Plant hormones

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.

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.

The governing concept is the role of plant hormones in growth regulation. Cytokinins primarily promote cell division and help form new tissues, whereas gibberellins are more strongly associated with stem elongation. Since the stated effect is increased division without direct stem lengthening, the chemical acts like cytokinin, so A is correct. Adrenaline and insulin are animal hormones and do not explain this plant response.

The governing concept is hormone specificity through receptors. Hormones may circulate widely in the blood, but they produce a response only in cells that possess the appropriate receptor and the biochemical machinery linked to it. The receptor recognizes the hormone’s shape and starts a cellular response. Therefore A is correct. Blood does reach target organs, hormones are not universally excluded from organs, and the brain does not always destroy them, so B, C, and D are false.

The governing concept is plant hormonal coordination through a directional growth response. In phototropism, light causes unequal distribution of auxin in the shoot, so cells on the shaded side generally elongate more. This unequal growth produces bending toward the light; bending is therefore the visible result, not the complete mechanism. Option A is correct. The other choices incorrectly invoke muscles, blood flow, or a brain.

A plant hormone acts according to its concentration, the tissue receiving it, and that tissue’s sensitivity and developmental state. Roots and stems have different structures, functions, and optimal hormone concentrations. Consequently, the same hormone may stimulate growth in one organ but inhibit or alter growth in another. Option A expresses this principle. The remaining choices are incorrect because roots and stems have different functions, plants are cellular, and hormones are not restricted to air.

The governing concept is phototropism, in which unequal auxin distribution causes unequal growth on the two sides of a young stem. Normally, auxin accumulates on the shaded side and promotes greater elongation there, so the stem bends towards light. If this unequal distribution is prevented, the difference in growth is reduced and option A follows. The other choices describe impossible or unrelated animal processes.

The governing concept is differential sensitivity to plant hormones. Roots and stems do not respond identically to the same auxin concentration: a level that promotes elongation in a stem may inhibit elongation in a root. Therefore option A correctly concludes that hormone action depends on both the responding organ and concentration. Auxin is a plant hormone, roots do respond to stimuli, and the stem does not always grow towards water.

The governing concept is differential sensitivity to a plant hormone. Roots and stems contain different tissues and respond to auxin at different concentration ranges. A level that promotes elongation in a stem may inhibit root elongation, because roots are generally more sensitive. Hence option A is correct. Roots do not contain brains, stems do not transport blood, and auxin is a plant hormone rather than an animal-only hormone.