Propagation/Greenhouse Management
Part 1 – 102 | Unit 1.3
Lecture 1: Seed Biology, Germination, & Development
a) Phase 1: Imbibition. Rapid initial uptake of water by the dry seed, followed by a brief
but gradual continuation of water uptake. This softens and swells the seed coat and
occurs even in seeds that are no longer viable.
b) Phase 2: Interim or lag phase. Water uptake greatly reduced; internal physiological
processes begin. From the outside, little appears to be happening, but this is a very
active physiological and metabolic period within the seed.
i. Activation of mitochondria within cells of the seed: Supporting increased cellular
respiration and energy production
ii. Protein synthesis: Translation of stored RNA to fuel continued germination
iii. Metabolism and use of stored nutrient reserves to fuel development
iv. Enzyme production and synthesis, leading to the loosening of cell walls around the
embryo and root radicle, which will ease subsequent cell enlargement, division, and
elongation
c) Phase 3: Root radical emergence. Initially, root radicle emergence results from cell
enlargement, but this is rapidly followed by cell division and elongation as the root
radicle pushes into the surrounding soil media.
d) The processes internal to the seed and the below-ground emergence of the root radicle
define the process of germination. However, from a grower’s perspective, we typically
discuss germination in relation to when the plumule or embryonic shoot emerges
above the soil surface. It is at this point that we’re most aware of germination and must
shift our management practices, particularly to manage for relative wet to dry swings in
the soil to prevent the presence damping off organisms and other pathogens (see more
in Lecture 2, Managing Environmental Conditions—Using Greenhouses to Optimize
Seedling Production).
- Early seedling development: Processes and shifting needs
a) Continued cell division-extension of root radical and root tip from base of embryo axis,
into the soil medium. Initial root development is unbranched and taproot-like.
b) Emergence of plumule or growing point of the shoot, from upper end of the embryo
axis. Initial, above-ground seedling development follows one of two patterns, either:
i. Epigeous germination: Ongoing elongation of the hypocotyl, raising the cotyledons
above ground where they provide stored nutrient transfer and initial photosynthesis,
until the emergence of the first set of true leaves. This normally occurs within 24
hours of above-ground emergence.
ii. Hypogeous germination: The hypocotyl does not continue to expand, and only the
epicotyl emerges above ground, soon followed by true leaves. The cotyledons deliver
nutrients for early development, but usually remain at or below the soil surface, and
photosynthesis comes exclusively from the true leaves.
c) Overall weight of seedling increases throughout developmental stages, while weight of
storage tissue decreases as stored nutrients are consumed by the growing seedling
d) Rate of respiration and volume of water uptake steadily increase with ongoing cell
division concurrent with the expansion of roots and above-ground shoots
e) As seedlings continue to develop through cell division and elongation, depending on
the root nature the species, a taproot, fibrous, or branched root system will develop,
with fine root hairs developing to increase the overall surface area available for
enhanced water and nutrient uptake
f) Development of true leaves, roughly concurrent with development of branched root
system in most species, begins process of effective photosynthesis, helping to fuel
continued growth