Geotropic pertains to the word geotropism, which is the movement of a plant in response to gravity (either downwards or upwards).
If you were to put a plant on its side, you would observe that, after a while, the roots would start growing downward while the stem would grow upward.
This response to gravity is called geotropism, or gravitropism. The roots exhibit positive geotropic growth, or growth with gravity, while the stem exhibits negative geotropic growth or growth against gravity.
Gravitropism (also known as geotropism) is a coordinated technique of differential growth by a plant in response to gravity pulling on it.
It also takes place in fungi. Gravity can be either “artificial gravity” or natural gravity. It is a general characteristic of all higher and many lower plants as well as other organisms.
Charles Darwin was one of the first to scientifically record that roots show positive geotropic (positive geotropism) and stems show negative geotropic (negative geotropism).
Which means that roots grow in the direction of gravitational pull (i.e., downward) and stems grow in the opposite direction (i.e., upwards). This behaviour can be easily illustrated with any potted plant.
When laid on its side, the growing parts of the stem begin to demonstrate negative geotropic (i.e., negative gravitropism), growing upwards.
However, herbaceous (non-woody) stems have the capability to bend to a degree, but most of the redirected movement takes place as a consequence of root or stem growth outside.
The mechanism is based on the Cholodny-Went model, which was presented in 1927 and has since been revised. Although the model has been criticized and still goes on to be refined, it has greatly stood the test of time.
Moreover, several parts of a plant grow in opposite directions because they have various functions. The roots of the plant need to grow into the ground so as to provide stability for the plant as well as access water and nutrients.
Also, the stem needs to grow upward toward the sunlight so that the plant can photosynthesize, or produce food from sunlight. An example of gravitropism is a tree from central Minnesota.
This tree has fallen over and due to gravitropism, it exhibits this arched growth. Gravitropism maintains the vertical direction of these trees.
These trees, typical of those in abrupt subalpine environments, are covered by in-depth snow in winter. As small saplings, they are stressed-out by the snow and bent almost flat to the ground.
During spring growth, and more so as bigger trees, gravitropism allows them to familiarize vertically over years of subsequent growth.
Negative Geotropic In Roots
In the process of plant roots growing in the direction of gravity by gravitropism, increased concentrations of auxin will move towards the cells on the bottom side of the root.
With this, it will suppress growth on this side while allowing cell elongation on the top of the root. As a result of this, curved growth will occur and the root will be directed downward.
However, root growth originates from the division of stem cells in the root meristem discovered at the tip of the root and the successive asymmetric proliferation of cells in a shoot-ward region to the tip known as the stretching zone.
Differential development during tropisms primarily involves changes in cell expansion versus changes in cell division, although a function for cell division in tropic growth has not been formally ruled out.
Gravity is felt at the root tip and this information must then be relayed to the stretching zone so as to conserve growth direction, mount adequate growth responses to changes in orientation, and persist in growing its roots in the exact same direction as gravity.
There is a large piece of evidence that demonstrates that roots bend in response to gravity due to a regulated movement of the plant hormone auxin, which is known as polar auxin transport.
This was clarified in the 1920s with the Cholodny-Went model. The model was independently presented by the Ukrainian scientist N. Cholodny of the University of Kyiv in 1927 and by Frits Went of the California Institute of Technology in 1928, both based on the work they had done in 1926.
Auxin exists in almost every organ and tissue of a plant, but it has been reoriented in the gravity domain and can originate differential growth resulting in root curvature.
Experiments show that auxin diffusion is characterized by a fast movement of auxin to the downward side of the root in reaction to a gravity stimulus at a 90-degree angle or more.
However, once the root tip arrives at a 40° angle to the horizontal of the stimulus, auxin distribution quickly moves to a more balanced arrangement.
This behaviour is defined as a “tipping point” mechanism for auxin conveyance in response to a gravitational stimulus.
