Which one of the plant growth regulators would you use if you are asked to (a) induce rooting in a twig (b) quickly ripen a fruit (c) delay leaf senescence (d) induce growth in axillary buds (e) ‘bolt’ a rosette plant (f) induce immediate stomatal closure in leaves.

To address each of the given scenarios, specific plant growth regulators can be utilized based on their physiological effects:

(a) Inducing rooting in a twig: To stimulate root formation in a twig, one would typically use auxins, particularly synthetic auxins such as indole-3-butyric acid (IBA) or naphthaleneacetic acid (NAA). These auxins promote adventitious root formation and enhance the rooting process in cuttings.

(b) Quickly ripening a fruit: Ethylene is the plant growth regulator commonly used to hasten fruit ripening. Applying ethylene gas or ethylene-releasing compounds can accelerate the ripening process by promoting the conversion of starches to sugars, softening of fruit tissues, and development of characteristic color and aroma.

(c) Delaying leaf senescence: Abscisic acid (ABA) is involved in regulating senescence processes in plants. While ABA typically promotes senescence, its precise role can vary depending on the plant species and specific physiological context. Manipulating ABA levels or applying ABA analogs may help delay leaf senescence in certain situations.

(d) Inducing growth in axillary buds: Cytokinins are plant growth regulators known for their role in promoting cell division and shoot growth. Applying cytokinins to axillary buds can stimulate their growth and branching, leading to the development of lateral shoots and branches.

(e) 'Bolting' a rosette plant: Bolting refers to the rapid elongation of the flowering stem in certain rosette plants, often in response to environmental cues such as changes in temperature or day length. Gibberellins are the plant growth regulators typically associated with promoting stem elongation and flowering. Applying gibberellins can induce bolting in rosette plants.

(f) Inducing immediate stomatal closure in leaves: Abscisic acid (ABA) is the primary plant growth regulator responsible for inducing stomatal closure in leaves. ABA levels increase in response to environmental stresses such as drought, triggering the closure of stomata to reduce water loss through transpiration. Applying ABA or ABA analogs can induce immediate stomatal closure in leaves, helping plants conserve water during periods of water stress.

In summary, the selection of plant growth regulators for specific applications depends on their physiological effects and desired outcomes in manipulating plant growth and development processes.