Do Plants Stop Growing in Complete Darkness?

Normal Plant in Darkness

• If a healthy green potted plant such as a tomato plant is put into a container where light is excluded, the plant begins to respond with several automatic responses. One is thinner, longer growth called etiolation. The plant is displaying negative geotropism, or growing away from the earth's gravity and, if any light is available at all, positive phototropism, growing toward any possible source of sunlight. The green color begins to pale because sunlight isn't available to fuel photosynthesis, and the green color fades from green to pale green to yellow to white. If a light source is not provided, the plant uses up all the available food it contains and dies.

Plants in Darkness Receiving Sugar

• Photosynthesis combines carbon dioxide and water in the presence of sunlight to produce sugar. Early experiments cited by L. Knudson and E. W. Lindstrom showed that normal green plants such as corn could be grown in the dark for some time if they are given water containing sugars. However, the plants eventually died. Knudson and Lindstrom grew albino barley that had no chlorophyll in the dark under various growing conditions and growing media. Plants given solutions of sucrose and glucose showed an increase of growth and in water culture lived a month longer than plants not receiving sugar. Sustained growth did not occur. More recently in a study entitled "Sucrose availability on the aerial part of the plant promotes morphogenesis and flowering of Arabidopsis in the dark," it was shown how Arabidopsis -- a plant in the mustard family -- could grow and flower in complete darkness when raised on vertical Petri dishes with agar medium containing sucrose.

Parasitic Plants

• Some plants don't need to photosynthesize at all and do not have their own chlorophyll. They live as parasites by harvesting the food made by other plants that do photosynthesize. Some examples are Indian pipes (Monotropa species), which connect to beneficial fungi that help tree roots get nutrients, and broomrapes (Orobanche species), which connect to roots of shrubs. As long as their connections to the host plant are in place, they grow in complete darkness underground, breaking through to sunlight to flower and produce seeds.

Bulbs and Seeds

• Both bulbs and seeds contain stored food. When proper conditions of temperature and moisture have occurred, bulbs and seeds are triggered to start growing. If the bulbs or seeds are then grown in complete darkness, they can produce foliage relying on the stored food and energy inside them. The foliage does not have any color, but in bulbs, flowers can be produced that often do have their own colors. Betty Thomson and Pauline Miller studied pea seeds that were grown in soil in a greenhouse or complete darkness after germination. They found that in the 16 days after germination, there was little difference in plant growth between the two situations as far length of leaves and internodes, and total number of leaves and leaf primordia.

Algae

• One-celled aquatic plants called algae have been modified by gene insertion to grow in complete darkness. The gene, introduced from human red blood cells, allows the algae to use carbon from sugars rather than carbon from photosynthesis to grow. This means they can grow in complete darkness if provided with sugar in their growing medium. The algae are commercially cultured to produce dietary supplements, pigments used in scientific labeling and feed for aquaculture animals such as fish and shrimp. The ability to grow in darkness allows the algae to be cultured in the dark in enclosed fermenters rather than in open ponds exposed to sunlight for greater control over product production and purity.