Coordination and response in plants

Unlike animals, plants can't move to find better conditions. Instead, they respond to their environment by growing. A 'tropism' is a directional growth response of a plant towards or away from a stimulus. The main stimuli plants respond to are light and gravity. A growth response towards a stimulus is called a positive tropism, while growth away from it is a negative tropism. For example, plant shoots grow towards light (positive phototropism) to maximise photosynthesis, and away from the pull of gravity (negative gravitropism) to grow upwards. Conversely, roots grow away from light (negative phototropism) and towards gravity (positive gravitropism) to anchor the plant and find water and minerals.

Plant tropisms are controlled by a group of hormones, the most important of which is auxin. Auxin is a chemical substance produced mainly in the growing tips of shoots and roots. From there, it diffuses to other parts of the plant. The primary role of auxin is to control cell elongation – the process where cells get longer, causing the plant to grow. A higher concentration of auxin generally leads to more cell elongation in shoots. This chemical control allows the plant to precisely adjust its growth pattern in response to environmental cues like light and gravity.

Phototropism is the growth of a plant shoot towards a light source. This response is controlled by auxin. Here is the step-by-step mechanism:

1. Light shines on the shoot from one side.
2. Auxin is produced at the very tip of the shoot.
3. Light causes the auxin to move from the lighter side to the more shaded side of the shoot tip.
4. This results in a higher concentration of auxin accumulating on the shaded side as it diffuses down the shoot.
5. The higher auxin concentration on the shaded side stimulates the cells there to elongate more rapidly than the cells on the lighter side.
6. This difference in elongation rate causes the shoot to bend and grow towards the light source. This ensures the leaves are in the best position for photosynthesis.

Gravitropism (or geotropism) is a plant's growth response to gravity. This is also controlled by auxin, but the effect is different in shoots and roots.

In Shoots (Negative Gravitropism): If a plant is placed horizontally, gravity causes auxin to accumulate on the lower side of the shoot. Just like in phototropism, this higher concentration of auxin stimulates cell elongation. The cells on the lower side grow faster than the cells on the upper side, causing the shoot to bend upwards, away from the pull of gravity.

In Roots (Positive Gravitropism): In roots, the opposite happens. If a root is growing horizontally, gravity also causes auxin to accumulate on the lower side. However, in roots, a high concentration of auxin *inhibits* cell elongation. Therefore, the cells on the upper side (with a lower auxin concentration) elongate more rapidly than the inhibited cells on the lower side. This causes the root to bend downwards, towards the pull of gravity.

Our understanding of plant hormones, particularly auxins, has led to important agricultural and horticultural applications. Synthetic versions of these hormones can be manufactured and used to control plant growth in specific ways.

Selective Weedkillers: Many common lawn weeds are broad-leaved plants (dicots), while grass is a narrow-leaved plant (monocot). Broad-leaved plants are much more sensitive to high concentrations of auxin than narrow-leaved plants. Synthetic auxins, such as 2,4-D, are sprayed onto lawns. They are absorbed by the broad-leaved weeds, causing them to grow uncontrollably and rapidly. This disrupts their transport systems and ultimately kills them, while the grass is largely unaffected.

Rooting Powders: When taking a cutting from a plant to grow a new one, it can be difficult to get roots to form. Rooting powders contain synthetic auxins. Dipping the base of a cutting into the powder encourages the rapid development of new roots, increasing the success rate of propagation.