انتحاء أرضي

Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feature of all higher and many lower plants as well as other organisms. 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 any 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 degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside. The mechanism is based on the Cholodny–Went model which was proposed in 1927, and has since been modified. Although the model has been criticized and continues to be refined, it has largely stood the test of time.

Example of gravitropism in a tree from central Minnesota. This tree has fallen over and due to gravitropism exhibits this arched growth.

A straight and vertical tree. The shape has been regulated by the gravitropic curvature at the base.

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 في الجذور

In the process of plant roots growing in the direction of gravity by gravitropism, high concentrations of auxin move towards the cells on the bottom side of the root. This suppresses growth on this side, while allowing cell elongation on the top of the root. As a consequence of this, curved growth occurs and the root is directed downwards. Source:

 In shoots

Apex reorientation in Pinus pinaster during the first 24h after experimental inclination of the plant.

In the process of plant shoots growing opposite the direction of gravity by gravitropism, high concentration of auxin moves towards the bottom side of the shoot to initiate cell growth of the bottom cells, while suppressing cell growth on the top of the shoot. This allows the bottom cells of the shoot to continue a curved growth and elongate its cells upward, away from the pull of gravity as the auxin move towards the bottom of the shoot.

 Gravity-sensing mechanisms

 Compensation

The compensation reaction of the bending Coprinus stem. C – the compensating part of the stem.

 انظر أيضاً

• Amyloplast – starch organelle involved in sensing gravitropism
• Astrobotany – the field of science concerned with plants in a spaceflight environment
• Clinostat – a device used to the effects of gravitational pull
• Random positioning machine – a device used to negate the effects of gravitational pull
• Free fall machine – a device used to negate the effects of gravitational pull
• Large diameter centrifuge – a device used to create a hyper-gravity pull
• Prolonged sine – reaction of plants to turning from their usual vertical orientation