How is geotropism/gravitropism related to roots and shoots?
Roots have a tendency to grow downwards, while shoots grow upwards. Gravitropism is a turning or growth movement by a plant or fungus in response to gravity. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull (i.e., downward) and stems grow in the opposite direction (i.e., upwards). This behavior can be easily demonstrated with a potted plant. When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing (biologists say, turning; see tropism) upwards. Herbaceous (non-woody) stems are capable of a small degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth in a new direction. Plants can sense the Earth's gravitational field. Geotropism is the term applied to the consequent orientation response of growing plant parts. Roots are positively geotropic, that is, they will bend and grow downwards, towards the center of the Earth. In contrast, shoots are negatively geotropic, that is, they will bend and grow upwards, or away, from the surface.
Geotropism, also known as gravitropism, is the response of plants to gravity. It dictates the direction in which the roots and shoots grow in relation to gravity. Roots exhibit positive geotropism, meaning they grow in the direction of the gravitational pull, while shoots display negative geotropism, growing against gravity.
To understand the relationship between geotropism and roots and shoots, we need to examine the underlying mechanisms involved:
1. Statoliths: Found in specialized cells called statocytes, statoliths are dense, starch-filled organelles that perceive gravity. These statocytes are typically located in the root cap of plants, which helps the roots sense the direction of gravity.
2. Redistribution of auxins: Auxins are plant hormones that play a crucial role in growth and development. They can move from one part of the plant to another, which aids in coordination and direction of growth. In response to gravity, auxins in the root cap get redistributed towards the lower side, while in shoots, they move towards the upper side.
3. Cell elongation: Once the auxins are redistributed, they initiate the process of cell elongation. In roots, increased auxin concentration on the lower side inhibits cell elongation, causing that side to grow slower. This results in the root bending towards gravity. Conversely, in shoots, auxin concentration on the upper side promotes cell elongation, causing the shoot to grow against gravity.
So, geotropism links roots and shoots by guiding their growth in the appropriate direction with respect to gravity. This enables roots to grow deeper into the soil and shoots to grow upwards, optimizing their functions of nutrient uptake and photosynthesis, respectively.