Gardening 101

How Plants Use Light for More Than Just Energy

Photochrome chart

Capturing the energy in sunlight and converting it into forms other organisms can use is the fundamental function of plants in nearly all ecosystems. Substances that absorb light are called pigments. Pigments vary in the wavelengths of light they absorb, reflecting the rest.

Chlorophyll a and chlorophyll b are the most abundant pigments found in plants, absorbing light in the violet, blue, and red wavelengths, reflecting back the green wavelengths we see. In photosynthesis, chlorophyll captures the energy of light and passes it along a series of enzymes (like a relay race), that ultimately convert that energy into forms that plant cells use to turn carbon dioxide into sugars, cellulose, and other complex organic compounds.

Plants contain another class of pigments called carotenoids, including carotene and xanthophyll. Carotenoids play some role in capturing and transferring light energy to chlorophyll for photosynthesis but mainly function to prevent light and oxygen byproducts from causing cellular damage.

Chlorophyll is far more abundant than the carotenoids, but since it is degraded first in autumn, the underlying orange and yellow colors of the carotenoids are revealed, producing the vibrant fall colors we see in temperate deciduous trees and shrubs (reds are produced by anthocyanins – that’s a tale for another time!).

Capturing light for energy, however, is not the only way plants use light. Special enzymes evolved to use light as a signal for physiological processes that affect several kinds of plant behavior.

Phytochromes are a type of photoreceptor that respond to red and far-red light. Phytochromes change forms depending on how much and what kind of light they are exposed to, oscillating between active and inactive forms, and therefore acting as a biological on-and-off switch. When exposed to red light, phytochrome is turned on to its active form; when exposed to far-red light or lacking red light, phytochrome is turned off into its inactive form.

Seeds of some species use light-activated phytochromes as a signal for germination. Ever wonder why you barely cover lettuce or radish seeds? While larger seeds have sufficient energy stores to allow the seedling to grow in the dark through several inches of soil, small seeds do not have as much stored energy, and therefore only germinate when close to the surface where they harness more energy from photosynthesis.

Sufficient levels of red light are required to activate phytochromes. A lack of red light, or the higher ratio of far-red light often found in shaded areas, will not trigger germination. In fact, the on/off switch behavior of phytochromes was discovered in experiments with tiny lettuce seeds – scientists found they could start and stop the germination process by flashing red and far-red light! This dependence on light for germination is called photodormancy, and is the reason why mulch can be so effective in preventing germination of weed seeds.

In other species, phytochromes regulate a behavior called shade avoidance, which promotes plants to grow taller to avoid light competition. This can be a disadvantage in agricultural settings, where lanky growth and sensitivity to crowding can reduce yields. Modern varieties of corn have been bred to have less-sensitive phytochromes, increasing their tolerance to crowding and increasing yields per acre.

Phytochromes also help some plants determine what time of year it is, and therefore when to trigger flowering. Due to the tilt of the Earth’s axis, the length of the day and night varies through the year, with nights becoming increasingly long in winter the farther you are from the equator.

Phytochromes can detect this change in day length and use it as a signal for fall flowering plants like mums, poinsettias, and Christmas cacti.

The reverse is also true for some spring-flowering species, like lettuce or coneflowers, while tomatoes and other species are considered ‘day-neutral’ plants because flowering is not strongly linked to changes in day length.

Matt Jones is the Horticulture Extension Agent at the NC Cooperative Extension Chatham County Center.

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