Seeds

Question 1

What is the definition of a seed?

Answer 1

A seed is a reproductive structure (propagule) which is formed by the maturation of the ovule of seed plants, following fertilization

Question 2

What are the advantages of seeds?

Answer 2

1. Seed coat (testa) provides protection e.g. from mechanical damage.
2. More reserves than a spore - endosperm or cotyledon provides nourishment for the seedling while it germinates.
3. Seeds can lie dormant until conditions for growth are more favourable (survive - winter, drought)
4. Seeds usually provide a repository of genetic variation (meiosis, cross pollination)
   - No genetic variation in cuttings and tuber
5. Means of dispersal – animals, wind, projectile, water..

Question 3

What makes up a seed?

Answer 3

• 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

Question 4

What are the comparisons between monocot and dicot seeds?

Answer 4

“Grasses”
• Onecotyledon • • Starch stored outside the •
embryo (endosperm) • Hypogealgermination
“Broad leafs” Twocotyledons
Energy stored within the embryo (most cases)
• Bothhypogealand epigeal germination

Question 5

What conditions are needed for seed germination?

Answer 5

1. Adequate water
2. Suitable temperature
3. Adequate oxygen
4. Adequate light level (if needed)
5. Absence of inhibitors/toxins

Question 6

Adequate Water

Answer 6

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)

Question 7

Suitable Temperature

Answer 7

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)

Question 8

Oxygen

Answer 8

• C6H12O6 +O2 → CO2 + H2O + “ENERGY” • respiration „reverse‟ of photosynthesis
• aerobic vs. anaerobic respiration
• rate of respiration dependent upon the water content of the seed

Question 9

Light

Answer 9

• 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

Question 10

Light and Temperature Interactions

Answer 10

• 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?

Question 11

Inhibiting Conditions

Answer 11

• 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)

Question 12

What does Seed Coat Dormancy mean?

Answer 12

• Impermeable to Water (physical)
• Impermeable to Oxygen and
CO2 (physical)
• Mechanical Resistance (physical)
• Germination-inhibiting chemicals
(physiological)

Question 13

How is Seed Coat Dormancy overcome?

Answer 13

• 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

Question 14

What does Physiological Dormancy mean?

Answer 14

• 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

Question 15

What are some advantages of seed dormancy?

Answer 15

• 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

Question 16

What are the processes of seed germination?

Answer 16

1. Imbibition
2. Osmotic forces
3. Enzyme re-activation
4. Seedling growth

Question 17

Imbibition

Answer 17

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

Question 18

Osmotic Forces

Answer 18

2. 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

Question 19

Enzyme re-activation

Answer 19

3. 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
4. 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”

Question 20

Seedling Growth

Answer 20

4. 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)

Question 21

Why roots then shoots?

Answer 21

• Survival & growth can not occur without water

* Endosperm or cotyledons contain energy to enable root growth then turn on photosynthesis

Question 22

Hypogeal Vs Epigeal Germination

Answer 22

• 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

Question 23

Meaning Of Terms

Answer 23

• 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)

Question 24

What is Hypogeal Germination?

Answer 24

• 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

Question 25

What is Epigeal Germination?

Answer 25

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)

Question 26

What is Etiolation?

Answer 26

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.