Chapter 28: External Growth Factors in Plants

Overview of External Growth Factors

External growth factors refer to the environmental stimuli that influence plant growth and development. These factors include light, gravity, water, and touch, which can trigger specific growth responses in plants.

Phototropism

Phototropism is the growth response of plants to light direction, enabling them to maximize light absorption for photosynthesis.

Went's Experiment

Went’s experiment involved oats kept in the dark. This seminal experiment demonstrated how auxin (specifically Indole-3-acetic acid or IAA) is distributed unequally in light and dark conditions, leading to differential growth responses (curvature) towards light (positive phototropism).

Brigg’s Experiment

Brigg's experiment showed the lateral movement of IAA in plants subjected to light and dark conditions, underscoring the role of auxin in phototropic responses:
- Light Condition: IAA accumulates on the side of the plant away from the light, promoting growth on that side.
- Dark Condition: The distribution of IAA is uniform or minimal.

Experimental Setup Details:

The experiments included various configurations of IAA concentrations (40 Units, 20 Units, etc.) applied under different light conditions (Light/Dark) to observe the resulting growth patterns.

Action Spectrum for Blue Light Stimulated Phototropism

A graph demonstrating the curvature of oat coleoptiles in relation to different wavelengths of blue light. The effectiveness of blue light triggering phototropism peaks around 436 nm.

Time-lapse Observation

A time-lapse photograph illustrates a corn coleoptile's growth toward unidirectional blue light, further demonstrating the phototropic response.

Effects in Arabidopsis Seedlings

A comparison of phototropic responses in wild-type versus mutant Arabidopsis seedlings was conducted, showing varied responses to blue light.

Blue-light Stimulates Stomatal Opening

Blue light has been shown to stimulate stomatal opening by activating guard cells, making them swell. This mechanism is crucial for regulating gas exchange and transpiration in plants.

Effectiveness of Wavelength for Stomatal Opening

The action spectrum for blue light in stomatal opening displays relative effectiveness across various wavelengths, with a peak around the blue region of the spectrum.

Stomatal Movement Reversibility

Stomatal movements in response to blue and blue-green light pulses are reversible, with a defined timeline showcasing the gradual opening and closing.

Gravitropism

Gravitropism (also known as geotropism) is the growth response of plants to gravity.

Types of Gravitropism

Negative Gravitropism: Exhibited by shoots, where the growth is directed upward against gravity.
Positive Gravitropism: Exhibited by roots, where the growth is directed downward in accordance with gravitational pull.

Statoliths - Gravity Sensors

Statoliths are specialized cells (amyloplasts) that sense gravity, contributing to the gravitropic response through the starch-statolith hypothesis, where starch-filled plastids move in response to gravitational forces.

Starch Sheath Hypothesis

The starch sheath in roots aids in gravitropic responses, while in shoots, its absence leads to an agravitropic response.
Revision of gravitropic response elements shows that the endodermis is necessary for roots but not for shoots.

Mechanism of Gravitropic Responses

Auxin plays a critical role in root growth, moving laterally to inhibit elongation on the lower side of a root, affecting how it bends downwards.
The mechanism is regulated by different cellular responses in the epidermis, cortex, and endodermis during gravistimulation.

Thigmotropism

Thigmotropism is the directional growth of plants in response to touch or physical stimuli.

Tendril Growth in Thigmotropism

Tendrils differentiate growth patterns on the inside versus the outside when in contact with a support structure. This is a form of memory for tactile stimulation that enables the plant to wrap around supports for stability.

Nastic Movements

Nastic movements are growth responses to stimuli that are not dependent on the direction of the stimulus.

Nyctinastic Movements

These are daily up and down movements influenced by the circadian rhythm, observed in various plant species.

Thigmonastic Movements

Triggered by touch, these movements include phenomena like closing in sensitive plants (e.g., Mimosa pudica).

Thigmomorphogenesis

Thigmomorphogenesis refers to changes in growth due to mechanical stimulation or touch. It induces gene expression changes, particularly for calmodulin, hypothesizing a calcium ion role in growth responses.

Solar Tracking (Heliotropism)

Certain plants exhibit the ability to track the sun’s movement across the sky, allowing maximization of light capture throughout the day.

Circadian Rhythms

Circadian rhythms represent intrinsic biological clocks that regulate plant metabolism and responses to day-night cycles.

Diurnal Movements

Leaf positions change throughout the day in alignment with light availability, demonstrating adjustment to optimize photosynthesis.

Photoperiodism

Photoperiodism describes plants’ responses to the length of day and night, crucial for seasonal behavioral adaptation.

Regulation of Flowering Responses

Short-day plants require longer periods of darkness, while long-day plants flower when light duration exceeds a critical threshold. Day-neutral plants do not have specific light requirements.

Phytochrome and Photoperiodism

Phytochrome is a critical photoreceptor in plants facilitating light detection and regulating growth responses like germination and flowering according to light exposure. It exists in two forms, Pr (red-light absorbing) and Pfr (far-red light absorbing).

Vernalization

A process influencing flowering wherein exposure to low temperatures stabilizes flowering signals, overriding dormancy mechanisms controlled by various hormones.

Dormancy

Dormancy is characterized by arrested growth regulated by environmental signals such as moisture and temperature, enabling plants to survive adverse conditions. Length of photoperiod can induce dormancy in certain species, blending with growth cycles.