PBL 5: Fruits and Seeds Flashcards
What is a fruit?
-Adaptations that result in the protection+distribution of seeds: any ovary and its accessory parts that develop, mature and contain seeds
-From the ovary wall the pericarp develops, which contains 3 layers:
1.) Exocarp: outer layer
2.) Mesocarp: middle layer (can be fleshy/dry)
3.) Endocarp: inner layer that surrounds the seeds (can be fleshy/dry)
True fruit
Contain only ovarian tissue
Accessory fruit
NO ovarian tissue is present
-made from inferior ovaries
Simple fruit
develops from a single ovary or from fused ovaries the same flower
Aggregate: separate ovaries (carpels) that fused together during fruit development
Multiple: during development, the ovaries from individual flowers of an inflorescence fuse together into one fruit
Dry fruit
have a dry mesocarp and not consumed by animals
Dry fruits: Dehiscent
fruit breaks open and releases seeds
-Single carpel (legumes): break open on both sides: beans
-Compound gynoceium:
TYPES AND EXAMPLES:
–capsule: opens in many ways, >2 carpels (poppy fruit)
–nut: only one ovule matures, pericarp is very hard (acorn)
Dry fruits: Indehiscent
Fruit doesn’t break open
-Single carpel
TYPES AND EXAMPLES:
–caryopsis: only one seed and seed coat fuses with fruit during maturation (corn)
–achene: fruit wall remains distinct, seed is attached to the pericarp (sunflower, strawberry)
– samara: one seed, wing-like structure attached (maple)
Development of seeds
Develop from the ovule where the endosperm rapidly undergoes cell division and zygote begins to grow:
1.) Globular stage: zygote grows into a small cluster of cells
-One part grows into the embryo
-other one grows into a short stalk, the suspensor which pushed the embryo into the endosperm
2.) Heart stage: embryo develops cotyledons
-In EUDICOTS (angiosperms): 2 cotyledons develop and provide nutrients for the embryo (absorb nutrients from the endosperm)
-In MONOCOTS: cotyledon that acts as digestive/absorptive tissue
-CONIFERS develop 2+ cotyledons
3.) Torpedo stage: development of radicle (embryonic root), epicotyl (embryonic stem) and hypocotyl (root-shoot-junction)
4.) Mature embryo becomes quiescent and partially dehydrates
-Funiculus (part where embryo is attached): breaks and leaves a hilum (scar)
-Micropyle (later, entry point for sperm) allows for water entry (MORE PRESENT IN DICOTS)
-Integuments (ovule wall): become the seed coat & endosperm comes from second fertilisation in double fertilisation
-Coleoptile: protects first LEAVES
-Coleorhiza: protects first ROOTS
Seed germination process
1.) Imhibition: water fills the seed
2.) Activation of enzymes that begin plant growth
3.) Seed grows a root to access water underground
4.) Effect of light: seed grows shoots that grow towards the sun
5.) Development of embryo axis into a seedling, grows leaves and begins photomorphogenesis
Requirements for seed germination
-seed needs to be viable (alive)
-favorable conditions needed (water, oxygen, light)
-some species require a dormancy period before germination
Quiescence
Inactivity of the seed because of unfavourable external conditions (will germinate once they are in favourable conditions)
Dormancy
seed inactivity (even in the presence of favourable external conditions)
-Impermeable seed coat: no entry for water/oxygen
-Mechanically resistant seed coat: prevents embryo from developing radicle and epicotyl
-Rudimentary embryo: hasn’t reached full maturity when seed is shed
-Physiological immaturity of embryo: not capable of germinating when mature.
Evolutionary/adaptive reasons for dormancy
-Survival to reach/cover large distances
-Reaches right period (e.g. after spring)
Overcoming dormancy
-Scarification: damage to seed coat (via animal, scratching, biting, digestion, microorganisms)
-Chemical treatment: gibberellin+ethylene
-Light treatment: exposure to far-red light
-Soaking in water to get rid of seed coat inhibitors
-Stratification: long cold treatment to deactivate inhibitors and activate growth hormones (gibberellins)
–Water imersion
–Photolight exposure
–Thermal exposure
