Crop Physiology Lesson: Plant Hormones and Regulators Phytohormone Part 2

Original video: https://youtu.be/4LKaeyVyV2Q

Lecture on: 17 November 2025

Part 1: https://youtu.be/3mblOhwxerU

This lecture, part of the Crop Physiology (AGR3301) course, provides a comprehensive overview of Plant Growth Regulators (PGRs), also known as Phytohormones. These are natural chemical messengers that control a plant's growth, development, and movement at minute concentrations.

PGR Classification Plant hormones are broadly classified into two groups based on their primary functions:

Growth Promoters: Involved in activities like cell division, cell enlargement, flowering, fruiting, and seed formation (e.g., Auxins, Gibberellins, Cytokinins).

Growth Inhibitors: Involved in responses to stress and activities like dormancy and abscission (e.g., Abscisic Acid, Ethylene)

1. Auxins (Indole-3-Acetic Acid, IAA) The discussion begins with the historical discovery of Auxins, starting with Charles Darwin's experiments on phototropism in Canary grass coleoptiles.

Discovery & Chemistry: Auxin (from Greek 'auxein': to grow) was isolated by F.W. Went from oat coleoptile tips and identified as IAA. They are produced mainly by the growing apices of stems and roots.

Key Functions: Auxin increases the plasticity of plant cell walls and is involved in stem elongation. However, high concentrations inhibit cell elongation in roots. Other roles include Apical Dominance, Gravitropism, Phototropism, and inducing Parthenocarpy (seedless fruit formation).

Horticulture: Synthetic auxins like IBA and NAA are used in horticulture, while 2,4-D is a common herbicide.

2. Gibberellins (GAs) Discovery: Gibberellins were discovered in association with the 'bakanae' (foolish seedling) disease of rice, caused by the fungus Gibberella fujikuroi. Gibberellic acid was one of the first GAs isolated and is the most studied form.

Key Functions: GAs stimulate both cell division and cellular elongation, leading to prominent effects on stem elongation. GAs promote seed germination (especially in cereals) by mobilizing reserves and are used commercially to increase fruit size (e.g., Thompson Seedless grapes) They also promote bolting (shoot elongation) in rosette plants prior to flowering

3. Cytokinins

Discovery: Cytokinins were discovered by Skoog and Miller after observing that tobacco cells would enlarge but not divide when supplied only with auxin; the addition of old herring-sperm DNA (rich in adenine derivatives) induced cell division. Kinetin was the first compound isolated in this class.

Key Functions: Cytokinins are chemically similar to adenine and primarily promote cell division (mitosis). They influence morphogenesis in tissue culture (high Cytokinin/Auxin ratios favor shoot formation). Cytokinins also promote lateral bud development (overcoming apical dominance) and delay leaf senescence.

Keywords: Phytohormones, Plant Growth Regulators, Auxin, Gibberellin, Cytokinin, Abscisic Acid, Ethylene, Plant Hormones, Apical Dominance, Stomatal Closure, Fruit Ripening, Cell Elongation, AGR3301.

Location:

Faculty of Agriculture, Universiti Putra Malaysia

Fakulti Pertanian, Universiti Putra Malaysia, 43000 Seri Kembangan, Selangor

https://maps.app.goo.gl/VePw71hmih6pbHkPA

2.983567621706455, 101.73466120334834

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