Seed Plants: Angiosperms

Outline

• Phylum Magnoliophyta

– Gametophyte Development

• Pollination

• Fertilization and Seed Development

• Specialization Trends in Flowering Plants

• Pollination Ecology

• Herbaria and Plant Preservation

Overview

• Angiosperms are plants with seeds contained within a vessel (carpel).

– Largest and most diverse phylum of the Plant Kingdom.

Phylum Magnoliophyta

• Flowering Plants

– Angiosperms are heterosporous.

• Female gametophytes are wholly enclosed within sporophyte tissue and reduced to only a few cells.

• At maturity, male gametophytes consist of a germinated pollen grain with three nuclei.

Phylum Magnoliophyta

• Development of Gametophyte

– Diploid megasporocyte cell differentiates from all other cells in the ovule.

• Undergoes meiosis and produces four haploid megaspores.

– Three degenerate

– Integuments differentiate and eventually become the seed coat.

• Leave micropyle at one end.

Gametophyte Development

• Central Cell Nuclei may become a binucleate cell, or may fuse together, forming a single diploid nucleus.

– One of the cells functions as the egg.

• Female gametophyte (megagametophyte) consists of a large sac usually containing eight nuclei in seven cells.

Mature Megasporocyte

Gametophyte Development

• While megagametophyte is developing, a parallel process that leads to the formation of male gametophytes occurs in the anthers.

Four patches of tissue differentiate from the main cell mass.

• Contain diploid microsporocyte cells which undergo meiosis and produce a quartet of microspores.

Lily Anther Cross Section

Gametophyte Development

• After meiosis, the haploid microspores in the pollen sac undergo several changes.

– Nucleus in each microspore divides by mitosis.

– Members of each quartet of microspores separate from one another.

– A two-layered wall develops around each microspore.

• Microspores are now pollen grains.

– Outer wall - exine.

Pollination

• Pollination is the transfer of pollen grains from an anther to a stigma.

• Fertilization involves the union of an egg and a sperm.

– May not occur until days or weeks or even months after pollination has taken place.

Fertilization and Seed Development

• After pollination, further development of the male gametophyte may not take place unless the pollen grain is:

– From a different plant of the same species.

– From a variety different from that of the receiving flower.

• Under suitable conditions, a pollen tube grows down between the cells of the stigma and style until it reaches the ovule micropyle.

Fertilization and Seed Development

• When the pollen tube reaches the micropyle, it continues to the female gametophyte and discharges its contents.

– Double Fertilization (Fusion) Occurs

• One sperm migrates from the synergid to the egg and forms a zygote.

• Other sperm cell migrates from the synergid and unites with the central cell nuclei producing a triploid endosperm nucleus.

Mature Male Gametophyte

Apomixis and Parthenocarpy

• Apomixis is development or fusion of gametes but with the normal structures otherwise being involved.

– Fruits that develop from ovaries with unfertilized eggs are parthenocarpic.

• Seedless (Navel oranges)

– Seedless watermelon are produced by crossing varieties with different numbers of chromosomes. (Not parthenocarpic)

• Fertilization and seed formation don’t occur.

Specialization Trends in Flowering Plants

• Fossil record suggests flowering plants first appeared about 160 million years ago during the late Jurassic period.

– Most botanists hypothesize primitive flowers had numerous spirally-arranged parts that were not fused together and were variable in number.

• Flowers were regular and contained both stamens and pistils.

Specialization Trends in Flowering Plants

• In advanced flowers, the receptacle has fused to the ovary.

– When the ovary is embedded in the receptacle, it is said to be inferior.

• Flower parts attached to to the top of the ovary are epigynous.

– When the ovary is produced on top of the receptacle, it is said to be superior.

• Flower parts attached around the base are hypogynous.

Specialization Trends in Flowering Plants

• Flowers have tended to become irregular.

• Complete flowers contain a calyx, corolla, stamens, and a pistil.

– Perfect flowers have stamens and a pistil.

• Imperfect flowers are missing either stamens or a pistil.

• Monoecious plants have both male and female imperfect flowers.

– Dioecious plants only have only male or female flowers.

Pollination Ecology

• Throughout the evolutionary history of flowering plants, the pollinators have evidently coevolved with plants.

– Twenty thousand bee species are included among current-day pollinators.

• Many bee-pollinated flowers are delicately sweet and fragrant.

– Flowers pollinated by beetles tend to have different, stronger odors.

Pollination Ecology

• Moth and butterfly-pollinated flowers also often have sweet fragrances.

– Night-flying moths tend to visit white or yellow flowers.

– Butterflies tend to visit bright blue, yellow, or orange flowers.

• Nectaries are at bottom of corolla tubes.

– Situated for specialized mouth parts.

Pollination Ecology

• Flowers visited by birds are often bright red or yellow, and usually have little, if any, odor.

