Seed development and dormancy Seed A fertilized ovule

Seed development and dormancy

Seed • • A fertilized ovule Protective outer covering (seed coat) Storage tissue (cotyledons, endosperm) Embryo (radicle and shoot) • Carries all the genetic information, that when properly translated, results in plants with characteristics similar to the parents

Seed Coat • Seed coat— surrounding the seed – Tough layer for protection – Prevents water loss

Pistil

Sticky Stigma

Embryo • Seed production begins in the ovary with the union of the pollen sperm nucleus and egg in the ovule • The embryo is a new plant resulting from the male and female gamete during fertilization • The embryo contains one or two cotyledons or seed leaves, a radicle or immature root, and an immature shoot • Epicotyl is above the cotyledons, hypocotyl is below the cotyledons • The endosperm is a nutritive source for the seed and developing plant

Seed Provenance • The geographic source of the seed • From hot to cold climates – may flower too early, may break dormancy too early (injury), and not harden off in fall (injury) • From cold to hot climates – may harden early (no fall color), may break bud and flower late

Provenance example • Acer rubrum has a natural range from Minnesota to Florida, but if you take an Acer rubrum from Florida and plant it in Minnesota, it will not harden off early enough and can sustain cold damage.

Seed collection • Ripe? – Experience – No more increase in fresh weight – Make a calendar • From a known source • Healthy (disease free) • Minimize hybridization? – Isolation!

Viability • Is the seed “good”? • Sound seeds sink, nonviable float (sometimes) • Germination test • Tetrazolium test – Living tissue – turns red – Dead tissue – no color change • X-ray

Seed storage • How long will the seed remain viable? • Varies with species and storage conditions • Viability can be prolonged by cool temperatures, low humidity, low oxygen • Keep environment constant • Under 41 °F (general)

A long time • The oldest carbon-dated seed that germinated was a Judean Date Palm from Herod the Great’s palace in Israel, dating from 2, 000 years ago • Most recently a 31, 800 (± 300) year old Silene stenophylla found in a squirrel’s nest in permafrost in northeastern Siberia was regenerated • http: //www. pnas. org/cont ent/early/2012/02/17/111 8386109

Seed dormancy • If the seed is viable and it has favorable environmental conditions, but there is a lack of germination, we call the seed dormant • If the seed simply needs to imbibe water to initiate germination, we call the seed quiescent

3 types of seed dormancy • Exogenous (outside the embryo) – Physical – water and/or oxygen can not be imbibed – Mechanical – embryo can not expand through – Chemical – germination inhibitors in the surrounding tissue (fruit) • Endogenous (inside the embryo) – Physiological – size and development of the embryo • Double – a seed experiencing both of the above

Overcoming exogenous physical seed dormancy • Must scarify seed coat before the seed will germinate 1. 2. 3. 4. 5. Mechanical 1. 2. Nick with a file Sandpaper or gravel in drum 1. Sulfuric acid 1. 2. Hot water to soften High temperatures to mimic fire Chemical Temperature Sow in summer (biological) Warm stratification (temperature and biological)

Overcoming exogenous physical seed dormancy (cont. ) • All of these techniques mimic nature • Freeze thaw cycles • Fire, hot desert areas • Decomposition by bacteria and fungi • Digestion by animals

Overcoming exogenous mechanical seed dormancy • Must break or weaken the shell and/or seed coat 1. Mechanical 2. Temperature 1. Hot water to soften 2. High temperatures to mimic fire 3. Sow in summer (biological) 4. Warm stratification (temperature and biological)

Overcoming exogenous chemical seed dormancy • Remove the tissue/fruit covering the seed • Leach seeds repeatedly in water • Example: some desert plants only germinate after hard rains – ensures enough water to germinate and grow – chemical inhibitors in seed coat are washed away

Overcoming endogenous seed dormancy • Embryo requires a period of after-ripening induced by cool, moist conditions • Placing harvested seeds in cool, moist conditions is called cold stratification • Length of time depends on species • Stratification environment must provide oxygen, moisture, and cool temps (41 °F) • 4 to 1 stratification media to seeds (moist sphagnum in a plastic bag works well)

Overcoming double dormancy • Must remove all obstacles in the proper order – exogenous first, then endogenous • Ex. Scarification, to allow imbibition of water, followed by cold stratification, to allow embryo maturation

Think about • How deep should a seed be planted?