Growth Regulators
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In order for a plant to respond positively or negatively to light, temperature, gravity, day length, etc., some hormone must be produced in one tissue and transported to another to cause the response. We looked at three classes of hormones: auxins, cytokinins, gibberellins.
A: IAA (indole acetic acid), an auxin, was shown to have two effects.
A1. IAA produced in the terminal bud inhibits lateral bud growth, (giving rise to apical dominance). As long as the apical bud is present, the apex (i.e. the uppermost terminal part) will be dominant (apical dominance), and all growth to the sides will be minimal. If you cut off the shoot apex, then the side (axillary) buds grow out. This was easy to see with the tobacco plants. This experiment shows why one snips off the shoot tips to get bushy growth in house plants.
Link to tobacco plant with intact apex
Link to tobacco plants with apex removed
Link to tobacco plant with apex removed two weeks earlier but with IAA in lanolin applied
Link to tobacco plants with apex removed but with lanolin only applied
A2. IAA prevents the formation of the abscission zone. This means as long as IAA is present the petiol will not fall off. The blade produces IAA; as long as it is present the petiole does not fall off. Thank goodness, the young plant needs the leaves on the plant, there will be plenty of time in the fall for IAA production to cease and the leaves can then fall off.
Link to view of Coleus plant with intact blades
Link to view of Coleus Plant with blades removed, but with IAA in lanolin applied
Link to views of Coleus Plant with blades removed, but with lanolin only applied
A3. The herbicide 2,4-D is a synthetic auxin. Pots of corn and wheat (both monocots) did not mind being sprayed with it. The dicot vegetables like radishes and beans looked sick. One uses 2,4-D in grain (monocot) fields to get rid of dicot (broad-leaved) weeds. Obviously you would not use it in a cabbage field. 2,4-D in high concentrations over-stimulates cell-division in phloem, leading to blockage as well as disrupting other cellular biochemical events. The reason for the selective killing of dicots while leaving monocots relatively unharmed is poorly understood but most likely many monocots have different chemical pathways that are not susceptible to 2,4-D.Link to view of mixed pots of corn and beans treated with 2,4-D
B. Gibberellic acid stimulates both cell division and cell elongation. The difference between the dwarf beans and peas and normal ones is due to the mutation of the single gene that codes for one step in the production of gibberellic acid. A "dwarf" plant is one which does not normally produce as much GA as a "normal" plant. So when the dwarf plants are sprayed with GA, they should elongate and appear "normal" .
Link to view of dwarf pea plants one of which has been treated with gibberellic acid