– Typically large flowers.

• Birds are highly active pollinators and tend to burn energy rapidly.

– Many bird-pollinated flowers produce copious amounts of nectar to assure repeated visits.

• Bats tend to visit flowers that open only at night.

Herbaria and Plant Preservation

• Herbaria are essentially libraries of dried, pressed plants, algae, and fungi, arranged and labeled.

– Properly prepared and maintained specimens may remain in excellent condition for 300 or more years.

Methods

Review

• Phylum Magnoliophyta

– Gametophyte Development

• Pollination

• Fertilization and Seed Development

• Specialization Trends in Flowering Plants

• Pollination Ecology

• Herbaria and Plant Preservation

• The reproductive adaptations of angiosperms include flowers and fruits

• Angiosperms

– Are commonly known as flowering plants

– Are seed plants that produce the reproductive structures called flowers and fruits

– Are the most widespread and diverse of all plants

Characteristics of Angiosperms

• The key adaptations in the evolution of angiosperms

– Are flowers and fruits

• The flower

– Is an angiosperm structure specialized for sexual reproduction

Flowers

• The flower

– Is an angiosperm structure specialized for sexual reproduction

• A flower is a specialized shoot with modified leaves

– Sepals, which enclose the flower

– Petals, which are brightly colored and attract pollinators

– Stamens, which produce pollen

– Carpels, which produce ovules

Fruits

• Fruits

– Typically consist of a mature ovary

• Can be carried by wind, water, or animals to new locations, enhancing seed dispersal

The Angiosperm Life Cycle

• In the angiosperm life cycle

– Double fertilization occurs when a pollen tube discharges two sperm into the female gametophyte within an ovule

– One sperm fertilizes the egg, while the other combines with two nuclei in the center cell of the female gametophyte and initiates development of food-storing endosperm

• The endosperm

– Nourishes the developing embryo

• The life cycle of an angiosperm

Angiosperm Evolution

• Clarifying the origin and diversification of angiosperms

– Poses fascinating challenges to evolutionary biologists

• Angiosperms originated at least 140 million years ago

– And during the late Mesozoic, the major branches of the clade diverged from their common ancestor

Fossil Angiosperms

• Primitive fossils of 125-million-year-old angiosperms

– Display both derived and primitive traits

An “Evo-Devo” Hypothesis of Flower Origins

• In hypothesizing how pollen-producing and ovule-producing structures were combined into a single flower

– Scientist Michael Frohlich proposed that the ancestor of angiosperms had separate pollen-producing and ovule-producing structures

Angiosperm Diversity

• The two main groups of angiosperms

– Are monocots and eudicots

• Basal angiosperms

– Are less derived and include the flowering plants belonging to the oldest lineages

• Magnoliids

– Share some traits with basal angiosperms but are more closely related to monocots and eudicots

• Exploring Angiosperm Diversity

• Exploring Angiosperm Diversity

• Exploring Angiosperm Diversity

• Exploring Angiosperm Diversity

Evolutionary Links Between Angiosperms and Animals

• Pollination of flowers by animals and transport of seeds by animals

– Are two important relationships in terrestrial ecosystems

• : Human welfare depends greatly on seed plants

• No group is more important to human survival than seed plants

Products from Seed Plants

• Humans depend on seed plants for

– Food

– Wood

– Many medicines

Seed Dormancy of Trees and Shrubs

by Jill Barbour

Germination Specialist

Organic Seed Dormancy

• Endogenous

• embryo characteristic prevents germination-epicotyl, hypocotyl, radical

• Exogenous

• characteristic of structures - seed coats, fruit walls, including endosperm or perisperm prevents germination

Types of Seed Dormancy

• Physiological

• Morphological

• Morphophysiological

• Physical

• Physical & physiological

• Chemical

• Mechanical

Physiological Dormancy

• Nondeep

• Intermediate

• Deep

Causes of Physiological Dormancy

• Covering restricts oxygen

• Inhibitors in coverings

• Embryo cannot break through physical barriers

• Endosperm restrict embryo growth

• Interaction between embryo and covering

• Abies alba, Castanea sativa, Corylus avellana, Euonymus europaeus, Juglans nigra, Juglans regia, Juniperus, Prunus avium,Rhamnus frangula, Vaccinium myrtillus

• Carpinus requires warm followed by cold stratification

• Elaeagnus umbellata- chemicals shortened prechilling & increase germination

Nondeep Physiological Dormancy

• Germinate over a narrow range of temperatures

• Excised embryos usually grow

• Broken by short periods of prechilling

• Require germination temperature above 15°C

• Broken by chemicals- potassium nitrate, thiourea, kinetin, ethylene, gibberellins

