Plant hormones

The governing concept is hormonal response to environmental stress. During water shortage, abscisic acid promotes stomatal closure, reducing transpiration and helping the plant conserve water. This protective action explains why it is called a stress hormone, so option A is correct. The other choices describe unrelated animal functions and do not represent the role of abscisic acid in plants.

The governing concept is phototropism, the bending of a plant shoot toward light. In a shoot illuminated from one side, auxin is redistributed toward the shaded side. Its higher concentration there promotes greater cell elongation, so that side grows faster and the shoot bends toward the light. Therefore option A is correct; the other locations do not explain directional shoot growth.

Cytokinin is the correct answer because it promotes cytokinesis and supports cell division in actively growing plant tissues. It works with other plant hormones to regulate growth and development. Auxin mainly promotes cell elongation and root-related growth, while abscisic acid generally inhibits growth and helps plants respond to stress. Adrenaline is an animal hormone, not a plant hormone, so it cannot be the suitable choice here.

Abscisic acid, commonly abbreviated as ABA, is associated with growth inhibition and plant responses to stressful conditions such as water shortage. During stress, it promotes stomatal closure, reducing water loss, and is therefore linked with wilting and dormancy-related responses. Gibberellin generally promotes stem growth, and cytokinin supports cell division and delays leaf ageing. Thyroxine is an animal hormone, so option C is the only suitable answer.

Gibberellin is a plant growth hormone that promotes stem elongation and can increase the length of internodes, the regions between successive leaves. Therefore, a plant treated with suitable gibberellin may develop a taller stem. Insulin, adrenaline, and thyroxine are hormones associated with animals rather than plant stem elongation. Auxin also affects plant growth, but it is not included among the choices, making gibberellin the unambiguous answer.

A shoot normally shows positive phototropism, meaning it grows towards a directional light stimulus. Auxin becomes distributed more on the shaded side of the stem, where it promotes greater cell elongation. Because the shaded side grows faster, the stem curves towards the illuminated side. It does not bend downward, move away from light, or remain straight when the light is one-sided.

Geotropism, or gravitropism, describes a plant growth response to gravity. A positive response occurs in the direction of the stimulus. Therefore, a root showing positive geotropism grows downward, along the direction of gravitational pull. Growth against gravity is negative geotropism and is typical of the shoot. Light response and touch-related shrinking describe different plant responses, so they are not correct here.

The governing concept is a tendril’s touch-guided growth response. A tendril is a slender, specialised structure that bends and coils when it contacts a support, helping the plant climb and obtain better light. A main root anchors and absorbs water, a petal attracts pollinators, and a cotyledon stores or supplies nourishment to the young seedling. Thus option A is correct.

The governing concept is positive phototropism controlled by auxin. With one-sided light, auxin becomes concentrated on the shaded side of a young shoot. In shoots, this promotes greater cell elongation on that side. The shaded side therefore grows faster than the illuminated side, producing unequal growth and bending the stem toward the light. Option B is correct; plant stems do not have blood flow, and neither cell death nor root pressure explains this directional curvature.

Phototropism is a directional growth response to light. In a shoot receiving light from one side, auxin is redistributed and usually promotes greater cell elongation on the shaded side. That side grows faster, so the shoot bends toward the light. The plant does not move by muscles or run, and leaves do not pump blood. Thus option A correctly identifies unequal growth as the cause.

Phototropic bending requires differential growth on the two sides of a shoot. Normally, one-sided light changes auxin distribution, causing greater elongation on one side and bending. If auxin is artificially maintained at equal levels, both sides receive a similar growth stimulus, so their elongation remains nearly equal and curvature decreases. Light can still enter the plant; therefore option A is correct.

Abscisic acid, or ABA, is a plant hormone associated with growth inhibition and responses to environmental stress. During water shortage, ABA promotes stomatal closure, reducing transpiration and helping conserve water; it can also promote dormancy or suppress growth in suitable situations. It does not produce insulin, transmit animal nerve signals or control heartbeat. Therefore option A is correct.

Gibberellin and cytokinin are plant hormones, but they promote different growth processes. Gibberellin mainly stimulates stem elongation, whereas cytokinin promotes cell division and supports the formation of new cells. Therefore, combining their functions shows that plant growth depends on both an increase in cell number and an increase in cell length. Options B, C, and D incorrectly describe these hormones.

During severe water shortage, a plant must reduce transpiration to conserve its limited water. Abscisic acid, or ABA, is a plant hormone that promotes stomatal closure under water stress. If ABA is deficient or the guard cells do not respond to it, stomata may remain open and excessive water loss can continue. Therefore option A is correct; the other hormones are mainly associated with animals.

Hormones are released into the bloodstream and can influence many target tissues. Their effects generally develop more slowly than nerve impulses but may persist for a longer time, making them suitable for growth, development, metabolism, and other sustained processes. Immediate withdrawal from heat and rapid pupil changes require nervous control. Therefore option A is the only appropriate choice.

The governing concept is differential sensitivity to plant hormones. The same chemical messenger can produce different responses because roots, stems, and shoots have different functions, hormone concentrations, receptors, and sensitivities. For example, a concentration that promotes growth in one organ may inhibit it in another. Thus option A is correct. Options B and C are false, and D is incorrect because roots and stems have different roles.

In a young shoot, the tip detects directional light and coordinates the distribution of auxin. Auxin becomes more concentrated on the shaded side, where it promotes cell elongation, causing the shoot to bend towards the light. Covering the tip can reduce light perception and disrupt this unequal distribution, so bending may decrease. Thus option A is correct.

Phototropism results from unequal growth on the two sides of a shoot. If cells on the shaded side elongate more, that side becomes longer and acts like the outer side of a bend. The shorter illuminated side becomes the inner side, so the shoot curves towards the light. Therefore option A is correct; the other choices describe unrelated or unsupported directions.

Auxin regulates plant growth, but different organs have different sensitivities to its concentration. A level that promotes cell elongation in a stem may inhibit elongation in a root. This organ-specific response explains why the same hormone can produce opposite effects, so option A is correct. Auxin is a plant hormone, roots do respond to hormones, and stem growth is not always toward gravity.

Cytokinin is a plant hormone whose important functions include promoting cell division and supporting growth in actively dividing tissues. If cytokinin activity becomes very low, cell division in regions such as meristems may be reduced. Therefore option A is correct. Emergency responses are associated with animal nervous or hormonal systems, while blood-sugar control and breathing rate are not cytokinin functions.