Plant growth regulators are basically plant hormones.Fauziya said:
We need to know about three plant hormones:
(i) Auxin (specifically IAA)
(ii) Gibberellic acid (GA)
(iii) Abscissic acid (very confusing spelling)
(i) Auxin
There are several chemicals known as auxins, in our case it is IAA. Auxins are produced in apical bud. They move from the apical bud to the lateral buds either from cell to cell or via phloem sap and xylem vessels. Cell to cell movement could involve symplast pathway (via plasmodesmata if I remember correctly) or could be active transport. Coming to the effect of auxin, when auxin is present in high concentration in lateral buds, it inhibits the growth of the lateral buds. So, the plant only grows upward and cannot branch sideways. Removal of apical bud allows lateral buds to grow. This is known as apical dominance. This happens because removal of apical bud means no meristematic region to produce auxin, so concn of auxin in lateral bud decreases eventually. Also, ABA and GA enhance IAA while cytokinins are antagonistic to IAA.
(ii) GA:
We need to know two effects:
(a) Stem elongation:
In plants such as peas, genes are partially responsible for determining how tall the plant grows. The dominant allele codes for an enzyme that catalyses the synthesis of active form of GA whereas recessive allele codes for inactive form of GA. Thus, homozygous recessive plants are relatively short. GA, on its part, induces cell DIVISION and ELONGATION in the STEM, so causing the plant to grow tall
(b) Seed germination:
When seeds absorb water by osmosis, embryo are stimulated to produe GA. This GA passes to the ALEURONE layer where it switches on the gene for transcription of certain enzymes, one of them being amylase. It also results in hydrolysis of proteins to amino acids. The amylase produced diffuses to the endosperm. Here it catalyses the hydrolysis of starch to maltose and then the hydrolysis of maltose to glucose. The glucose diffuses to the embryo plant and provides a source of energy for growth of embryo plant. (Thus, we start with an embryo and end up at the embryo.)
(iii) ABA
We need to know the role of ABA in stomatal closure (role of ABA in leaf abscission not required)
ABA is known as a stress hormone as it is secreted by plants when subjected to difficult environmental conditions (which for A level invariably means low water conditions due to low or high tempr). ABA binds to the ABA receptors on the plasma membrane of the guard cells. On binding, it inhibits the proton pump. This stops hydrogen ions being pumped out, so potassium ions do not enter any further and rather the cell loses K+ ions. Thus the water potential of the guard cells increases and thus the cell loses water to the surrounding cells by osmosis down the water potential gradient. This reduces turgor and makes the guard cells flaccid, eventually closing the stomata
