Seeds Flashcards
What is the definition of a seed?
A seed is a reproductive structure (propagule) which is formed by the maturation of the ovule of seed plants, following fertilization
What are the advantages of seeds?
- Seed coat (testa) provides protection e.g. from mechanical damage.
- More reserves than a spore - endosperm or cotyledon provides nourishment for the seedling while it germinates.
- Seeds can lie dormant until conditions for growth are more favourable (survive - winter, drought)
- Seeds usually provide a repository of genetic variation (meiosis, cross pollination)
- No genetic variation in cuttings and tuber - Means of dispersal – animals, wind, projectile, water..
What makes up a seed?
• seed = embryo + seedcoat (testa) OR seed = embryo + testa + endosperm
• embryo = plumule + radicle + cotyledon(s)
• plumule : the first bud of an embryo; part of the
embryonic axis above the cotyledonary node, almost the
same as epicotyl
• radicle : embryonic root
• hypocotyl: region below the cotyledon node and true root
is where stem & root anatomy „swap over‟
• cotyledon :first leaf of an embryo; 1 (monocot) or 2
(dicot) per seed; functions in:
– foodstorageor
– absorptionfromtheendospermor
– photosynthesis
What are the comparisons between monocot and dicot seeds?
“Grasses”
• Onecotyledon • • Starch stored outside the •
embryo (endosperm) • Hypogealgermination
“Broad leafs” Twocotyledons
Energy stored within the embryo (most cases)
• Bothhypogealand epigeal germination
What conditions are needed for seed germination?
- Adequate water
- Suitable temperature
- Adequate oxygen
- Adequate light level (if needed)
- Absence of inhibitors/toxins
Adequate Water
Rate at which seeds take up water (imbibition) based upon:
1.Permeability of the seed coat (testa)
scarification (scratching of water impermeable seed coat – e.g. your Acacia seeds)
low temperature treatment (stratification)
2.Seed composition.
starchy seeds take up water quickly (corn or wheat) oily or fatty seeds take up slower (canola or peanut)
3.Whether the water is in the form of: liquid
vapour (high humidity)
Suitable Temperature
Based upon 3 “CARDINAL” temperatures
1. Minimum temperature below which no germination
2. Optimal temperature = highest % germination in the shortest time
3. Maximum temperature above which no germination
Beyond the maximum are lethal temperatures = seed death (~50oC)
Oxygen
- C6H12O6 +O2 → CO2 + H2O + “ENERGY” • respiration „reverse‟ of photosynthesis
- aerobic vs. anaerobic respiration
- rate of respiration dependent upon the water content of the seed
Light
• Most seeds are light independent relative to germination
• Exceptions:
– germinate only in the dark
– germinate only in the light
– germinate only after a short light exposure
– germinate only after illumination with Far-Red light
Light and Temperature Interactions
• Light or temperature alone may be dangerous as germination triggers
• Combinations often allow for more ideal growth conditions
• Small seeds (e.g. Canola)
– Often sown shallow because of:
• Small energy reserves (barely enough energy to reach the light)
• Requirement for light to germinate • Which came first?
Inhibiting Conditions
• Internal
– Compounds present in the seed coat
• External
– Gases – excess CO2 – Salts
– Chemicals
Most of these can be washed or LEACHED away by large amounts of water.
• Water, temperature, O2 and no toxins/inhibitors seeds still may not germinate
• termed dormancy
– Imposed by the seed coat (physical and/or physiological
– Imposed by the embryo (physiological)
What does Seed Coat Dormancy mean?
• Impermeable to Water (physical) • Impermeable to Oxygen and CO2 (physical) • Mechanical Resistance (physical) • Germination-inhibiting chemicals (physiological)
How is Seed Coat Dormancy overcome?
• Human intervention
– Cutting / scratching the seed coat – Chemical (acid) treatment
• Natural means
– Scarification (scratching) of the seed coat by soil or animals etc.
– Freeze-thaw cycles
– Fire
– Cycles of water to leach out inhibitors
What does Physiological Dormancy mean?
- Embryo is not yet fully developed (e.g. orchid seeds require symbiotic fungi)
- Requirement for energy storage
- Requirement for light (specific wavelength)
- Requirement for chilling (stratification) (Hormones: Abscisic acid deactivation, Gibberellin synthesis)
- Combinations of some or all of the above
What are some advantages of seed dormancy?
- Can delay germination until environmental conditions are favourable.
- Creation of a seed bank
- Can help synchronize germination to a particular time of the year
- Can facilitate seed dispersal
What are the processes of seed germination?
- Imbibition
- Osmotic forces
- Enzyme re-activation
- Seedling growth
Imbibition
1. Imbibition • Process of taking up water • Seed need not be viable (alive) • Simply soaking up water • Must enter through the seed coat • Increases seed volume – often ruptures seed coat
Osmotic Forces
- Osmotic forces – Osmotic pressure forces water into cells of the seed
• Re-hydration of membranes and protoplasm of the seed cells
• Seeds regain pre-desiccation size and shape
• Can get seed “leakiness‟ – some pathogens can detect this and attack
Enzyme re-activation
- Enzyme re-activation
• First stage that requires a viable seed (living)
• Mobilizes stored energy by breaking down starches (grains), proteins (beans) and fats (oilseeds) to simple energy rich compounds - Enzyme re-activation cont.
• Inmonocots,theendosperm(starch)istheenergy storage but sugar is required for energy
• surroundedbythealeuronelayer
• thintissueproducingtheenzymeα-amylase
• α-amylaseconvertsstarchtosugar
• sugarisusedtopowergermination
Anyone consuming a “Cleansing Ale” would have partaken of the result of the conversion of starch to
sugar known as “MALTING”
Seedling Growth
- Seedlinggrowth
• Metabolic rate increases
• Embryo increases in size
• Water absorption continues through “root”
• Shoot portion grows upward
• Germination ends when the seedling becomes photosynthetically self sufficient (stops living on stored nutrient)
Why roots then shoots?
- Survival & growth can not occur without water
* Endosperm or cotyledons contain energy to enable root growth then turn on photosynthesis
Hypogeal Vs Epigeal Germination
• All the 4 processes of germination are common to both hypo- & epigeal germination
• The difference is which part of the seedling elongates:
– Hypocotyl – region of plant axis between the roots and cotyledons (below the cotyledons), or
– Epicotyl – region of shoot above the cotyledons
Meaning Of Terms
- Hypo = Below
- Geal = Ground
- Epi= Above
- Cotyl = Cotyledon
- So in hypogeal germination the cotyledon remains below ground because the shoot above it elongates (ie the epicotyl)
- In epigeal germination the cotyledon is taken above ground because the shoot below it elongates (hypocotyl)
What is Hypogeal Germination?
- Hypogeal germination, the epicotyl elongates
- Cotyledons remain under the soil
- First “leaf-like” organs are photosynthetically active true leaves
- e.g. pea and corn germination
What is Epigeal Germination?
Epigeal germination, the Hypocotyl elongates
• Cotyledonsarecarriedabovethesoil
• First“leaf-like”organsarethecotyledonsand
are photosynthetically active (are green) but are not specialised for photosynthesis (are thick and solid – poor gas exchange)
• e.g.Beans(themusicalfruit!i.e.withpod)
What is Etiolation?
Etiolation - a means of getting to the light!
• If a seedling grows in the absence of light it:
• Elongates its internodes • Does not turn green
• Does not produce energy
• Doesitgrow?
• Seedlingsincreaseinsize but technically do not grow.
