Tropism

Sensitivity refers to the capacity of living organisms to react to stimuli. While plants don’t exhibit movements akin to animals, certain parts of them do react to stimuli. These responses are categorized as tropic responses or tropisms.

Tropisms represent growth movements influenced by directional stimuli. For instance, a shoot might grow towards a source of light while moving away from the pull of gravity. Typically, these growth movements occur in response to the orientation of light or gravity. Responses to light are termed phototropisms, while responses to gravity are referred to as gravitropisms or geotropisms

Phototropism and gravitropism are examples of how plants chemically control their growth in response to environmental stimuli.

Phototropism

Phototropism is the directional growth response of plants in response to light. When a shoot is exposed to light, the hormone auxin plays a central role in mediating phototropic responses. Light stimulates the synthesis of auxin in the shoot tip, leading to an accumulation of auxin on the side of the shoot away from the light source. This uneven distribution of auxin causes cells on the shaded side to elongate more rapidly than cells on the illuminated side. As a result, the shoot bends towards the light source.

The movement of auxin is regulated by a protein called PIN3, which actively transports auxin from cell to cell. In response to light, PIN3 redistributes auxin to the shaded side of the shoot, promoting cell elongation in that region and causing the shoot to curve towards the light.

Gravitropism

Gravitropism, also known as geotropism, is the growth response of plants in relation to gravity. In shoots, gravity stimulates the redistribution of auxin, leading to differential growth rates and curvature of the shoot. When a shoot is placed horizontally, gravity causes auxin to accumulate on the lower side of the shoot. This accumulation inhibits cell elongation on the lower side while promoting elongation on the upper side, causing the shoot to grow upwards (negative gravitropism) or downwards (positive gravitropism), depending on the orientation.

The movement of auxin in response to gravity is regulated by specialized cells called statocytes, which contain starch-filled organelles called statoliths. When the orientation of the plant changes, statoliths settle on one side of the cell, triggering changes in auxin transport and distribution.

In both phototropism and gravitropism, the chemical control of plant growth involves the regulation of auxin distribution and transport. These processes allow plants to adapt their growth patterns in response to environmental cues, optimizing their ability to capture light for photosynthesis and anchor themselves in the soil.

Advantages of Tropic Responses

• By exhibiting positive phototropism, shoots grow towards sources of light, positioning their leaves optimally for photosynthesis. This maximizes the plant’s ability to capture sunlight and produce energy through photosynthesis.
• Flowers are brought into an exposed position, increasing their visibility to pollinators such as flying insects. This enhances the likelihood of successful pollination, leading to reproduction and genetic diversity within plant populations.

Negative Gravitropism in Shoots

• Negative gravitropism in shoots results in vertical growth, lifting leaves and flowers above the ground. This vertical orientation allows plants to compete effectively for light and carbon dioxide, essential resources for photosynthesis.
• Flowers positioned higher above the ground are more accessible to insect or wind pollinators, increasing the likelihood of successful pollination and seed production. Additionally, effective seed dispersal may occur from fruits positioned on a long, vertical stem.

Positive Gravitropism in Roots

• Positive gravitropism in roots directs their growth towards gravity, facilitating penetration into the soil, which serves as their anchorage and source of water and mineral salts.
• By growing downwards, roots can explore a larger volume of soil, enhancing their ability to absorb water and nutrients. This increases the plant’s overall efficiency in resource acquisition and contributes to its growth and development.
• Lateral roots, while not positively gravitropic, grow at right angles or slightly downwards from the main root. This branching pattern allows roots to extensively explore the surrounding soil and anchor the plant securely in place.