3. Light: Energy, Information, and Timing

Visible light from the sun is the ultimate source of energy for most life on Earth, and it is crucial for a number of key plant processes, including: chlorophyll synthesis, photosynthesis, phototropism (movement toward light), photoperiodism (timing of seasonal activities), and transpiration.

Light Intensity: Sun Lovers and Shade Dwellers

Different plants are adapted to thrive in different light intensities.

• “Sun plants,” like loblolly pine, are often found in open areas because their rate of photosynthesis continues to increase with light intensity up to full sunlight.
• “Shade plants,” like red oak, often grow on the forest floor. They are adapted to reach their maximum rate of photosynthesis at one-third or less of full sunlight and may even show slight decreases at higher intensities.

Research by Björkman and Holmgren (1963) on golden-rod (Solidago virgaurea) confirmed that these differences are genetic. They found that shade-adapted plants were more efficient at photosynthesis in weak light, while sun-adapted plants were more efficient in strong light, a clear adaptation to their native habitats.

Light Quality: A Spectrum of Signals

Plants don’t just respond to how much light they receive, but also to its color, or wavelength. Different colors of light act as signals that trigger specific biological responses.

• Photosynthesis: This process primarily uses red light (around 655-672 nm) and blue-violet light (around 440 nm), which are the wavelengths most strongly absorbed by chlorophyll.
• Phototropism (movement toward light): The bending of a plant stem toward a light source is primarily a response to blue-violet light (400–490 nm).
• Flowering Control: For many plants whose flowering is controlled by day length, red light in the 620-640 nm range is the most effective for interrupting the dark period and altering the flowering response.

Day Length: The Plant’s Calendar (Photoperiodism)

Photoperiodism is the response of an organism to the relative lengths of the light and dark periods. In the 1920s, Garner and Allard discovered that this mechanism controls flowering in many species. They classified plants into three main categories:

• Short-day plants: Flower only when the day length is below a certain critical maximum.
• Long-day plants: Flower only when the day length is above a certain critical minimum.
• Day-neutral plants: Flowering is not determined by day length.

This timing mechanism involves a special pigment called phytochrome, which acts like a biological light switch. Daylight flips the switch to “on” (Pfr), and darkness allows it to slowly flip back to “off” (Pr). The plant measures the length of the night by how much of the pigment has flipped back to the “off” state, using this information to decide when it’s the right season to flower.

This ability to measure day length is a critical adaptation for plants in different latitudes. For example, strains of sideoats grama (Bouteloua curtipendula) from southern Texas are short-day plants, ensuring they flower before the hottest part of summer. In contrast, strains from North Dakota are long-day plants, which allows them to take advantage of the very long summer days at northern latitudes to complete their life cycle.