Physical dormancy
Unlike physiological dormancy, which has its mechanisms inside the seed, physical dormancy is initiated by a mechanism outside the seed. Only about fifteen plant families (including Anacardiaceae, Cannaceae, Convolvulaceae, Fabaceae, and Malvaceae) exhibit physical dormancy. The mechanism for this type of dormancy is a hard physical barrier, which surrounds either the fruit or the seed. This prevents water from entering the seed, and therefore prevents imbibition. Inside this physical barrier, seeds can survive for a long time, but something else needs to happen in order to break the dormancy. Here are some examples:
In fire-prone areas of Australia, some species from the genus Banksia keep their seeds inside a woody fruit for years. During a forest fire, the heat of the fire will open up the fruit, which breaks the dormancy of the seeds inside. They fall to the ground, where, once the fire has passed through, conditions are optimal for the seeds to germinate.
In semi-arid areas of Central America, the legume Stylosanthes humilis has seeds with water-impermeable coats and a pore, called a water gap, which closes at seed maturity. The water gap opens in response to fluctuations in temperature, which breaks the dormancy by enabling the seed to take in water and oxygen.
Case study: Acacias and giraffes
South Africa’s Acacia burkeii is a legume whose seeds have hard seed coats. The physical dormancy this gives them is important for survival. The process starts when a giraffe eats acacia seeds. As the seeds pass through the animal’s digestive tract, the acidic conditions, microorganisms and enzymes in the animal’s gut have the effect of scarifying their hard seed coats. Meanwhile, the giraffe may travel far from the parent plant, before depositing the scarified seeds in feces. This not only breaks dormancy, but also distributes the seeds a long way from the parent plant, reducing the risk of competition between young seedlings and the parent plants.
Morphological dormancy