Propagation/Greenhouse ManagementUnit 1.3 | Part 1 – 101
Lecture 1: Seed Biology, Germination, & Development
ii. Maximum: Each species has an uppermost temperature at which germination
can occur. Above this threshold, injury or dormancy are often induced. Nearing
this threshold, percent of germination often declines and days to emergence may
increase.
iii. Optimal: Every species has on optimal temperature and corollary temperature range
in which the percent germination is highest and days to emergence is the lowest.
This is the target range to strive for when managing greenhouse facilities or sowing
seeds outdoors.
iv. In addition to optimal temperatures, some species either require or benefit from day-
night temperature fluctuation. Many small-seeded species, which best germinate
near the soil surface, benefit from the temperature fluctuation that normally occurs
at the soil surface. Germination may be inhibited in species requiring temperature
fluctuation if seeds are buried too deeply, as temperatures typically remain more
constant at depth.
b) Moisture: All seeds require moisture to initiate metabolic processes and support
germination. Seeds imbibe water from the soil pores in direct contact with the seed; as
this soil dries, moisture is replaced by capillary action from nearby soil pores, helping
facilitate germination. For most seeds, field soil or propagation media should be
maintained at or above 50%–75% of field capacity during the germination phase, and
have a firm, fine texture to provide good seed-to-soil contact.
c) Aeration: Soil/media must allow for gas exchange to and from the germinating embryo
i. Oxygen (0 2 ) dissolved into the soil media is required to facilitate embryonic
respiration
ii. Carbon dioxide (CO 2 ), a byproduct of respiration, must be able to dissipate and move
away from the seed
Note that good soil structure enhances gas exchange, whereby gases can move into
and out of the soil via the pore spaces between soil particles. Avoiding overwatering
and allowing for adequate infiltration of water and subsequent dry down between
irrigations also promote gas exchange. Excessive irrigation and/or poorly drained
soils can limit germination and development when oxygen is crowded out of the
pore spaces by persistent moisture.
d) Light can either induce or release dormancy, depending on the species. The effect of
light on sensitive species results either from light quality (wavelength) or photoperiod
(the duration of exposure.) Most cultivated crops express minimal or no sensitivity to
light during germination, in large part due to millennia of grower and breeder selection
for consistency and reliability of germination.
i. Most species germinate best under dark conditions by being slightly buried in the
soil medium, and in some cases (e.g., Phacelia, Allium, Phlox) germination may be
inhibited by light. Light inhibition is particularly common in desert species, where
germination in the presence of light would likely lead to desiccation and death due
to the normally dry conditions of the soil surface.
ii. Seeds of certain species (e.g., Lactuca, Begonia, Primula, Coleus) require exposure,
however brief, to light to induce germination. This is particularly common amongst
small-seeded species and is thought to be an evolutionary mechanism to prevent
germination when seed is buried deeply in the soil, where a germinating seed may
exhaust its resources before emerging above ground to begin photosynthesizing.
iii. The effect of light on germination should not be confused with necessity of light
for seedling development. All seedlings require sunlight for photosynthesis and
continued development.
- Physiological steps in germination: A three-phase process leading to the emergence of
roots and above-ground growth