SEED GERMINATION Outside, In   When you look at a seed, what you are actually seeing is the seed coat. The seed coat performs much of the same work that your own coat performs …  It provides protection against entry of parasites,  It protects against mechanical injury  In some seeds, it buffers against unfavorably high or low temperatures.  It stops germination until the right time  What would happen if the seed coat failed in fall? Parts of a seed Inside…   Inside of the seed coat, an embryo is protected. The embryo is an immature plant with all of the parts of an adult plant. A close looks shows leaves and roots, albeit very small leaves and roots  The leaves of the embryo are called plumules  The leaves are sheathed by a cotyledon  The embryonic roots are called radicles  The embryonic stem is called the hypocotyl   The seed is filled with nourishment for the new plant; this source of nutrition is called the endosperm. Parts of a seed Germination   The first step of germination is the absorption of water – a lot of water Absorption of water activates enzymes in the seed that stimulate growth.  These enzymes break down starches in the endosperm into sugars that can be used for energy  The deciding factor in whether or not a seed germinates is whether or not energy is available for growth and cell division. Stages leading to cell division Mitchondria reconstituted Respiration Initially anaerobic Later aerobic Soluble sugars ATP RNA activated Polysomes Protein synthesis (0.5h) Enzymes (proteins) DNA synthesis (45h) Mitosis (70h) Seed Dormancy  If moisture is the key to starting seed germination, it should be clear what is necessary for seed dormancy A waterproof seed coat  No oxygen  Chemical inhibitors   For germination to occur, these must be negated Coat broken down; oxygen available  Water absorbed  Growth promoters activated, inhibitors are inhibited.  Germination STAGE EVENTS PREGERMINATION (a) (b) (c) (d) GERMINATION (a) Rupture of seed coat. (b) Emergence of seedling, usually radicle first. POST GERMINATION (a) Controlled growth of root and shoot axis. (b) Controlled transport of materials from food stores to growing axis. (c) Senescence (aging) of food storage tissues. Rehydration – imbibition of water. RNA & protein synthesis stimulated. Increased metabolism – increased respiration. Hydrolysis (digestion) of food reserves by enzymes. (e) Changes in cell ultrastructure. (f) Induction of cell division & cell growth. Germination Factors  Water is clearly the most important factor in germination; an adequate continuous supply of water is necessary for complete emergence.  Water functions a triggering enzyme for starch conversion into sugar, turgor pressure for moving the radicle root down and the cotelydons up, and for transporting nutrients and enzymes within the seed

Seed Germination

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