• Light required for germination

• Arbutus unedo –can germinate in dark

• Ulmus glabra- no prechill

• Vaccinium- long period of light required, GA reduces length of light

Intermediate Physiological Dormancy

• Excised embryos will grow

• As much as 6 months prechilling needed

• Gibberellins, kinetin, thiourea can shorten prechilling requirement

• Acer negundo, Acer pseudoplatanus, Acer saccharum, Corylus avellana, Fraxinus americana, Fraxinus pennsylvanica

• Fagus sylvatica – ethylene accelerated and increased germination at 15°C, at 5°C chemicals no better than water soak on germination, GA₃ increased germination of unchilled seeds at 15°C, 10 weeks prechill negate chemical effect (Seed Sci 2004, p21-33)

Deep Physiological Dormancy

• Excised embryos do not grow or produce abnormal seedlings (Prunus will)

• Long prechill requirement

• Chemicals do not affect germination of intact seeds

• Sorbus aucuparis – secondary dormancy induced above 20°C, germinates best at 1-3°C

• Acer platanoides, Acer tartaricum, Malus domestica,

• Prunus persica – 90 days prechill

• Prunus mahaleb – 100 days prechill

• 3 to 5°C best germination temperature for Prunus mahaleb, Prunus padus

Morphological Dormancy

• Morphology of embryo not developed

• Temperate families- Apiaceae, Ranunculaceae

• Tropical families – Annonacease, Arecaceae, Degeneriaceae, Lactoridaceae, Monimiaceae, Myrsticaceae, Winteraceae

Morphophysiological Dormancy

• Underdeveloped embryos

• Embryo growth and dormancy break required

• Embryo grows first then dormancy broken or both at same time

• Vary warm, moist and cold stratification periods

• Viburnum- epicotyl dormancy, warm for radical then cold for epicotyl

• Fraxinus excelsior, Magnolia acuminata

Physical Dormancy

• Present in 15 angiosperm families

• Large embryos with food reserve in embryo not endosperm

• Hilum impermeable in Cercis siliquastrum

• Impermeable in seed coats- micropyle, hilum, chalazal area, impermeable palisade cells

• Embryo is not dormant

• Air drying during development intensifies hardness

• Cytisus scoparius – dry heat(65°C) for 2 minutes, or acid for 30 minutes

• Crataegus in warm climates only endocarp dormant

• Robinia pseudoacacia, Laburnum anagroides

Physical & Physiological Dormancy

• Embryo dormancy usually broken first

• Germinate at low temperatures (5, 10, 15°C)

• Prechilling breaks physiological dormancy

• Hot water, acid, or mechanical scarification effective before prechilling

• Cercis siliquastrum – 16 weeks prechilling = 77% germination(Jordan source)(2004 Seed Sci p 255-260)

• Cersis canadensis, Cotinus coggygria, Cotinus obovatus, Sambuscus

• Tilia- endosperm is inhibitor, excised embryos grow

• Crataegus – 3 month periods of cold-warm-cold-warm-cold=55% germination, apomixis common

Chemical Dormancy

• Inhibitors in embryo, endosperm, seed coat

• Leaching or seed coat removal

• Seed may have physiological dormancy too so need prechilling

• Abscisic acid inhibits germination when applied exogenously

• Nickel (20 mg/liter) increased germination of Picea abies

Mechanical Dormancy

• Stony endocarps

• Embryos with deep physiological dormancy -require long prechilling

• Anacardiaceae, Cornaceae, Juglandaceae, Nyssaceae, Oleaceae

• Cornus sanguinea – 94% germination at 12 weeks prechilling, 81% germination at 12 weeks warm + 12 weeks cold stratification(2004 Seed Sci p 1-4)

• Cornus mas- 18 week warm + 15-18 weeks cold stratification (Tylkowski 1991)

• Cornaceae not morphologically dormant

• Elaeagnus angustifolia – snip both ends

• Rosaceae - warm maturation temperature prior to collection reduced dormancy

Mattoral Germination Conditions

• Mean optimum germination temperature for trees about 21°C – during cool season when soil is moist

• Mean optimum germination temperature for shrubs about 19°C

• Shrub seed germinate in light and dark

• No shrub seed has morphological dormancy (underdeveloped embyros)

Boreal & North Temperate Subalpine

• Pinus cembra- 90-270 days of prechilling

• No morphological, morphophysiological, physical dormancy in species

• Pinus mugo, Picea abies – nondormant

• Small seeds usually from mesic areas, sometimes no endosperm(Ulmus), Sequoia, Sequoiadendron, Picea, Populus

• Large seeds from xeric areas – Pinus edulis

• Embryo size indicator of evolutionary development of Angiosperms, Magnolia precursor for Angiosperms with primitive embryo

• Alpine seeds not very dormant, but may have undeveloped embryos, poor pollination = many empty seeds, short time for development of reproductive structures

• Dry summers, cold winters – many dormancy mechanisms to prevent germination, Juniperus