Botany · Ch 15 — Plant Growth and Development
Plant Growth Regulators
Plant Growth Regulators
Plant Growth Regulators, or PGRs, are the chemical messengers of the plant world - organic compounds made in tiny quantities in one part of the plant body and transported to another part, where they steer specific physiological processes. Five classical groups are recognised as coordinating plant growth and development: auxins, gibberellins, cytokinins, ethylene and abscisic acid (Figure 15.10). When these substances are made naturally by the plant itself, they are specifically called phytohormones; manufactured chemicals that mimic a hormone's action, and sometimes even its molecular structure, without being produced by the plant, are called synthetic PGRs instead. Two further groups, brassinosteroids and polyamines, have more recently also been recognised as behaving in a hormone-like way. PGRs are broadly sorted by whether they prom …
What this figure shows. A branching classification chart headed 'Plant Growth Regulators (PGRs)', splitting into 'Natural (Phytohormones)' - listing Auxin, Gibberellin, Cytokinin, Ethylene and Abscisic acid as growth promoters and inhibitors - and 'Synthetic', listing NAA, 2,4-D and 2,4,5-T, summarising the natural-versus-synthetic PGR grouping described i …
Characteristics of Phytohormones; Synergistic and Antagonistic Effects
Across all five classical groups, phytohormones share a common set of traits: they are usually produced at the growing tips of roots, stems and leaves; they move from their site of production to their site of action through the plant's conductive tissues; they act in extremely small (trace) quantities; they are all organic compounds; unlike animal hormones, there are no dedicated glands or specialised cells set aside just to secrete them; and their overall job is to influence physiological activity by promoting, inhibiting or otherwise modifying growth. When two different PGRs work together, reinforcing each other's effect on the same process, that relationship is called a synergistic effect - auxin working alongside either gibberellins or cytokinins is a standard example. When two PGRs instead pull a process in opposite directions, with one promoting and the other …