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Botany PSC103 > Seeds > Flashcards

Seeds Flashcards

1
Q

What is the definition of a seed?

A

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

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2
Q

What are the advantages of seeds?

A
  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..
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3
Q

What makes up a seed?

A

• 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

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4
Q

What are the comparisons between monocot and dicot seeds?

A

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

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5
Q

What conditions are needed for seed germination?

A
  1. Adequate water
  2. Suitable temperature
  3. Adequate oxygen
  4. Adequate light level (if needed)
  5. Absence of inhibitors/toxins
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6
Q

Adequate Water

A

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)

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7
Q

Suitable Temperature

A

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)

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8
Q

Oxygen

A
  • C6H12O6 +O2 → CO2 + H2O + “ENERGY” • respiration „reverse‟ of photosynthesis
  • aerobic vs. anaerobic respiration
  • rate of respiration dependent upon the water content of the seed
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9
Q

Light

A

• 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

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10
Q

Light and Temperature Interactions

A

• 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?

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11
Q

Inhibiting Conditions

A

• 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)

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12
Q

What does Seed Coat Dormancy mean?

A 
• Impermeable to Water (physical)
• Impermeable to Oxygen and
CO2 (physical)
• Mechanical Resistance (physical)
• Germination-inhibiting chemicals
(physiological)
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13
Q

How is Seed Coat Dormancy overcome?

A

• 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

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14
Q

What does Physiological Dormancy mean?

A
  • 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
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15
Q

What are some advantages of seed dormancy?

A
  • 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
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16
Q

What are the processes of seed germination?

A
  1. Imbibition
  2. Osmotic forces
  3. Enzyme re-activation
  4. Seedling growth
17
Q

Imbibition

A 
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
18
Q

Osmotic Forces

A
  1. 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
19
Q

Enzyme re-activation

A
  1. 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
  2. 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”
20
Q

Seedling Growth

A
  1. 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)
21
Q

Why roots then shoots?

A
  • Survival & growth can not occur without water

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

22
Q

Hypogeal Vs Epigeal Germination

A

• 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

23
Q

Meaning Of Terms

A
  • 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)
24
Q

What is Hypogeal Germination?

A
  • 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
25
Q

What is Epigeal Germination?

A

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)

26
Q

What is Etiolation?

A

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.

Botany PSC103 (25 decks)

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