Plant Development: Seed Germination

• Plant development encompasses the entire life cycle of a plant, from seed germination to vegetative growth, flowering, fertilization, and seed production.
• Plant development is a continuous process influenced by both internal genetic factors and external environmental cues.

Seed Germination

• Seed germination is the process by which a dormant seed resumes active growth, leading to the emergence of a seedling.
• It begins with water imbibition, which activates metabolic processes within the seed.
• Water uptake leads to the swelling of the seed and the rehydration of cellular components.
• Germination depends on favorable environmental conditions, including adequate moisture, suitable temperature, and often, the presence of light or darkness.
• The radicle, or embryonic root, is typically the first structure to emerge from the seed, followed by the plumule, which gives rise to the shoot.
• Seed dormancy is a state in which seeds are prevented from germinating even under optimal conditions.
• Dormancy can be caused by a variety of factors, including a hard seed coat, the presence of germination inhibitors, or the requirement for a period of after-ripening.
• Scarification, stratification, and hormone treatments can break seed dormancy.
  • Scarification: Physical or chemical abrasion of the seed coat to allow water imbibition.
  • Stratification: Exposing seeds to a period of cold, moist conditions.
  • Hormone treatments: Application of gibberellins or other plant growth regulators.

Root Growth

• Root growth is essential for anchorage, water and nutrient uptake, and interaction with soil microorganisms.
• The root apical meristem (RAM) is responsible for the continuous production of new cells.
• The root cap protects the RAM as the root grows through the soil and also perceives gravity.
• Root cells elongate and differentiate into specialized cell types, including epidermal cells, cortical cells, and vascular tissue.
• Root hairs, extensions of epidermal cells, increase the surface area for water and nutrient absorption.
• Root growth is influenced by gravity (gravitropism), light (phototropism), and water (hydrotropism).
• Auxin plays a key role in gravitropism by redistributing to the lower side of the root, inhibiting cell elongation in that region and causing the root to bend downward.
• Mycorrhizae, symbiotic associations between roots and fungi, enhance nutrient uptake, particularly phosphorus.
• Root architecture is plastic and adapts to local soil conditions.
• Nutrient availability, water stress, and the presence of beneficial or harmful microbes can affect root branching and elongation.

Leaf Development

• Leaf development occurs at the shoot apical meristem (SAM).
• Leaf primordia initiate on the flanks of the SAM and develop into mature leaves.
• Leaf development involves three main stages: initiation, blade expansion, and differentiation.
• The adaxial-abaxial axis (top-bottom) is crucial for proper leaf development.
• Genes such as PHABULOSA and REVOLUTA specify adaxial identity, while KANADI genes specify abaxial identity.
• The proximal-distal axis (base-tip) and the mediolateral axis (midrib-margin) are also important for leaf shape and venation.
• Leaf shape is highly variable among plant species and is influenced by genetic and environmental factors.
• Simple leaves have a single, undivided blade, while compound leaves have multiple leaflets.
• Leaf size and shape are affected by light intensity, temperature, and nutrient availability.
• Shade leaves tend to be larger and thinner than sun leaves to maximize light capture in low-light environments.
• Leaf senescence is the process of aging and eventual death of leaves.
• During senescence, nutrients are remobilized from the leaves to other parts of the plant, such as developing seeds or storage organs.
• Chlorophyll is broken down, resulting in the yellowing or browning of leaves.
• Leaf abscission is the process by which leaves detach from the plant. An abscission zone forms at the base of the petiole, weakening the connection between the leaf and the stem.