seed and fruit development•seed tissues--cotyledons, endosperm, perisperm--nourish the seedling...
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SEED AND FRUIT DEVELOPMENT
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Seeds:
• Include embryo from fertilized ovum (axis plus
cotyledons), accessory tissues, seed coat
• Accessory tissues: endosperm from fertilized polar
nuclei; sometimes some nucellus ("perisperm", from
megasporangium)
• Seed coat: from integuments
• Cotyledons, endosperm, perisperm store food for
germination: lipid, starch, protein
Seed development:
• Cytokinins in endosperm stimulate cell division of the
fertilized egg
• Auxins produced by embryos stimulate expansion of
cells
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Stages of development of an embryo in an Arabidopsis (mouse-ear cress) seed
cotyledon
heart
globular
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Courtesy of Prof. John Harada, UCD
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In a bean (Phaseolus) seed, the major nutritive tissue is the cotyledon.There is no endosperm remaining
In a grass (Setaria) seed, the majornutritive tissue is endosperm
The castor bean(Ricinus) seedhas both endospermand cotyledon
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Fruits: developed ovaries
• Before fertilization, carpel wall (ovary) protects the
ovule and embryo sac and guides the pollen
tube
• After fertilization, the carpel wall changes function
to aid in seed dispersal (and sometimes to
help time germination of the seed)
• Cytokinins in endosperm stimulate cell division
• Auxins produced by embryos stimulate expansion
of cells
• Ethylene stimulates ripening (maturation) of ovary
wall to produce fruit
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Types of fruits
--fleshy or dry
--dehiscent (opens to release seeds) or indehiscent
--simple (from one ovary), aggregate (from several
ovaries on one flower), multiple (from inflorescence)
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Types of fruits
--fleshy or dry
--dehiscent (opens to release seeds) or indehiscent
--simple (from one ovary), aggregate (from several
ovaries on one flower), multiple (from inflorescence)
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Types of fruits
--fleshy or dry
--dehiscent (opens to release seeds) or indehiscent
--simple (from one ovary), aggregate (from several
ovaries on one flower), multiple (from inflorescence)
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A pea pod is a dry, dehiscent, simple fruit
A cucumber berry is a fleshy,indehiscent, simple fruit
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A strawberry is an aggregate fruit--thetiny individual fruits, achenes, are dry,indehiscent, simple fruits. Thefleshy receptacle develops from the stem
The dandelion head might be considered a multiple fruit. TheIndividual parts are dry,Indehiscent, simple fruits
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Why fruits?
They promote dispersal of seed by…
Wind: Winged (ash, elm) and plumed (dandelion)
Water: air-filled (sedges), fibrous (coconut)
Animals: burred, barbed fruits (star thistle, cockelbur); fleshy berries (tomato: the seeds pass through guts); nuts (hidden by squirrels, birds); sticky fruits (mistletoe)
Ulmus
elm
star-thistle
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Seed germination
Needs correct conditions (varies by species)
• Temperature (warm enough)
• Temperature history (chilling: “stratification”—
common in high latitudes, unlikely in Vietnam)
• Seed coat breaching (“scarification”)
• Acid bath (animal stomach)
• Light (red or far-red)
• Water (leaches inhibitor [ABA], or simply for turgor
pressure)
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Seed germination involves the “mobilization” of storage compounds:their conversion to compounds that can be metabolized for energy and growth
Example: barley seed germination
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Summary
•Seeds are plant embryos surrounded by dry integument tissue
•The embryos are dormant when the seeds are mature
•The seed tissues often control germination of the embryonic plant
•Seed tissues--cotyledons, endosperm, perisperm--nourish theseedling when it germinates
•A simple fruit is a mature ovary; aggregate and multiple fruits represent many ovaries
•Fruits promote the dispersal of seeds by wind, water, or animal transport
•Seed germination involves the mobilization of stored compounds