tailieunhanh - Plant physiology - Chapter 17 Phytochrome and Light Control of Plant Development

HAVE YOU EVER LIFTED UP A BOARD that has been lying on a lawn for a few weeks and noticed that the grass growing underneath was much paler and spindlier than the surrounding grass? The reason this happens is that the board is opaque, keeping the underlying grass in darkness. Seedlings grown in the dark have a pale, unusually tall and spindly appearance. This form of growth, known as etiolated growth, is dramatically different from the stockier, green appearance of seedlings grown in the light (Figure ). Given the key role of photosynthesis in plant metabolism, one might be tempted. | Chapter 7 Phytochrome and Light Control of Plant Development HAVE YOU EVER LIFTED UP A BOARD that has been lying on a lawn for a few weeks and noticed that the grass growing underneath was much paler and spindlier than the surrounding grass The reason this happens is that the board is opaque keeping the underlying grass in darkness. Seedlings grown in the dark have a pale unusually tall and spindly appearance. This form of growth known as etiolated growth is dramatically different from the stockier green appearance of seedlings grown in the light Figure . Given the key role of photosynthesis in plant metabolism one might be tempted to attribute much of this contrast to differences in the availability of light-derived metabolic energy. However it takes very little light or time to initiate the transformation from the etiolated to the green state. So in the change from dark to light growth light acts as a developmental trigger rather than a direct energy source. If you were to remove the board and expose the pale patch of grass to light it would appear almost the same shade of green as the surrounding grass within a week or so. Although not visible to the naked eye these changes actually start almost immediately after exposure to light. For example within hours of applying a single flash of relatively dim light to a dark-grown bean seedling in the laboratory one can measure several developmental changes a decrease in the rate of stem elongation the beginning of apical-hook straightening and the initiation of the synthesis of pigments that are characteristic of green plants. Light has acted as a signal to induce a change in the form of the seedling from one that facilitates growth beneath the soil to one that is more adaptive to growth above ground. In the absence of light the seedling uses primarily stored seed reserves for etiolated growth. However seed plants including grasses don t store enough energy to sustain growth indefinitely. They require light energy

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