Plant Hormones - Tropisms: Phototropism & Geotropism (GCSE Biology)
Tropisms: Phototropism & Geotropism
Tropisms
Plants require hormones too. They are needed for the optimal growth of the plant. They help to move the plant towards light or towards water in order to aid processes like photosynthesis and to make sure that it has an adequate water supply. This is vital as plants are unlike many other organisms in the fact that they can’t really move in our sense of the world. The only way in which they can adjust their position for the optimum conditions requires the use of these hormones. These movements are known as tropisms.
Phototropism
- Phototropisms involve moving in relation to light. A phototropism is a movement involving light. Photo is a word with a Greek origin meaning light. Tropisms can be positive or negative, so a positive phototropism involves moving towards a light source. This helps the plant take in as much light as possible for photosynthesis. A negative phototropism on the other hand, involves the plant moving away from light.
- Different tropisms occur in different types of the plant. In a plant, a positive phototropism takes place in the stem, causing the plant to grow towards the light. Whereas, the root of the plant grows away from the light. This is a negative phototropism. This allows the roots to get as far into the soil as possible in order to reach as much water as it can.
Geotropism
- Geotropisms involve moving in relation to gravity. Geotropisms are also known as gravitotropisms. The stem of a plant moves away from gravity in order to grow upwards in a negative geotropism, whereas the root of a plant moves towards gravity to move downwards. This is a positive geotropism.
Hormones in Tropisms
Auxins
- Tropisms require auxins. The hormone that changes the direction of the growth of the plant is an auxin. They do this by elongating plant cells through increasing and decreasing cell division, based on where they are active. Auxins are made in the tips of the roots and stems and move through the plant via diffusion.
- Auxins in the stem of a plant increase growth. When light hits a plant, one side of the stem will bear the brunt of the light, whereas another side will be shaded. The auxin will concentrate on the side in the shade. This will cause an elongation of the shaded side. This will bend the plant towards the light.
- In the root, auxins will reduce growth in the roots. The auxin will congregate in the shaded side side of the root, however here will reduce growth, so the plant moves away from the light.
- A similar principle occurs with geotropisms. When the stem is horizontal, the auxin will concentrate in the bottom half of the stem, causing an increase in growth of the bottom part. This will cause the stem to bend upwards. In the root, the auxin will again congregate in the bottom of the root. However, the auxins will reduce growth here, causing the root to bend toward gravity. An example of an auxin is IAA, or Indole Acetic Acid.
Plant hormones, also known as phytohormones, are chemical substances produced by plants that control and coordinate their growth, development, and response to environmental stimuli.
Phototropism is a plant’s growth response to light. In particular, it is the tendency of plants to grow towards a source of light, which is mediated by the hormone auxin.
Geotropism, also known as gravitropism, is a plant’s growth response to gravity. It is the tendency of plants to grow either towards or away from the Earth’s gravitational pull, which is mediated by the hormone auxin.
Auxin is a plant hormone that plays a crucial role in regulating phototropism and geotropism. It is produced in the plant’s apical meristem and transported downwards, where it accumulates on the shaded side of the plant, causing it to bend towards the light source.
When a plant is exposed to light, auxin is produced in the plant’s apical meristem and transported downwards to the lower parts of the stem. The auxin accumulates on the shaded side of the stem, causing the cells on that side to elongate and the stem to bend towards the light source.
When a plant is placed on its side, the auxin in the plant’s stem accumulates on the lower side due to gravity. This causes the cells on the lower side to elongate more than the cells on the upper side, resulting in the stem bending upwards, towards the source of gravity.
Phototropism and geotropism help plants to optimize their growth and survival. Phototropism allows plants to orient their leaves towards the light source, which maximizes photosynthesis and promotes efficient use of resources. Geotropism helps plants to anchor themselves in the soil, resist wind and other external forces, and orient their roots downwards to absorb water and nutrients from the soil.
To observe phototropism, we can place a potted plant near a window and monitor how the stem and leaves bend towards the light source. To observe geotropism, we can place a potted plant on its side and monitor how the stem bends upwards towards the source of gravity, while the roots grow downwards.
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