective at causing de-etiolation, and its effect can be reversed by far-red light.Plants shaded by other plants receive light rich in far-red, because leaves absorb red light. They respond by growing taller with longer internodes, the shade avoidance response.