Plant Tropisms and Hormonal Responses: Phototropism, Geotropism, Hydrotropism, and Thigmotropism

How do plants increase their chances of survival?

through adaptations, such as responding to environmental cues like light and gravity, and developing specialized structures to cope with stress like drought or flooding.

Plant adaptations when they loose their leaves

Changes in day length, causes changes in light and temperature.

During autumn, the glucose needed to release the energy needed to protect the plant from freezing, is greater than the glucose produced by photosynthesis.

Trees lose leaves and remain dormant until spring

A protective layer is produced to prevent the entry of pathogens and water loss since the xylem and phloem have been sealed.

Auxin

A plant growth hormone that promotes cell elongation in shoots, leading to phototropism and geotropism. Produced by

Phototropism

A growth response to light.

Geotropism

A response to gravity.

Hydrotropism

The growth response to water, where roots grow towards areas of higher moisture.

Thigmotropism

A response to touch/physical contact.

Tropisms

Directional growth responses of plants to environmental factors.

Abscisic acid (ABA)

Inhibits growth and promotes stomatal closure in response to drought stress.

Gibberellins

Stimulates stem elongation and germination.

Ethylene

Involved in fruit ripening and abscission.

Cytokinins

Promotes cell division.

Statoliths

Dense, starch-filled organelles that signal the direction of gravity.

Moisture gradient

The stimulus for hydrotropism in plants.

Positive geotropism

Roots show positive growth, meaning they grow downwards.

Steps of geotropism in roots

1️⃣ Statoliths in root cap sense gravity; 2️⃣ Auxin redistributes to lower side; 3️⃣ Differential growth causes bending; 4️⃣ Roots grow downwards.

Steps in phototropism caused by auxin redistribution

1️⃣ Light source is detected; 2️⃣ Auxin accumulates on the shaded side; 3️⃣ Cells on the shaded side elongate more; 4️⃣ Stem bends towards the light.

Directional growth response

The response of plants to environmental stimuli called tropisms.

In thigmotropism

Tendrils and climbing stems wrap around supports.

Auxin accumulation

Auxin accumulates on the shaded side of a plant stem in response to light, causing it to bend towards the light source.

Primary hormone for phototropism

Auxin regulates cell elongation and growth in phototropism.

Hormones in geotropism

In geotropism, auxin and gibberellins redistribute to cause differential growth.

Plant tropisms

Regulated by hormones like auxin and gibberellins.

Abscisic Acid

Inhibits growth.

Water Source Proximity

Triggers auxin redistribution in hydrotropism.

Auxin Distribution

Promotes cell elongation in hydrotropism.