Plant hormones that respond to light govern the growth of plants from germination through the growing season. Some plants require full sun; others grow in the shade. Their light needs determine where you should plant them outside and whether or not you can grow them indoors.
Light and Seed Germination
Tiny seeds are normally sown and left uncovered. Even if you sprinkle compost or vermiculite on small seeds that need light, sunlight will travel a short distance through the soil. The exposure to light does not need to be long or continuous for some seeds. Sow medium and large seeds just below the surface. If you buy seeds that germinate only in darkness, sow them deeper, and cover them with black plastic to block all light until they germinate.
Photosynthesis is the process by which plants use energy from sunlight to produce chemical compounds that plants use for energy. The green pigment chlorophyll is associated with the conversion of sunlight into energy. Photosynthesis, typically conducted by plant leaves, releases oxygen into the air. When leaves turn red or yellow in autumn, they are no longer using light for photosynthesis.
Some seeds germinate only in darkness; others require light. Seedlings grown without light tend to be tall and thin, lacking the chlorophyll they need for photosynthesis. Plant hormones respond to changing light conditions as the growing season progresses. While sunlight contains light in all ranges of the spectrum, red light dominates in the early morning and late afternoon, and blue light dominates in the middle of the day. Plants typically need blue light in their growing and flowering stages, and the intense spring and summer sun of lengthening days gives them lots of that. Less intense midday autumn sunlight contains more red spectrum light and less blue spectrum. Plant hormones in deciduous plants respond to the changing quality of light and shorter days by dropping their leaves in preparation for dormancy.
Photoperiodism is the effect that daily hours of light and dark have on flowering. Short-day plants flower only when the daylight hours are below a certain number. Long-day plants flower only when the daylight hours are above a certain number. The number of daylight hours has no effect on day neutral plants. Onions (Allium cepa) give us a practical example. Imagine a line drawn between San Francisco and Washington D.C. The length of summer days increases as you go farther north of that imaginary line; their length decreases as you go farther south. Long-day onions, planted in late winter or early spring, require 14 to 16 hours of daylight and are planted in areas well north of that line. Short-day onion cultivars, planted in the fall, need no more than 12 hours of daylight and are planted in areas well south. Intermediate cultivars require 13 to 15 hours of daylight and are appropriate for the transition zone.
Phototropism occurs when auxin, a plant hormone, causes cells on the dark side of a stem to grow longer. The elongated cells cause some plants, such as sunflowers (Helianthus annuus), to follow the sun from the time it rises until it sets. Some vines, including English ivy (Hedera helix), grow away from the sun, making it easier for them to climb toward dark, solid objects such as trees and walls.
- Mad Sci Network: What Is the Effect of Light on Plant Growth
- Iowa State University: Photomorphogenesis
- Millar Research Group: Photoperiodism
- Clermont College, University of Cincinnati: Photosynthesis
- University of Hamburg: Phototropism
- Thompson-Morgan.com: The Effect of Light on Germination and Seedlings
- Master Gardeners of Santa Clara County: Onions: Description and Growing Tips
- Mad Sci Network: If There Is No Direct Contact With Light, Does Phototropism Still Occur?
- Photos.com/Photos.com/Getty Images