Plants respond to gravity. Plant Responses to Non-Light Stimuli Plants respond to gravity. Without anchoring downward and growing a stem upward, a seed will soon run out of stored energy and perish. Plants possess a mechanism to sense gravity in a response called gravitropism. Within plant cells, starch particles called statoliths are contained within the cytoplasm; there, the free-moving, heavy-starch granules sink within the cytoplasm under the force of gravity, allowing the plant to move in a downward direction. Principles of Biology

Plant Responses to Non-Light Stimuli Figure 1 Mutation for gravitropism. The plant in photo B cannot sense gravity as the plant in photo A does. Plant B has a mutation for the gene coding the plant hormone auxin, interfering with directional growth. Principles of Biology

Plants respond to touch. Plant Responses to Non-Light Stimuli Plants respond to touch. Plants respond to mechanical pressure such as touch by changing the way they grow, a response known as thigmomorphogensis. When a plant responds to touch by growing in a certain way or direction, it is called thigmotropism. The competitive advantage of responding to touch is seen with the tendrils of a morning glory or pea plant. These plants grow, extending tendrils that seek out an object to touch, and then wrap around it so the plant can continue to grow vertically because it lacks the structures to do so independently. Principles of Biology

Plant Responses to Non-Light Stimuli Figure 2 Thigmotrophic behavior of kudzu vines. Notice how the kudzu vines cover the trees and bushes in the foreground of the photo. The vines have also started to climb upward into the taller trees. Principles of Biology

Plants respond to chemicals and temperature. Plant Responses to Non-Light Stimuli Plants respond to chemicals and temperature. Chemotropism is a directional plant movement in response to chemical cues. Chemotropism is seen in pollen tubes that grow toward the ovary in response to chemicals the ovaries release. Non-directional responses by plants are called nastic movements. Tulips and crocus flowers will open when stimulated by increased temperatures in a response called thermonasty. Plants may exhibit chemonasty, non-directional movement responses to a chemical stimulus. Nastic movements tend to be rapid and easily observed, whereas tropisms are often slower responses that take place over time. Principles of Biology

Plants respond to stress. Plant Responses to Non-Light Stimuli Plants respond to stress. For plants, stress comes in the form of changes to their environment such as a drowning flood, a fire, or a drought, threatening the plant's survival. During drought, the stomata close, decreasing transpiration as water levels in the leaf reduce turgor pressure. Lower water levels also trigger cells to produce abscisic acid, a plant hormone that acts on guard cells to prevent reopening of stomata. When some plants sense a lack of oxygen, they produce ethylene, which causes cell death in the cortex of root tissues. The spaces left by the sacrificed tissue allow oxygen back in. Principles of Biology

Plant Responses to Non-Light Stimuli Figure 3 Mangrove plant growing in water. By extending the root system into the air, the mangrove has adapted to life in water year round. Principles of Biology

Plants respond to stress. Plant Responses to Non-Light Stimuli Plants respond to stress. Stress to plants can occur in high-salt conditions. When there are high levels of salt in the soil, water moves out of the roots into the soil, drying the roots. Some plants have adapted specifically to grow in high-salt environments. These plants, called halophytes, have evolved salt glands that have sodium pumps to actively remove the salt out of the plant on a continuous basis. Principles of Biology

Exceptions to the rule. Plant Responses to Non-Light Stimuli Excessive heat poses a threat to plant survival. Cells produce heat-shock proteins that function to protect other proteins from falling apart or denaturing from the heat. Plants rapidly produce more heat-shock proteins during heat stress conditions, enabling survival. Intense cold poses a very serious risk to plants because the entire cell function can potentially cease if the cell membrane fluid mosaic loses its mobility. One survival mechanism to counteract the threat of intense cold involves increasing the concentration of solutes in the cytoplasm, which can prevent the formation of ice crystals within cells. One survival mechanism to counteract the threat of intense cold involves the synthesis of antifreeze proteins, proteins that interact with ice, preventing growth and complete crystal formation. Many plants have evolved the ability to become dormant during long periods of intense cold. Principles of Biology