March 28, 2024

This is the response of plant parts to the stimulus of gravity. Plant shoots grow away from gravity of the earth and are said to be negatively geotropic. Roots grow towards gravity and are said to be positively geotropic.

 

Experiments to demonstrate geotropism

1) Using germinating seedling (African yam bean seeds)

Method:

Some African yam bean seeds are sown between moist filter papers in a beaker. As soon as the radicles and plumules emerge, some of the young seedlings are carefully removed and placed on a flat board in the dark, with the radicle and plumule lying horizontally. (The purpose of keeping the experiment in the dark is to remove all responses due to light). Some of the seedlings are left in a vertical position to serve as control.

 

Result

After 24 hours the radicle, now the root, deviates from its horizontal position and grows vertically downwards, while the shoot grows vertically upwards away from the force of gravity.

The roots and shoots of those seedlings kept in a vertical position remain straight.

Conclusion: Shoot is negatively geotropic while root is positively geotropic.

 

2) Using a potted plant and klinostat

Seeds of cowpea are sown in small pots (one seed per pot).

After the appearance of the first foliage leaves, the pots are placed in the dark with the seedlings in a horizontal position. A klinostat with a seedling is also kept working in a horizontal position to serve as a control. The whole set-up is left for about 24 – 36 hours.

 

Result

After 24 hours the shoot of the seedling in the pot curves and grows vertically upwards (away from gravity). If the soil is carefully washed off and the whole seedling is examined, the root is also seen to have curved and grown downwards (towards gravity).

The shoot of the working klinostat remains in a horizontal position and does not curve. It will however curve if the klinostat is left stationary.

 

Explanation of result

Auxin produced at the terminal bud of the shoot under the influence of gravity is distributed to the lower half of the plant body lying horizontally. The auxin concentration which promotes the growth of the shoot is too much for the root and will inhibit its growth. The auxin distributed into the lower half of the plant increases the growth rate of the shoot in that lower half while inhibiting the growth of the root in the lower half where it is concentrated. Due to these unequal growths, the shoot bends away from gravity while the root whose growth is more on the upper half bends towards gravity. The effect of gravity on the root is opposite to its effect on the shoot.

The shoot in the working klinostat does not bend as the rotation of the klinostat equally rotates the plant body which receives the auxin equally at any one time. Growth is therefore uniform and gravity has no effect on it.

The advantage of this kind of response is that no matter the position of a seed in the soil, during germination the radicle is always positively geotropic while the plumule is always negatively geotropic. The breathing roots of mangrove trees are however negatively geotropic. They grow above water to obtain atmospheric oxygen for respiration.

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