In plant physiology, which of the following is not a plant hormone phytohormone involved in the regulation of growth and development?

Introduction / Context:
Plant hormones, also called phytohormones or plant growth regulators, are chemical messengers that control growth, development, and responses to the environment. Typical examples include auxins, gibberellins, cytokinins, ethylene, and abscisic acid. This question asks you to identify which listed substance is not a plant hormone but instead belongs to animal endocrine physiology.

Given Data / Assumptions:

• Options include gibberellic acid, auxins, cytokinins, thyroxine, and abscisic acid.
• Gibberellic acid, auxins, cytokinins, and abscisic acid are standard names in plant physiology.
• Thyroxine is a well known thyroid hormone in vertebrates.
• We assume standard high school or undergraduate botany and zoology classifications.

Concept / Approach:
Plant hormones are small organic molecules produced in minute quantities that regulate cell elongation, division, seed germination, flowering, fruit ripening, and stress responses. Auxins promote cell elongation, gibberellins promote stem elongation and breaking of dormancy, cytokinins promote cell division, and abscisic acid is involved in stress responses and stomatal closure. Thyroxine, however, is a thyroid hormone secreted by the thyroid gland in animals and regulates basal metabolic rate and development. It is not a plant hormone and does not function in plant growth regulation.

Step-by-Step Solution:
Step 1: Recall that auxins, especially indole acetic acid, are core plant hormones controlling cell elongation and apical dominance. Step 2: Remember that gibberellic acid is a gibberellin involved in stem elongation, seed germination, and flowering. Step 3: Recognize that cytokinins promote cell division and delay leaf senescence, so they are also plant hormones. Step 4: Abscisic acid is another plant hormone that acts mainly as a growth inhibitor and stress hormone, especially in drought. Step 5: Thyroxine, in contrast, is a vertebrate thyroid hormone that regulates metabolism and is not produced by plants. Step 6: Therefore, the substance that is not a plant hormone is thyroxine.

Verification / Alternative check:
Plant physiology chapters typically list five major classes of plant hormones and do not include thyroxine anywhere. Zoology or human physiology chapters discuss thyroxine under endocrine glands, where it is clearly identified as a hormone derived from the amino acid tyrosine with iodine atoms, secreted by the thyroid gland. This separation of thyroxine into animal physiology and its absence from plant growth regulator lists confirms that it is not a plant hormone.

Why Other Options Are Wrong:
Gibberellic acid is a well known plant hormone in the gibberellin group, so option A is a true plant hormone. Auxins are the first discovered class of plant hormones, so option B is also a plant hormone. Cytokinins regulate cell division and are standard plant hormones, so option C is correct as a plant hormone. Abscisic acid regulates dormancy and stress responses, so option E is also a plant hormone. None of these fit the requirement of being non plant hormones.

Common Pitfalls:
Students sometimes confuse names ending with in or one and think that any hormone like term could belong to plants. Another pitfall is to forget that many hormones with similar sounding names exist across plant and animal kingdoms. The key is to recall which chemicals are actually listed under plant growth regulators in your syllabus and to recognize that thyroid hormones are strictly animal hormones.

Final Answer:
The compound that is not a plant hormone is Thyroxine.