If you've ever noticed your houseplants pressing against the window, admired the vibrant colors of autumn or seen seedlings grow spidery and fall over, then you have observed plants responding to light. Because plants are designed to take energy from the sun and convert it into sugars that the plants use for energy, from their very beginnings they grow in such a way as to maximize their exposure to light and, therefore, create more energy.
Germination and Growth
For some species, light governs the behavior even of the embryo inside of the seed. Harvard University researcher Lewis Lipp found that some species of tree seeds required light in order for the germination process to begin. Other plants germinate only when light stimulates a certain part of the seed, which assures that the seed is correctly situated in the soil for the plant to have the best chance of survival.
Once germinated, the seedlings' behavior continues to be influenced by light. If you've ever started seeds indoors without adequate light, you might have noticed that the seedlings become stretched, often to the point that they cannot support themselves and topple over. Botanists call this "etoliation," a process by which the light-starved plant elongates itself in search of the light it needs to continue to grow.
Flower production is controlled by light. Darkness that follows a period of light stimulates the production of a hormonelike substance called florigen that generates flowers. Plants differ in the amount of light and darkness required for flowering. Some plants, called short-day plants, require longer periods of darkness than long-day plants, accounting for the seasonal differences in flowering observed between species.
Phototropism refers to the tendency of plants to grow toward a light source. Houseplants will press against window glass, while plants grown at the edge of a shady patch reach toward the sunny section of the garden. As with flowering, it is actually darkness that stimulates the light-seeking behavior. The side of the plant stem that is mostly in shadow experiences a boost in the production of auxin, a hormone that increases growth in that part of the plant, causing the plant to angle itself toward the light. Phototropism helps plants reach toward the light source they need to generate energy.
Pigment and Coloration
As the days shorten, the leaves of deciduous trees and some plants receive less exposure to light, signaling the coming of winter and cold conditions that threaten the tender tissues of the leaves. To survive the winter, trees must lose their leaves, and the changing duration of light each day signals the tree to begin biochemical processes in the leaves that result in the spectacular colors of autumn. The leaf veins pinch shut, and the leaves fall from the trees before they can be damaged by harsh winter weather. In other species, exposing young leaves to light stimulates the production of anthocyanin, a chemical that turns the leaves red and offers some protection to the tender young tissue from the sun's harsh rays.
In the mid-18th century, German scientist J.G. Zinn observed that bean leaves rose and fell throughout the day, even when the plants were not exposed to light. It is common knowledge that humans and animals experience natural rhythms to their behavior, but the discovery of similar tendencies in plants was surprising. Circadian rhythms in plants are set by the plants' genetics but controlled by external stimulus. So the Circadian rhythms allow the plant to respond most effectively to the light stimulus it is receiving, in much the same way that most people's sleep-wake rhythms tend to organize themselves around the start of the day.