Germination and Emergence
Seedling Recruitment:
Germination and Emergence
Germination and Establishment: differentiation of germination and establishment often
arbitary, hard to define:
-germination events may be evident ultrastructurally 30 min after wetting: emergence of
radicle happens later, not reliable indicator of germination
-commonly germination determined by appearance of radicle, or emergence from soil
-commonly establishment determined by appearance of photosynthetic tissue from seedling
(cotyledons or leaves) and are capable of independent growth on own
"Safe-site" concept:
-the "environmental sieve" of safe and unsafe sites
-only a small fraction of the seed bank germinates to give seedlings
-presence, absence and density of seedling population depends on availability of seed and
frequency of "safe sites"
A safe site is a place a seed might be that provides:
-stimuli required for relief of seed dormancy
-conditions required for the germination processes to proceed
-the resources (water, oxygen) which are consumed in the course of germination
-place where specific hazards are absent: predators, competitors, toxic soil constituents
(e.g. herbicides, allelochemicals) and pre-emergence pathogens
The micro-scale of seed events:
-the conditions actually permitting establishment must be seen at the scale of the size of
a seed;
-at this very small scale the environment is extremely heterogeneous
-a worm cast is a mountain
-a fallen leaf is a shade from light
-a raindrop is a cataclysm
Periodicity of germination
-major weed germination "flush" of seedlings
-some species have more than one 1 flush per season
-the time of this flush varies by species; by environment
Adequate seed-soil contact necessary for successful seed imbibition of moisture
Moisture
-some species germinate over a large range of water tensions, some very specific tensions;
same for wet-dry cycles
-intensity of water evaporation and loss of water (air humidity, wind, etc.) can not
exceed absorption for imbibition
Soil factors affect contact, water availability to seed for uptake
-soil porosity
-soil particle size
-physical factors relating to whether soil in process of wetting or of drying, etc.
Compression of soil-seed increases germination in many cases
-example: animals trampling field pressed soil, greater seedling emergence of Plantago
lanceolata (plantain) in compressed soil areas
-example: grazing animals in pasture can press freshly fallen seed into ground
differentially in autumn, germination following spring increased for these compacted seeds
of Ranunculus sp. despite fact hoof marks no longer apparent
Soil surface microtopography:
Change in topography does not have an all-or-nothing effect on germination:
-changes in surface roughness do not result in the complete elimination of a species from
an area; it just changes the frequency safe sites encountered, the frequency that a
particular species germinates
-some species thrive in smooth soil surface microtopographies, others in rough; ratio of
species present thusly determined
Microtopography influences on the relative establishment of species by the ways the
seed rests on soil:
Example of two bromegrass species illustrates this:
-Bromus rigidus predominates on smooth soil surface: it has a larger seed,
straight, needlelike awn; and will lay flat with good soil contact on smooth soil
-B. madritensis predominates on rougher soil surface: it has a smaller, lighter
seed with curved awn; on a smooth surface the seed rolls over on convex seed side with
embryo pointing ineffectively into air; on a rough surface it can curl around soil clods
for good soil contact
Common lambsquarters example: Chenopodium album seed tuberculate and rough and tend
to remain on the side of the soil clod where they first landed
Mustard sp. example: Brassica napus seed are smooth and round and roll down into
crevices
Seed shape/structure:
Shape of seed, how it lies relative to water contact points affects germination potential:
-example: Panicum turgidum: seed has convex and flat sides; germination much better
if they land with flat side down
Seed dispersal structures also play a role in germination and soil contact. For example
the role of the rigid pappus of many species in Compositae family:
-example: Dandelion: achene with pappus lies at angle to soil, evidence this is superior
angle for germination; pappus opens and closes hygroscopically, also altering angle of
seed to soil
-example: Senecio sp. (common groundsel): pappus collapses and becomes
mucilaginous, aiding soil contact
Seeds with micropyle closer to water source contact point often have better germination
Large seeds, rough seeds, tuberculate seeds (small rounded protuberance, hump on seed):
-ratio of seed surface area to surface available for water contact
-more contact results in better germination
Mucilage covered seeds absorb water more efficiently (e.g. groundsel)
Seed polymorphisms: several different soil-substrate contact variants within a species
-tuberculate and non-tuberculate seeds (common in Caryophyllaceae)
-small and large seed (common in Compositae)
Emergence
Harder for seed buried to emerge than for seed on surface: extra work load emerging from
depth
Greater emergence depths takes longer generally, uses more of the seed reserve; small
seeds often cannot emerge from deeper depths (relationship of seed reserves available and
depth of seed)
Soil crusting as an impediment to seedling emergence
-clay layers harder to penetrate
-some weed species better adapted (e.g. spike pointed seedlings like grasses) to
penetration than others (e.g. round pointed seedlings like dicots with hypocotyl hooks or
large, blunt cotyledons
-capping restrictions due to impeding gas flow to seeds or physical barrier
-numbers of seedling working together can produce more force together as emerging unit
than individual
Buried seeds lack light requirement for germination from depth
Chemical Factors affecting Recruitment
Most seeds don't need chemicals for germination and establishment: fulfilled by seed food
reserves
Toxic chemicals can inhibit germination: evaporation exceeds precipitation and disolved
chemicals (salts, etc.) can accumulate in remaining water, inhibit germination
Soil pH: acidic soil conditions inhibit weed species differentially
Soil chemical conditions enhance susceptibility to soil pathogens: many germinate and die
right away
Herbicides:
-in-crop treatments present at time of recruitment;
-"carryover" in soil of previous applications
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