Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

7. Plant Growth and Development
7.2 Juvenile Stage

The juvenile stage in seed plants begins with the germination of seeds or vegetative propagation Organs and is characterized by the rapid accumulation of vegetative biomass. During this period, plants are incapable of sexual reproduction. The seed germination process is divided into phases: imbibition, radicle emergence (sprouting), heterotrophic seedling growth, and the transition to an autotrophic mode of Nutrition.

Imbibition. Water uptake by the seed serves as the triggering factor for germination. It occurs due to an increase in the permeability of the seed coats to water and through the Hydration of Biopolymers within the Cells. As a result, osmotic pressure (imbibition pressure) develops, causing the seed coats to rupture. Swelling is essentially independent of Temperature, O2 content, or light.

Radicle emergence begins when seed moisture reaches 40–65%. It occurs through the growth and elongation of the embryonic ROOT or hypocotyl, which pushes the root tip out of the seed. In grasses, it additionally pierces the root sheath, the coleorhiza. Cell Division generally occurs later. Elongation growth is enabled by a decrease in ABA levels during imbibition. The Emergence of the root ensures the anchorage of the germinating seed in the soil and improves water uptake.

Heterotrophic growth. Following the growth of the root, the SHOOT develops. Germinating in the dark, both root and shoot orient themselves primarily along the gravitational vector. The growth

of the axial Organs of the embryo and seedling is supported by phytohormones. The growth of the embryonic radicle is accompanied by the appearance of distinct zones of cell division, elongation, and differentiation. The root itself begins to synthesize Cytokinins and GAs, which are transported to the shoot. The shoot elongates due to the stretching of the hypocotyl (in dicots) or mesocotyl (in grasses). The hypocotyl bends sharply to form an apical hook, which facilitates its movement through the soil. Indole-3-acetic acid is synthesized in the bud of dicots and the apex of the coleoptile (a colorless sheath leaf) in grasses. Once the etiolated shoot reaches the soil surface, photogrowth and photomorphogenetic responses are triggered: hypocotyl or mesocotyl elongation is sharply inhibited, while the growth of the epicotyl (the first true internode) and leaves is promoted. The Ethylene content in the hook region decreases, causing the hook to straighten. The plant turns green and transitions to a phototrophic mode of nutrition. As the main, lateral, and adventitious roots continue to grow, shoots form, and the stem thickens, the plant accumulates significant vegetative mass by the end of the juvenile period. The duration of the juvenile period varies among plants, ranging from a few weeks (in annual grasses) to decades (in trees).

The juvenile stage is characterized by a complete absence of flowering. However, the lack of flowering is not necessarily an indicator of juvenility, as many mature plants do not flower for extended periods. Juvenile growth exhibits a higher capacity for root formation, a property widely utilized in horticulture. The juvenile state is maintained by a specific hormonal balance. The transition from juvenile to mature growth is most pronounced in woody plants. In the beech tree, for instance, all developmental phases—from juvenility to maturity—coexist.