AIM:Seeds and Seed Germination © 2008 Paul Billiet ODWS

Seed structure Cotyledon Plumule Seed coat or testa Radicle Micropyle Seed coat: Outer layer of the seed in angiosperms Micropyle: Pore through which the seed absorbs water. Cotyledon: Where the endosperm is found; provides nutrients. Plumule: leaf in early development Radicle: emerges first, develops into root system. © 2008 Paul Billiet ODWS

Dormancy Metabolism falls Number of organelles per cell falls Dehydration – water content falls Vacuoles in cells deflate Food reserves become dense crystalline bodies Viability refers to how long the seed can remain before it cannot produce a seedling. Dormancy is a period of time in which the seed ‘rests’ before germination. © 2008 Paul Billiet ODWS

Maintaining dormancy Physical barriers The seed coat (testa) is waxy = waterproof and impermeable to oxygen Physical state – dehydrated Chemical inhibitors present e.g. salts, mustard oils, organic acids, alkaloids Growth promoters absent © 2008 Paul Billiet ODWS

Seed viability Viability: When a seed is capable of germinating after all the necessary environmental conditions are met. Average life span of a seed 10 to 15 years. Some are very short-lived e.g. willow (< 1 week) Some are very long-lived e.g. mimosa 221 years Conditions are very important for longevity Cold, dry, anaerobic conditions These are the conditions which are maintained in seed banks © 2008 Paul Billiet ODWS

Germination: The breaking of dormancy The growth of the embryo and its penetration of the seed coat Break down of barriers Abrasion of seed coat (soil particles) Decomposition of seed coat (soil microbes, gut enzymes) Cracking of seed coat (fire) Change in physical state - rehydration Seed absorbs water and seed coat gets burst. It is the first sign of germination. 2. There is an activation of enzymes, increase in respiration and plant cells get duplicated. A chain of chemical changes starts which leads to development of plant embryo. 3. Chemical energy stored in the form of starch is converted to sugar, which is used during germination process. Soon, embryo gets enlarged and seed coat burst opens. 4. Growing plant emerges out. Tip of root first emerges and helps to anchor the seed in place. It also allows embryo to absorb minerals and water from soil. Some seeds require special treatment of temperature, light or moisture to start germination. Destruction and dilution of inhibitors Light, temperature, water Production of growth promoters © 2008 Paul Billiet ODWS

Monocots vs. Dicots

Germination in Dicots Radicle (primary root) emerges from seed. Hypocotyl emerges from seed and grows up through soil. The two cotyledons protect the pumule from damage. The primary root emerges through the seed coats while the seed is still buried in the soil. The hypocotyl ("below the cotyledons") emerges from the seed coats and pushes its way up through the soil. It is bent in a hairpin shape — the hypocotyl arch — as it grows up. The two cotyledons protect the plumule (immature leaves) — the epicotyl ("above the cotyledons") and first leaves — from mechanical damage. Once the hypocotyl arch emerges from the soil, it straightens out. This response is triggered by light. Both red light, absorbed by phytochrome and blue light, absorbed by cryptochrome can do the job. The cotyledons spread apart exposing the epicotyl with the apical meristem at its tip, and two primary leaves In many dicots, the cotyledons not only transfer their food stores to the developing plant but also turn green and make more food by photosynthesis until they drop off.

Germination in Monocots Primary root pierces seed (and fruit) and grows down. Primary leaf grows up, protected by the coleoptile. When grass seeds — like corn (maize) or oats (shown here) — germinate, the primary root pierces the seed (and fruit) coverings and grows down; the primary leaf of the plant grows up. It is protected as it pushes up through the soil by the coleoptile — a hollow, cylindrical structure. Once the seedling has grown above the surface, the coleoptile stops growing and the primary leaf pierces it.

Factors affecting seed germination Over watering can prevent the plant from getting enough oxygen. If seed is deeply planted in soil, it may use all of its stored energy before reaching the surface. Dry conditions can prevent germination, as the seed will not get enough moisture. Some seeds have such hard seed coats that oxygen and water can’t get through it. If soil temperature is extremely low or high, then it can affect or prevent the germination process.