Plant Physiology: Flowering and Growth Regulators

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Questions and Answers

What is the consequence of a dark period being shorter than a critical length for flowering?

Plants will die.

No flowering occurs. (correct)

Flowering can occur.

Flowering may be delayed.

What happens when a flash of red light interrupts the dark period?

The red light has no effect.

Flowering occurs only if the dark period is exactly 12 hours.

No flowering occurs.

Flowering occurs. (correct)

What cancels the effects of a red light flash during the dark period?

A subsequent flash of blue light.

A longer red light exposure.

A subsequent flash of far-red light. (correct)

A longer dark period.

What is the outcome when a red flash follows a far-red flash?

Flowering occurs if the dark period is short enough. Signup and view all the answers

How does a plant respond when exposed to alternating red and far-red lights?

It will respond to the last flash of light. Signup and view all the answers

What are gibberellins primarily used for in commercial applications?

To delay fruit ripening Signup and view all the answers

Which of the following accurately describes cytokinin functions?

Delays aging of plant cells Signup and view all the answers

What indicates that roots are healthy and active to shoots?

Production of cytokinins Signup and view all the answers

What was isolated from young maize grains and named zeatin in 1963?

Cytokinin Signup and view all the answers

How do cytokinins affect lateral bud growth?

They promote lateral bud growth Signup and view all the answers

Which of the following methods is a commercial use of cytokinins?

Prolonging the shelf life of cut flowers Signup and view all the answers

What was the first discovered substance that induced cell division in tissue culture?

Kinetin Signup and view all the answers

For cytokinins to effectively signal shoots about root health, what must be true?

Roots must actively produce cytokinins Signup and view all the answers

What is the primary role of auxins in plant growth?

Cause bending towards light Signup and view all the answers

Which substance was discovered by F.W. Went associated with auxin activity?

Naphthaleneacetic acid (NAA) Signup and view all the answers

Gibberellins are primarily synthesized in which part of the plant?

Meristems of apical buds, roots, and young leaves Signup and view all the answers

Which of the following is a common commercial use of synthetic auxins?

Induce root formation in stem cuttings Signup and view all the answers

What effect do gibberellins have on seed dormancy?

They help break seed dormancy Signup and view all the answers

Which chemical structure is associated with gibberellins?

Terpene structure Signup and view all the answers

Which of the following statements about gibberellins is true?

They promote parthenocarpy Signup and view all the answers

Who was responsible for the discovery of gibberellins?

E. Kurosawa Signup and view all the answers

Which type of plants flower when the dark period exceeds a certain critical length?

Short-day plants Signup and view all the answers

What role do phytochromes play in flowering plants?

They act as photoreceptors for controlling flowering. Signup and view all the answers

What happens to a long-day plant if the level of Pfr is high?

It stimulates flowering. Signup and view all the answers

What is florigen primarily responsible for?

Transmitting information from leaves to flower buds. Signup and view all the answers

What triggers the formation of active florigen in short-day plants?

An increase in dark periods. Signup and view all the answers

Which of the following is an example of a long-day plant?

Spinach Signup and view all the answers

Day-neutral plants are characterized by flowering that is:

Unrelated to photoperiodism. Signup and view all the answers

What is the effect of a low amount of Pfr in short-day plants?

It triggers flowering. Signup and view all the answers

What is one of the primary functions of abscisic acid in plants?

Causing stomatal closure during water stress Signup and view all the answers

Which plant hormone works antagonistically to abscisic acid?

Auxins Signup and view all the answers

In which part of the plant is abscisic acid primarily synthesized?

Leaves, stems, fruits, and seeds Signup and view all the answers

How does abscisic acid affect seed dormancy?

It inhibits germination until conditions improve Signup and view all the answers

What happens to guard cells when abscisic acid is present during water stress?

They lose K+ ions causing stomatal closure Signup and view all the answers

Which of the following is a commercial application of abscisic acid?

Inducing dormancy in nursery stock before shipment Signup and view all the answers

What effect does high levels of abscisic acid have on fruits and leaves?

Promotes their abscission Signup and view all the answers

What triggers the germination process in seeds that contain abscisic acid?

The washing away of abscisic acid by rainfall Signup and view all the answers

What is the primary purpose of phototropic control of flowering in plants?

To synchronize flowering with favorable environmental conditions Signup and view all the answers

Which of the following statements is true about long-day plants?

They can flower with shorter dark periods Signup and view all the answers

How do short-day plants typically respond to dark periods?

They require a long uninterrupted dark period to flower Signup and view all the answers

What effect does introducing a light flash during the dark period have on flowering?

It interrupts the flowering process of short-day plants Signup and view all the answers

Which of the following describes the conclusion from Experiment 1?

The length of uninterrupted darkness controls flowering Signup and view all the answers

In the context of flowering, what do SD and LD stand for?

Short-day and long-day Signup and view all the answers

What is a consequence of ensuring that plants of similar species flower at the same time?

It facilitates cross-pollination and cross-fertilization Signup and view all the answers

Which statement about the manipulation of light and dark periods by horticulturists is accurate?

They can manipulate light exposure to induce flowering in both types of plants Signup and view all the answers

Study Notes

Coordination and Control in Plants

Plant hormones, also known as growth substances or plant growth regulators, are organic chemicals.
These chemicals exist in very low concentrations in plant tissues.
They act as messengers, stimulating, inhibiting, or modifying growth and development.
Plant hormones are typically synthesized in specific regions of the plant, such as embryos, apical meristems of shoots and roots, young leaves and developing seeds.

