Geotropism: what it is, types and examples in plants

Geotropism, also called gravitropism, is a type of directional growth in plants in response to gravity. This characteristic is manifested even when tilting a plant, where the roots and shoots readjust their direction of growth according to the gravitational force. This process is common in both higher and many lower plants, and even in some fungi.

Its mechanism is based on an uneven distribution of auxin: in the roots, the lower side with more auxin grows less, causing a downward curvature; The opposite happens in the stems, growing more on that side and orienting upwards.

We recommend that you learn more about Tropism: what it is, types and examples, where geotropism is included.

It can be positive or negative:

Positive geotropism

Positive geotropism occurs when a part of the plant grows in the same direction as gravity, that is, downward. The clearest example is the growth of roots, which, by receiving more auxin on their lower side, inhibit their elongation in that area, causing a downward curvature.

Some modified stems also present positive geotropism, such as ginger rhizomes and potato tubers, which grow underground. Furthermore, certain flowers such as peanuts lengthen their peduncle after fertilization and bury it, developing the fruits underground through geocarpy.

Negative geotropism

Negative geotropism refers to the growth of plant organs in the opposite direction to gravity, that is, upward. This type of response is mainly observed in stems and branches, which must overcome the gravitational force to direct their leaves and flowers towards the light. In these cases, auxin accumulates on the lower side of the stem, promoting greater cell elongation in that area and generating an upward curvature.

Some underground organs also present negative geotropism. Pneumorrizas, respiratory roots of mangroves such as Avicennia germinans and Laguncularia racemosagrow upward to facilitate gas exchange in water-saturated soils.

Geotropism works through a complex interaction between gravity, plant hormones and specialized cellular structures. When a plant is placed on its side, the hormone auxin builds up at the bottom of the stem due to gravity. This accumulation stimulates greater cell elongation on that side, causing the stem to bend upward. On the other hand, in the roots, auxin inhibits growth at the bottom, causing them to curve downward.

The main gravity sensor in plants are statoliths, dense amyloplasts that store starch and sediment at the base of specialized cells called statocytes. These statoliths are present in roots, stems and inflorescences, and their sedimentation activates mechanosensitive channels that trigger the redistribution of auxin.

• Root: These grow in the direction of gravity due to the accumulation of auxin in the lower part of the root, which inhibits its growth in that area and allows elongation in the upper part. Here you can learn about the Root Parts and their functions.
• Outbreaks: Geotropism acts in the opposite way. Auxin accumulates at the bottom of the sprout, stimulating cell elongation in that region, causing the sprout to curve upward, away from gravity.
• Fruit: In the case of bananas, when exposed to light, auxin migrates to the shaded side of the fruit, which stimulates the differential growth of its cells and produces its characteristic upward curvature, which also responds to gravity. In this other post you can learn about the Parts of the fruit and their functions.

• Geotropism is essential for plants to adjust their growth in response to gravity, since they cannot move like other organisms.
• Thanks to this mechanism, the roots are oriented downward to absorb water and minerals, which also contributes to the stability of the plant.
• The shoots develop in the opposite direction to gravity to better capture sunlight, facilitating photosynthesis, flowering and fruiting.
• This process is regulated by auxin, a phytohormone that promotes or inhibits cell elongation depending on its concentration and location. Examples such as soybean hypocotyls demonstrate how the asymmetric distribution of auxin activates specific genes (SAURs) that induce shoot curvature. At the cellular level, amyloplasts act as gravity sensors by moving within specialized cells, activating mechanisms that are not yet fully understood.

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