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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