Plant Hormones vs. Animal Hormones

Feature	Plant Hormones	Animal Hormones
Glands	No specialized glands for hormone synthesis	Produced by specialized endocrine glands
Transport	Transported from the site of production to other parts of the plant	Mainly transported through the bloodstream to target cells/organs
Transport speed	Generally slow	Relatively rapid
Specificity	Less specific, may affect different tissues and organs	More specific, affecting only target cells/organs
Primary function	Primarily involved in growth and development	Regulate various bodily functions, including metabolism, reproduction, growth and development

Major Plant Hormones

There are five major plant hormones: Auxin, Abscisic Acid, Cytokinin, Gibberellins, and Ethylene.
These hormones play crucial roles in plant growth and development.

Auxins

Auxins influence cell division and elongation by impacting vacuolation and elongation.
Natural auxins, like Indole-3-acetic acid (IAA), are produced naturally by plants.
Auxin's effects vary depending on the target tissue.
- Examples include elongation of shoots, cell division and differentiation in vascular cambium, fruit development in ovaries, and lateral root formation.
Auxins are involved in phototropism (plant growth towards light) and geotropism (plant growth in response to gravity).
More auxin is produced on the shaded side of a shoot or coleoptile, causing cells on that side to elongate faster than those on the illuminated side, leading to the bending of the plant towards light.
Similarly, auxins accumulate in the lower half of the stem and root, causing downward growth in response to gravity.
Auxins promote fruit growth (parthenocarpy). Applying IAA to the stigma can trigger fruit development without fertilization, leading to seedless fruits.
Auxins also inhibit the growth of lateral buds (apical dominance), which helps in directing growth towards the main stem.
Auxins also inhibit abscission (dropping of leaves, flowers, and fruits). In aging leaves or ripened fruits, auxin production decreases, and abscisic acid increases, leading to abscission zone formation at the base of the leaf petiole or fruit stalk.

Gibberellins

Gibberellins promote cell elongation and stem growth.
They're synthesized in meristems of apical buds, roots, young leaves, and embryos in seeds.
Gibberellins have a terpene structure, a group of plant chemicals related to lipids.
Gibberellins interact with IAA to stimulate stem growth.
They can substitute red light to promote flowering in long-day plants, while inhibiting it in short-day plants.
They also aid in breaking seed dormancy.
Gibberellins also promote fruit growth and parthenocarpy.

Cytokinins

Cytokinins promote cell division and differentiation in the presence of auxin.
They are produced in actively growing tissues such as embryos in seeds, fruits, and roots.
Cytokinins are transported in the xylem to various parts of the plant.
They promote lateral bud growth, counteracting the effect of apical dominance.
They delay senescence (aging) of plant cells, maintaining nucleic acid levels, stimulating protein synthesis, and delaying chlorophyll breakdown.
Cytokinins can signal to shoots that roots are healthy and active, impacting shoot growth and leaf function based on root health.

Abscisic Acid (ABA)

ABA slows down metabolism and inhibits growth in most plant parts, acting antagonistically to auxins, gibberellins, and cytokinins.
It's synthesized in leaves, stems, fruits, and seeds.
ABA is mainly transported in the phloem and can diffuse from the root cap to root cells.
ABA triggers stomatal closure in response to water stress, aiding in water conservation.
It also causes dormancy in some seeds.
This hormone promotes abscission of fruits and leaves.

Ethylene

Ethylene is a gaseous plant hormone, a metabolic by-product of most plant organs, particularly in ripening fruits and aging leaves.
It triggers fruit ripening.
Ethylene increases as fruit ripening occurs (positive feedback).
It triggers release of hydrolytic enzymes, breaking down cell wall components, chlorophyll, and softening the fruit.
These enzymes also convert starches and fruit acids into sugars, contributing to the bright colors, scents, and sugars associated with ripe fruit, attracting animals to eat the fruit and disperse seeds.
Ethylene is used commercially to hasten fruit ripening after harvest.

Phytochromes

Phytochromes are photoreceptors for red light responses.
They exist in two photo-interconvertible isomeric forms (Pr and Pfr).
Pr absorbs red light, and Pfr absorbs far-red light.
Sunlight triggers the conversion of Pr to Pfr, and in the dark Pfr reverts to Pr.
The ratio of Pr to Pfr affects various plant processes such as photoperiodism, seed germination, and flowering.

Photoperiodism and Flowering

Photoperiodism is the plant's response to the relative lengths of daylight and darkness.
Different plants have different photoperiodic requirements for flowering (short-day, long-day, and day-neutral plants).
Phytochromes play a key role in photoperiodism, as the balance between Pr and Pfr triggers or inhibits flowering.
Florigen, a chemical messenger, is secreted from the leaves in response to light conditions and carried to the flower buds, signaling flowering.

Other Hormones

Additional hormones, such as brassinosteroids and oligosaccharins, have various roles in plant growth and development.

Commercial Uses

Auxins, gibberellins, cytokinins, ABA, and ethylene all have important commercial applications in agriculture, horticulture, and other industries.

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Description

This quiz focuses on key concepts in plant physiology, particularly the effects of light on flowering and the roles of various plant hormones like gibberellins and cytokinins. Explore how different light treatments influence plant responses and the implications for agricultural practices. Test your knowledge on how these growth regulators affect plant development.