Effects • They promote cell elongation in stem and coleoptile. • At higher concentrations auxins inhibit the elongation of roots but extermely lower concentrations promotes growth of root. • Suppression of growth in lateral bud by apical bud due to auxin produced by apical bud is termed as apical dominance . • Auxin prevents abscission.
• It is used to eradicate weeds. Example: , -D and , , -T. • Synthetic auxins are used in the formation of seedless fruits (Parthenocarpic fruit). • It is used to break the dormancy in seeds.
Auxin in the Avena coleoptile Coleoptile placed on Agar Block Auxin diffuses in to agar block Decapited stump Auxin containing agar block in one side of stump Diffusion of Auxin from agar block Figure . : Avena Curvature Test . . Gibberellins .
Discovery The effect of gibberellins had been known in Japan since early where certain rice plants were found to suffer from ‘ Bakanae ’ or foolish seedling disease. This disease was found by Kurosawa ( ) to be caused by a fungus Gibberella fujikuroi . The active substance was separated from fungus and named as Types of Auxin Natural Synthetic Auxin occuring in plants are called “Natural auxin” . Indole Acetic Acid (IAA) .
Indole Propionic Acid (IPA) . Indole Butyric Acid (IBA) . Phenyl Acetic Acid (PAA) These are synthesized artificially and have properties like Auxin. .
, -Dichloro Phenoxy Acetic Acid ( , -D) . , , -Trichloro Phenoxy Acetic Acid ( , , -T) . Napthalene Acetic Acid (NAA) phloem and also occur in xylem due to lateral movement between vascular bundles. .
Bioassay (Dwarf Pea assay) Seeds of dwarf pea are allowed to germinate till the formation of the coleoptile. GA solution is applied to some seedlings. Others are kept under control. Epicotyl length is measured and as such, GA stimulating epicotyl growth can be seen.
. Physiological Effects • It produces extraordinary elongation of stem caused by cell division and cell elongation. • Rosette plants (genetic dwarfism) exhibit excessive internodal growth when they are treated with gibberellins. This sudden elongation of stem followed by flowering by the