The photoperiodic response

The photoperiodic response



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The Photoperiodic Response

So, back to our original question: Can plants tell time? We’ve seen that they are able to measure, and respond to, the relative lengths of day and night. So they must have some capacity to "count" numbers of hours. Scientists have determined that a plant’s leaves are responsible for doing the counting. Leaves contain a light-sensitive protein pigment called phytochrome. This pigment occurs in two stable forms, one sensitive to visible red light, one sensitive to far-red light (on the edge of the visible spectrum). A pigment molecule can convert from one form to the other, depending on the type of light it receives. In total darkness, however, the far-red-sensitive form slowly reverts to the other form. The length of total darkness, then, determines the ratio of the two forms. This is how plants "count" the number of hours of darkness. And it is because of phytochrome’s ability to convert from one form to another that the plant is able to detect when any type of light breaks the dark period.

Only when a plant’s darkness requirement is met will the leaves release certain plant growth regulators. The substances travel from the leaves through the stem to the apical buds, stimulating some of those buds to switch from leaf production to flower production.

Even with all the groundbreaking work going on in botanical research—including things like cloning and genetic engineering—commercial flower growers still must raise and lower the shades in their greenhouses and use precise lighting systems to force their photoperiodic flowering plants to initiate flower buds. A breakthrough in research on these flower-inducing hormones—and some type of product that could mimic the response—would certainly be welcomed by commercial growers!

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