Plant Reproduction and

Multiplication

1. Sexual Reproduction

2. Asexual Reproduction

3. Micropropagation

Sexual Versus Asexual Propagation

� Sexual propagation

- plants are propagated from seeds

- gives rise to a seedling population

� Asexual (vegetative) propagation

- plants are propagated from vegetative tissue or organs

- gives rise to a clonal population

Sexual Reproduction

Reproductive Development in

Angiosperms

Dilkes et al. 2004. Plant Cell. 16: 3174-3180

Embryo Development in Arabidopsis

Laux et al. (2004) Plant Cell 16:S190-S202

Transition of Seed from Development to Germination

vivipary

recalcitrant

orthodox

non-dormant dormant

Fre

sh w

eig

ht

Seed germinationSeed development

Maturation drying

Cell expansion

Histodifferentiation

Fre

sh w

eig

ht

Time

Orthodox seeds dry to ~10% of moisturecontent towards the end of seed development

Vivipary and recalcitrant seeds start togerminate before completing the maturationdrying stage

Hartmann et al. (2002) Hartmann and Kester’s Plant Propagation – Principles and Practices 7th ed. Prentice Hall, New Jersey, USA.

Early Stages of Seed Germination

� Some seeds germinate rapidly (non-dormant seeds) while others do not readily germinate (dormant seeds)

� Germination starts with the uptake of water by the dry seed and is

completed when the radicle elongates and breaks through the

layers surrounding the embryo

a. Water uptake by imbibition

b. Lag phase of germination

c. Radicle protrusion

Hartmann et al. (2002) Hartmann and Kester’s Plant Propagation – Principles and Practices 7th ed. Prentice Hall, New Jersey, USA.

Abiotic Factors Influencing Seed Germination

� Temperature

- temperature is positively correlated to germination rate

- optimal temperature for non-dormant seeds: 25 – 30oC; some require 15oC

� Water

- water stress can reduce the rate of germination

� Air

- O2 is required for the respiratory process in germinating seeds

- increased CO2 in the soil may inhibit seed germination

� Light

- some are light sensitive whereas others are light dependent

Hartmann et al. (2002) Hartmann and Kester’s Plant Propagation – Principles and Practices 7th ed. Prentice Hall, New Jersey, USA.

Patterns of Seed Germination

Hypocotyl is the stemsection between thecotyledons and theradicle.

Epicotyl is the sectionbetween the cotyledonsand the first true leaves.

Epigeous germination (a):

hypocotyl elongates, forms a hook and raises

the cotyledons above the ground

Hypogeous germination (b,c):

hypocotyl fails to elongate, only the epicotyl

emerges above the ground, while cotyledons

remain below ground

Definition of Seed Dormancy

“A dormant seed does not have the capacity to germinate in aspecified period of time under any combination of normal physicalenvironmental factors that are otherwise favourable for itsgermination”

[Baskin JM, Baskin CC. (2004) A classification system for seed dormancy. Seed Science Research 14: 1–16]

Classification of Seed Dormancy

i. Physiological dormancy (PD)

- most abundant and prevalent form

- factors within the embryo that inhibit germination - requires treatments (scarification, after-ripening in dry storage, moist-chilling stratification, GA) to break dormancy

- examples: A. thaliana, Helianthus annuus, Lycopersicon esculentum, Nicotiana spp.

ii. Morphological dormancy (MD)

- embryos are underdeveloped but differentiated - embryos are underdeveloped but differentiated

- embryos are not (physiologically) dormant but simply need time to grow and germinate

iii. Morphophysiological dormancy (MPD)

- underdeveloped embryos which are physiologically dormant

- requires treatments (warm and/or cold stratification, GA application) to break dormancy

iv. Physical dormancy (PY)

- caused by water-impermeable layers of palisade cells in the seed or fruit coat

- requires mechanical or chemical scarification to break dormancy

v. Combinational dormancy (PY + PD)

- water-impermeable coats with physiologically dormant embryos

Baskin JM, Baskin CC. (2004) A classification system for seed dormancy. Seed Science Research 14: 1–16

Morphology of Embryo and Endosperm in Angiosperm Seeds

ND = non-dormancy

PD = physiological dormancy

MD = morphological dormancy

MPD = morphophysiological dormancy

PY = physical dormancy

PY+PD = combinational dormancy

Seed types:

i. Basal - B1 to B4 (abundant endosperm and a tiny embryo)

Finch-Savage & Leubner-Metzger (2006) New Phytologist 171 (3): 501-523

endosperm and a tiny embryo) ii. Axile – LA (linear axile embryo)iii. Peripheral - Piv. Dwarf seeds - MAv. Foliate axile - FA1 to FA4 (nutrient

stored in cotyledon, little or no endosperm)

[Martin (1946) American Midland Naturalist 36: 513–660]:

Seed Dormancy Release and Germination

(a) Nicotiana sp. seed germination – two steps: testa rupture followed by endosperm rupture

Dormancy release and germination promotion occur during after-ripening dry storage (room temperature for several months) or via the light-GA pathway during imbibition

- GA, ethylene and brassinosteroids (BRs)

promote endosperm rupture

(βGlu I = class I β-1,3-glucanase)

Brassica napus seed germination – a one-stepevent: testa rupture plus initial radicle elongation

Finch-Savage & Leubner-Metzger (2006) New Phytologist 171 (3): 501-523

Mature seed

without

endosperm

- ABA does not inhibit testa rupture

- ABA inhibits subsequent radicle growth

promote endosperm rupture- ABA inhibits endosperm rupture but not testa rupture

Terminology

• After-ripening“the loss of the dormant state over some period of time through exposure of

the seeds to a set of environmental conditions after maturation and

separation from the parent plant” (Simpson, G.M. 1990. Seed Dormancy in Grasses. New

York: Cambridge University Press)

• Scarification• Scarification“The physical or chemical treatment given to some seeds in order to weaken

the seed coat sufficiently for germination to occur”

(www.hcs.ohio-state.edu/mg/manual/glossary.htm)

• Stratification

“Storing of seeds at low temperatures under moist conditions in order to

break dormancy”(www.hcs.ohio-state.edu/mg/manual/glossary.htm)

End of Lecture