Lesson-2B (Complementary Video-note) Seed Germination and Dormancy 26OCT2026
In Lesson 2A we saw how seeds are built, how they survive storage, and why a living seed may stay dormant. Lesson 2B picks up the story at the moment conditions turn favourable, and follows the seed all the way to an independent seedling. This is a complementary video-note: a focused recording to revise alongside the class lecture, with edited subtitles and chapter markers.
What a seed needs
A non-dormant seed needs water, oxygen and a suitable temperature to germinate, and some species also need light or darkness. Water starts the process (imbibition). Oxygen powers aerobic respiration, which supplies energy as adenosine triphosphate (ATP). Each species also has its own cardinal temperatures, a minimum, an optimum and a maximum, which set where germination works best.
Three phases of water uptake
When a dry seed meets water, its water content rises in three phases. Phase I is rapid, purely physical uptake; even a dead seed soaks up water, like a sponge. In phase II the water content levels off, but this is the busiest time inside a living seed: mitochondrial respiration, DNA repair and protein synthesis all restart. Germination is complete when the radicle breaks through the seed coat, and phase III, the growth phase, follows. In flooded soil, where oxygen is short, fermentation helps keep the seed supplied with energy.
Hormones and the food store
Two hormones work against each other. Abscisic acid (ABA) keeps the seed dormant, while gibberellin (GA) breaks dormancy and promotes germination. Cues such as light, chilling and water tip the balance towards GA.
GA also switches on the enzymes that break down the food store by hydrolysis, a process that uses water. Amylases turn starch into sugars, proteases turn proteins into amino acids, and lipases turn oils into fatty acids and glycerol. The fatty acids are then converted into sucrose. The products travel to the growing axis: the radicle, hypocotyl, epicotyl and plumule.
Barley: the embryo gives the signal
In barley, the embryo releases GA, which signals the aleurone layer to produce α-amylase. The enzyme breaks down starch in the endosperm, and the scutellum absorbs the sugars to feed the growing seedling. A classic prediction task shows the embryo’s role: a half-grain with the embryo removed breaks down little starch, but adding GA restores α-amylase and starch breakdown.
Light as a switch: phytochrome
Some seeds only germinate after light. The sensor is phytochrome, a pigment that switches between two forms. Red light (about 660 nm) converts it to the active form, Pfr, and far-red light (about 730 nm) switches it back. In 1952, Borthwick and colleagues gave Grand Rapids lettuce seeds alternating flashes of red and far-red light. Sequences ending in red gave near 100% germination; sequences ending in far-red gave much less. The last flash decides. Other seeds, such as habbatus sauda (Nigella sativa), germinate better in darkness, which affects how deep they should be sown.
From seedling to crop
Seedlings emerge in two ways. In epigeal germination, as in beans, the hypocotyl hooks and lifts the cotyledons above the soil. In hypogeal germination, as in peas, the epicotyl hooks and the cotyledons stay underground. Cereals push up a protective coleoptile, helped in maize by the mesocotyl.
The dry mass of the seed plus seedling first falls as reserves are used up, then rises once photosynthesis exceeds respiration. At that point the seedling has made the switch from heterotroph to autotroph.
These ideas link directly to farm problems: storage losses, pre-harvest sprouting, uneven emergence and planting depth. Understanding the physiology helps growers manage water, oxygen, temperature and light, so that every seed planted has the best chance of becoming a healthy crop.
Lecturer: Dr Amin Yaapar, Jabatan Sains Tanaman, Fakulti Pertanian, Universiti Putra Malaysia
Complementary Note:
AGR3301_Lecture2_Revision_Companion_Seed Germination Dormancy.pdf
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Video Transcript:
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