Plant Physiology - Plant Growth and Development (Seed Germination, Plant Growth Regulators - Auxin, Gibberellin, Cytokinin, Ethylene, ABA)

Seed Germination: The process where a seed develops into a seedling. It requires essential conditions such as $H_2O$ (for hydration and enzyme activation), $O_2$ (for aerobic respiration), and an optimum temperature (usually $25^{\circ}C$ to $30^{\circ}C$).

Types of Germination: In Epigeal germination, the hypocotyl elongates and pushes the cotyledons above the soil (e.g., Bean, Castor). In Hypogeal germination, the epicotyl elongates, and cotyledons remain below the soil (e.g., Pea, Maize).

Auxins (e.g., $IAA$ - Indole-3-acetic acid): Produced at shoot and root apices. They promote apical dominance (inhibiting lateral bud growth) and mediate phototropism by migrating to the shaded side of the stem.

Gibberellins ($GA$): A group of acidic PGRs (e.g., $GA_3$) that promote internodal elongation, 'bolting' (sudden stalk elongation before flowering in rosette plants), and break seed dormancy by inducing $\alpha$-amylase synthesis.

Cytokinins: Hormones synthesized in regions of rapid cell division (roots, developing shoots). They promote cytokinesis (cell division), delay leaf senescence (Richmond-Lang effect), and help overcome apical dominance.

Ethylene ($C_2H_4$): A gaseous plant growth regulator. It is primarily involved in fruit ripening, promotes senescence, and induces horizontal growth of seedlings.

Abscisic Acid ($ABA$): Known as the 'stress hormone' or growth inhibitor. It induces stomatal closure during water stress and promotes seed dormancy, acting as an antagonist to $GA$.