Phototropism

What is phototropism?

Phototropism is the directional growth or bending of plants toward a light source.

What were Charles and Francis Darwin's key findings regarding phototropism?

They demonstrated phototropism and identified the coleoptile (the protective sheath enclosing the embryonic shoot) as the primary site for perceiving light stimuli. They suggested a diffusible chemical substance produced in the coleoptile tip influenced growth below the tip, causing bending.

What is a coleoptile and why is it important in phototropism?

A coleoptile is a protective sheath enclosing the embryonic shoot of grasses (like oats or maize) in seedlings. Darwin and Darwin identified it as the primary site for perceiving light stimuli in plant responses to light.

What was the significance of the Boysen-Jensen Experiment (1910-1913)?

Boysen-Jensen confirmed Darwin's hypothesis. They demonstrated that a permeable material (gelatin) inserted below a decapitated coleoptile tip allowed bending toward light, while an impermeable material (mica) blocked the signal. This provided strong evidence for a diffusible substance from the tip transmitting the growth signal.

How did Fritz Went (1926-1928) further investigate the diffusible substance?

Went incubated excised coleoptile tips on small agar blocks to collect the growth-promoting substance. Asymmetrical placement of these agar blocks on decapitated coleoptiles in the dark induced differential cell elongation and bending, leading to the identification and isolation of auxin.

What is auxin and what are its roles?

Auxin (specifically Indole-3-acetic acid or IAA) is the first identified plant growth hormone, discovered by Fritz Went. It's a major regulator for processes like cell elongation, phototropism, gravitropism (growth in response to gravity), apical dominance (inhibition of lateral bud growth by the apical bud), and the initiation of adventitious roots.

Explain the 'acid growth hypothesis' for auxin action.

Auxin stimulates proton (H^+) pumps in the plasma membrane, actively pumping H^+ ions into the cell wall. This acidification activates expandin enzymes, which loosen cell wall fibers. The loosened wall allows turgor pressure to cause the cell to absorb water and expand, leading to irreversible cell elongation.

What is the action spectrum for phototropism, and what pigments are involved?

The action spectrum for phototropism shows peak absorption in the blue light range (approximately 400-500 nm, with specific peaks around 450 nm). This indicates that blue light is critical for initiating the response, activating specific blue-light photoreceptors called phototropins, which then trigger a signaling cascade.

What are phototropins?

Phototropins are specific blue-light photoreceptors that absorb light in the blue range (400-500 nm) and are responsible for initiating the phototropic response in plants.

What is phytochrome and what are its two forms?

Phytochrome is a family of photoreceptors crucial for various aspects of plant development and environmental sensing. It exists in two photoreversible forms: phytochrome red (Pr), the inactive form, and phytochrome far-red (P{fr}), the active form.

Describe the interconversion of phytochrome forms.

Phytochrome red (Pr) absorbs red light (peak ~660 nm) and rapidly transforms into the active phytochrome far-red (P{fr}) form (660\ nm\ Pr \to P{fr}). Conversely, P{fr} absorbs far-red light (peak ~730 nm) and rapidly reverts to Pr (730\ nm\ P{fr} \to Pr). In darkness, P{fr} also slowly reverts to Pr over several hours.

What role does P_{fr} play in plant development?

P{fr} is generally the biologically active form of phytochrome, initiating signaling pathways. Its presence, absence, and the Pr to P_{fr} ratio dictate various physiological effects, including stimulation or prevention of flowering, promotion or inhibition of seed germination, and control of seedling de-etiolation.

What is photoperiodism?

Photoperiodism is the physiological response of plants to the relative lengths of day and night, specifically the duration of the uninterrupted night length, which dictates crucial life cycle events such as flowering and seed germination. Plants use phytochrome and circadian rhythms to sense and respond to these changes.

Why is critical night length more important than critical day length for plant flowering?

Plants primarily measure the critical uninterrupted night length to determine their flowering response. Laboratory experiments show that interrupting a long night with a brief flash of light can decisively control flowering, demonstrating the sensitivity to the dark period.

Define Short-Day Plants (SDP).

Short-day plants (SDP), also known as long-night plants, require a long, uninterrupted period of darkness (i.e., short days) to flower. If their critical night length is interrupted by light, flowering may be inhibited. Examples include chrysanthemums, poinsettias, and rice.

Define Long-Day Plants (LDP).

Long-day plants (LDP), also known as short-night plants, require a period of darkness shorter than a specific critical length (i.e., long days with short nights) to flower. If the critical night length is extended or interrupted in a way that creates a shorter perceived night, flowering is promoted. Examples include spinach, iris, and wheat.

Define Day-Neutral Plants (DNP).

Day-neutral plants (DNP) are unaffected by the length of the day or night and flower once they reach a certain developmental stage or size, regardless of photoperiod. Examples include corn, tomatoes, and dandelions.

How does flashing red light during a long night affect SDPs and LDPs?

Flashing red light during the middle of a long night converts Pr to P{fr}. For short-day plants (SDPs), this interruption 'shortens' the perceived night, often inhibiting flowering. For long-day plants (LDPs), this same interruption can promote flowering by shortening the perceived night.

How do final exposures to red (R) and far-red (FR) light affect light-sensitive seed germination?

In light-sensitive seeds, a final exposure to red light stimulates germination because it initiates P{fr} formation. Conversely, a concluding pulse of far-red light prevents germination by promoting Pr formation, demonstrating phytochrome's photoreversibility and the last light exposure's dictation of biological outcome.