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Timing crucial for survival of plants

Getting caught at the beginning of winter without enough firewood is a lesson in the value of planning ahead. Cutting and splitting must begin in the summer, if the job’s to be done by fall.

Plants, too, must prepare for winter, well before the weather gets cold. The seeds that plants produce in the autumn are the result of processes which began months ago. But starting the process too early, before the weather is settled, the snow has melted, and pollinators have emerged, can be as bad as starting too late. How does a plant “know” when to begin?

Plants time their actions pretty much as we do – with clocks and calendars.



Clocks and calendars are surprisingly common in living beings. Well before the weather grows cold, birds can tell that it’s time to fly south; bears, that it’s time to put on weight; and it’s an internal clock that causes humans to suffer jet lag when we race across time zones.

Pigments are chemicals that absorb some colors and reflect others. The green of chlorophyll, the yellows and oranges of carotenes, the reds and blues (think apples and cabbage) of anthocyanins are familiar; less so is the blue of phytochrome (from Greek, “plant pigment”).




When a pigment absorbs light, the energy may simply become heat; the molecule grows warm, like a car seat in the sun. But some pigment molecules are delicately balanced springs – hit them with light and they flip, changing from one shape to another.

The retina of the human eye contains cells sensitive to dim light – the cells responsible for night vision. Within these cells are pigment molecules shaped like the letter “V” … when they’re in the dark. Exposed to light, however, the molecules spring open and straighten out. The sudden change in shape triggers changes in other molecules, ultimately, sending out a signal: “Eye to brain, eye to brain – I see something!”

Phytochrome, too, changes shape when struck by light. The activated molecule can then trigger reactions in molecules nearby.

The reaction that activates phytochrome can go the other way, too. Indeed, the activated molecule reverts back to the inactive form on its own … in the dark. So, while the activated molecule builds up during the day, it disappears during the hours of darkness. When the balance of activated and inactivated phytochrome is just right – when the day is the right length – the plant flowers. (Exposing plants to varying periods of artificial light and darkness, horticulturists can force the plants to bloom any time of the year the market demands).

Evolution has fine-tuned the phytochrome molecule, such that it can tell a plant not just how much light or dark it’s receiving, but why the light is bright or dim.

Sunlight that has filtered through leaves is missing the colors absorbed by chlorophyll – precisely the colors a plant needs most. Phytochrome has evolved so as not to be activated by the remaining, ineffectual colors; rather, the residual colors deactivate the activated form. Even though it’s receiving light, the plant can determine that the light is of poor quality. “Knowing” that it’s being shaded by its neighbors, it can respond by putting on a burst of growth to try to overtop them.

Many seeds detect light; like a plant deciding whether or not to flower, such seeds can decide whether or not to germinate. Sensing itself heavily shaded by vegetation, a seed may decide to delay germination until next spring, when it will have a better shot at reaching the sun.

Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).


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