Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Phytohormones: These are organic substances produced naturally in plants which influence physiological processes at very low concentrations. Major types include Auxins, Gibberellins, Cytokinins, Abscisic Acid (ABA), and Ethylene.
Auxins: Primarily synthesized at the shoot apices. The most common natural auxin is Indole-3-Acetic Acid (). They promote cell elongation and are responsible for apical dominance and phototropism.
Gibberellins: Found in embryos and roots. They help in the elongation of internodes, breaking seed dormancy, and increasing fruit size. is a well-known example.
Cytokinins: These hormones promote cell division (cytokinesis). They are found in regions where rapid cell division occurs, such as root tips and developing seeds. They help in delaying senescence (aging) in leaves.
Abscisic Acid (ABA): Often called the 'stress hormone' or growth inhibitor. It promotes seed dormancy, causes the wilting of leaves (abscission), and triggers the closure of stomata during water stress.
Ethylene (): The only gaseous plant hormone. It is primarily responsible for the ripening of fruits and the abscission of leaves and flowers.
Tropic Movements: Growth movements in response to a directional stimulus. These include Phototropism (light), Geotropism (gravity), Hydrotropism (water), Thigmotropism (touch), and Chemotropism (chemicals like or sugars).
📐Formulae
💡Examples
Problem 1:
Explain the mechanism behind the bending of a plant shoot toward a unidirectional light source (Phototropism).
Solution:
When light shines from one side, Auxin () diffuses from the tip toward the shaded side of the shoot. The higher concentration of auxin on the shaded side causes the cells there to elongate more rapidly than the cells on the lit side. This differential growth causes the shoot to bend toward the light.
Explanation:
Auxins promote growth in shoots but inhibit growth in roots. The redistribution of is the primary cause of directional growth in response to light stimuli.
Problem 2:
Why is Abscisic Acid () referred to as the 'stress hormone' in plants?
Solution:
During environmental stress, such as drought, levels increase significantly. It induces the closure of stomata to prevent excessive water loss through transpiration and promotes seed dormancy to ensure survival during unfavorable conditions.
Explanation:
acts as an antagonist to growth-promoting hormones like Auxins and Gibberellins to conserve plant resources.
Problem 3:
A gardener wants to produce seedless grapes and increase the length of the grape stalks. Which hormone should be applied?
Solution:
Gibberellins () should be applied.
Explanation:
Gibberellins are known to promote internodal elongation and can induce parthenocarpy (development of fruit without fertilization), which results in seedless fruits.