Faculty of Science, School of Sciences, Natabua Campus Lautoka BIO706 Embryology Lecture 30: Seeds and Seed Germination

Seeds and Seed Germination

Seed structure Cotyledon Plumule Radicle Micropyle Seed coat or testa

Seed maturation Takes place in the fruit on the parent plant Endospermous seeds: Retain the endosperm tissue, which eventually dies but it is surrounded by a layer of living cells, the aleurone layer. Non-endospermous seeds: The endosperm tissue is absorbed by the cotyledons. These then become the food reserve for the seed.

Dormancy Metabolism falls Number of organelles per cell falls Dehydration – water content falls Vacuoles in cells deflate Food reserves become dense crystalline bodies

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

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

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 Destruction and dilution of inhibitors Light, temperature, water Production of growth promoters

Germination STAGE EVENTS PREGERMINATION Rehydration – imbibition of water. RNA & protein synthesis stimulated. Increased metabolism – increased respiration. Hydrolysis (digestion) of food reserves by enzymes. Changes in cell ultrastructure. Induction of cell division & cell growth. GERMINATION Rupture of seed coat. Emergence of seedling, usually radicle first. POST GERMINATION Controlled growth of root and shoot axis. Controlled transport of materials from food stores to growing axis. Senescence (aging) of food storage tissues.

Stages leading to cell division Respiration Initially anaerobic Later aerobic Mitchondria reconstituted Soluble sugars ATP RNA activated Polysomes http://www.rbgsyd.nsw.gov.au/ Protein synthesis (0.5h) Enzymes (proteins) DNA synthesis (45h) Mitosis (70h)

The control of food reserve hydrolysis Control by growth promoters such as gibberellin and growth inhibitors such as abscisic acid These directly affect the genes for enzyme synthesis or the activity of the enzymes themselves The growth substances are affected by environmental factors (e.g. light, temperature, humidity)

The control of food reserve hydrolysis Negative feedback control of enzymes The action of the enzyme also limited by substrate Once all the starch in an amyloplast is hydrolysed the enzyme stops work Therefore the release of the stored food is adjusted to suite the demand Starch + H20 Maltose  - amylase Negative feedback

The mobilisation of food reserves Carbohydrates Starches (amylopectin & amylose) Amylases Maltose and glucose Proteins e.g. Zein Proteases Amino acids Lipids Oils Lipases Fatty acids & glycerol The food reserves are stored as large insoluble macromolecules They are hydrolysed using enzymes to smaller soluble molecules for transport

Germination of seeds depends on imbibition, the uptake of water due to the low water potential of the dry seed. This causes the expanding seed to rupture its seed coat and triggers metabolic changes in the embryo that enable it to resume growth. Enzymes begin digesting the storage materials of endosperm or cotyledons, and the nutrients are transferred to the growing regions of the embryo.

The first organ to emerge from the germinating seed is the radicle, the embryonic root. Next, the shoot tip must break through the soil surface. In garden beans and many other dicots, a hook forms in the hypocotyl, and growth pushes it aboveground. Stimulated by light, the hypocotyl straightens, raising the cotyledons and epicotyl.

As it rises into the air, the epicotyl spreads its first foliage leaves (true leaves). These foliage leaves expand, become green, and begin making food for photosynthesis. After the cotyledons have transferred all their nutrients to the developing plant, they shrivel and fall off the seedling.

Light seems to be main cue that tells the seedling that it has broken ground. A seedling that germinates in darkness will extend an exaggerated hypocotyl with a hook at its tip, and the foliage leaves fail to green. After it exhausts its food reserves, the spindly seedling stops growing and dies.

Peas, though in the same family as beans, have a different style of germinating. A hook forms in the epicotyl rather than the hypocotyl, and the shoot tip is lifted gently out of the soil by elongation of the epicotyl and straightening the hook. Pea cotyledons, unlike those of beans, remain behind underground.

Corn and other grasses, which are monocots, use yet a different method for breaking ground when they germinate. The coleoptile pushes upward through the soil and into the air. The shoot tip then grows straight up through the tunnel provided by the tubular coleoptile.

The tough seed gives rise to a fragile seedling that will be exposed to predators, parasites, wind, and other hazards. Because only a small fraction of seedlings endure long enough to become parents, plants must produce enormous numbers of seeds to compensate for low individual survival.

This provides ample genetic variation for natural selection to screen. However, flowering and fruiting in sexual reproduction is an expensive way of plant propagation especially when compared to asexual reproduction.

Courtesy: Pearson Education, Inc Courtesy: Pearson Education, Inc., publishing as Benjamin Cummings, relevant books and Internet sources Questions are welcome