Underground: Seeds and Roots

What do seeds contain?

An embryo, nutrient source, and a protective covering

Embryogenesis

The development of an embryo

What are the embryogenesis steps in eudicots?

1) The developing seed contains the endosperm and the zygote (The endosperm is triploid while the zygote is diploid)

2) The zygote divides to form the proembryo which is anchored to the micropyle by the suspensor. The endosperms has begun to divide.

3) The embryo proper is in the globular stage, a spherical mass of cells. Endosperm continues to divide.

4) The embryo proper is in the heart stage - the heart shape gives rise to cotyledons. The endosperm continues to divide.

5) The embryo proper is in the torpedo stage - the shoot apical meristem is between the cotyledons, and the root apical meristem is anchored to the suspensor.

6) The mature embryo includes the radicle, the hypocotyl, the epicotyl, and the ovule has developed into a seed coat.

What three stages does the embyro proper undergo in eudicots?

The globular stage, the heart stage, and the torpedo stage

Hilum

Where the seed was attached to the endocarp (inner layer of the fruit)

Micropyle

Where the pollen tube entered

What is the seed coat divided into?

Testa (outer coat) and tegmen (inner coat)

What forms the seed coat?

Integument of the ovule

What does the embryonic axis include?

The plume and the radicle

Plume

A young shoot containing:

- the shoot apical meristem

- the leaf primordia (developing leaves)

Radicle

The embryonic root (not always apparent)

Hypocotyl

The region between cotyledon attachment point and radicle

Epicotyl

The region between cotyledon attachment point and shoot tip

Scutellum

Name of the single cotyledon

It has vascular connections to the rest of the embryo

Aleurone

Outer layer of endosperm, secretes enzymes during germination

Disperal methods

Wind (drifting or spinning), water (floating), animals (ingested or sticky), mechanic (bursting, blastng)

How can seeds delay germination

Barriers in the seed coat and the embryo

Barriers in the seed coat

Physical dormancy: impermeability to water or oxygen

Chemical dormancy: compounds in seed coat inhibit germination

Barriers in the embryo

Physiological dormancy: embryo requires environmental conditions to complete maturation

Endodormancy: internal biochemical processes must be met before germination can begin

Ecodormancy: external factors are not optimal for germination

Germination requirements

Sufficient light (usually for smaller-seeded species)

Proper temperature (sometimes including a period of cold - stratification - or heat)

Oxygen (essential for cellular respiration)

Water (imbibition is important to start vigorous metabolism and water can leach away inhibitors)

Steps to initiate germination

Imbibition (water uptake)

Enzyme activation

Breakdown of starch in to sugar

Two germination types

Epigeous and hypogeous

Epigeous

The hypocotyl elongates, and cotyledons extend above ground

Hypogeous

The epicotyl elongates, and the cotyledon(s) remain below ground

Germination in epigeous eudicots

Ex beans

The hypocotyl is shaped like a hook with the plume pointing downwards (plume hook)

The plume in encased by cotyledons to be protected from damage

After emergence, the hypocotyl elongates

Germination in hypogeous eudicots

Ex peas

The epicotyl froms a hook

The cotyledons and hypocotyl remain underground

Germination in epigeous monocots

Ex onions

The single monocot bends into a hook and emerges before the coleoptile

Germination in hypogeous monocots

The cotyledon remains below ground and the coleoptile emerges first

Two types of root systems

Tap root system and fibrous root system

Root functions

Anchorage, support, absorption, nutrient storage

Root anchorage

Roots keep the plant in place AND help stabilize the soil, preventing erosion

Root support

Provide the foundation for upright growth (particularly tap roots)

Root absorption

Absorb the water and nutrients needed by the plant

- Symbiotic interactions: roots form connections with rhizobia bacteria and micorrhyzal fungi, which can help it obtain nutrients

Root nutrient storage

Can swell and store high-energy compounds like starch

Tap root system

Persists throughout the life of the plant

Has a primary root (tap root), forming a central axis, from which lateral roots branch at irregular intervals

- Branching responds to resources in the soil

- The primary root growth vertically

- The later roots grow relatively parallel to the soil

Fibrous root system

Begins with the radicle; however, the primary root stops growing and rots away

New roots from directly from the stem tissue (adventitious roots)