Negative Geotropic In Shoots
Gravitropism is an essential part of plant growth, orienting its function to maximize contact with sunlight as well as verifying that the roots are growing in the correct direction.
Growth due to gravitropism is mediated by changes in concentration of the plant hormone auxin within plant cells.
As plant shoots mature, high concentrations of auxin shift towards the bottom of the shoot to instigate cell growth in those cells while subduing cell growth on the top of the shoot.
This faster development of the bottom cells influences the upward curved growth and elongation, abusing the shootist cells, away from the direction of gravitational pull.
As plants mature, gravitropism begins again to direct growth and development, along with phototropism.
While amyloplasts continue to guide plants in the correct direction, plant organs and functions depend on phototropic responses to confirm that the leaves are receiving sufficient light to achieve basic functions such as photosynthesis.
In total darkness, mature plants have little to no sense of gravity, unlike seedlings that can still train themselves to have the shoots grow upward until light is attained when development can begin.
Differential sensitivity to auxin assists in explaining Darwin’s original observation that stems and roots react in the opposite way to the forces of gravity.
In both roots and stems, auxin builds up towards the gravity vector on the downward side. In roots, this results in the inhibition of cell proliferation on the lower side and the concomitant curvature of the roots that is towards gravity (positive geotropic).
In stems, the auxin also piles up on the downward side; however, in this tissue, it boosts cell expansion, which results in the shoot curving up (negative geotropic).
A recent study showed that for gravitropism to take place in shoots, a lot of propensity, instead of an inadequate gravitational force, is necessary.
This discovery sets aside gravity-sensing mechanisms that would depend on detecting the pressure of the weight of statoliths.
Negative Geotropism In Fruit
A few species of fruit exhibit negative geotropic (negative geotropism). Bananas are one well-known example.
Once the canopy that wraps the fruit dries, the bananas will begin to curve upwards towards sunlight; this is what is known as phototropism.
The specific chemical that stimulates the upward curvature is a phytohormone in the banana, which is called Auxin.
When the banana is first exposed to sunlight after the leaf canopy dries, one face of the fruit is shaded. On exposure to sunlight, auxin in the banana migrates from the sunny side to the shaded side.
Since auxin is a powerful plant growth hormone, the increased concentration promotes cell division and causes the plant cells on the shaded side to grow. This asymmetrical distribution of auxin is responsible for the upward curvature of the banana.
Gravity-sensing Mechanisms
Banana Fruit Exhibiting Negative Geotropic (Negative Geotropism).
Plants possess the ability to sense gravity in several ways, one of which is through statoliths. Statoliths are dense amyloplasts, organelles that synthesize and store starch involved in the perception of gravity by the plant (gravitropism), that collect in specialized cells called statocytes.
Statocytes are located in the starch parenchyma cells near vascular tissues in the shoots and in the columella in the caps of the roots.
These specialized amyloplasts are denser than the cytoplasm and can sediment according to the gravity vector.
The statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensitive channels.
The gravitropic signal then leads to the reorientation of auxin efflux carriers and the subsequent redistribution of auxin streams in the root cap and root as a whole.
Auxin moves toward higher concentrations on the bottom side of the root and suppresses elongation.
The asymmetric distribution of auxin leads to differential growth of the root tissues, causing the root to curve and follow the gravity stimuli.
Statoliths are also found in the endodermic layer of the hypocotyl, stem, and inflorescence stock.
The redistribution of auxin causes increased growth on the lower side of the shoot so that it orients in a direction opposite that of the gravity stimuli.
Final Words.
Negative geotropic (negative geotropism) is a fundamental aspect of how organisms interact with their environment and respond to the force of gravity.
From plants and fungi to animals, organisms exhibit unique growth patterns and behaviours that help them navigate their surroundings and access essential resources for survival.
By studying the mechanisms behind negative geotropics (negative geotropism), researchers can gain valuable insights into the complexities of life on Earth and develop innovative solutions for addressing environmental challenges and improving human health.
