Ideally we would like to classify dormancy
We humans like to categorise things, and seed dormancy is no exception. Ever since seed scientists discovered that a number of different types of seed dormancy exist, they have been interested in trying to assign the different types to groups within a system of categories.
Ideally, classifying different types of dormancy by attributing them to different factors, locations or durations can not only help us to categorise plant species, but also improve our ability to germinate dormant seeds.
In reality it’s not that simple
Figure 1. Typical multi-seeded stone of Leptecophylla juniperina (Ericaceae). Stone fruited Ericaceae usually exhibit dormancy and are problematical to germinate.
However nature rarely co-operates with our idealism, and despite numerous attempts to classify different dormancy types, the subject remains controversial and no universally accepted dormancy classification scheme exists. Classifying different dormancy types is proving difficult, largely because many dormancy mechanisms are not well understood yet. In addition, dormancy is not an ‘all or nothing’ seed characteristic. The amount of dormancy a seed exhibits, otherwise known as the seed’s dormancy status, can change over time and vary within and between seed lots.
In 1990 Simpson1 declared that seed classification terminologies were ‘…unable to cover the reality of the great range of interactions between seed morphology, physiology and biochemical functions, and the diverse environments which are always in flux, that together determine the nature of dormancy in any one particular seed.’
Today many people still believe that on the whole, the type of seed dormancy expressed by a plant species cannot be described using a single term.
Dormancy: inside or outside?
Despite the controversy surrounding dormancy classification, most seed scientists do agree that the location of different dormancy mechanisms can be either inside the dormant seed and/or around the outside of the dormant seed.
In 1969 M. G. Nikolaeva published what is still regarded by many today as the most comprehensive attempt to classify the different types of seed dormancy2. Nikolaeva was the first person to allocate names and numerical symbols to the different types of dormancy, and the first to distinguish between:
These terms are still considered useful and relevant today (table 1).
Endogenous dormancy may be due to physiological or morphological characteristics of the seed embryo, or a combination of both (table 1). The different types of endogenous dormancy are physiological dormancy (PD), morphological dormancy (MD) and morpho-physiological dormancy (MPD).
Exogenous dormancy may be due to a physical, chemical or mechanical characteristic of the seed coat or fruit (table 1). The different types of exogenous dormancy are physical dormancy (PY), chemical dormancy and mechanical dormancy.
N.B. The following pages describe how we classify different types of exogenous and endogenous dormancies, based on our current understanding. However this information is designed to introduce you to the idea of dormancy classification and is not meant to be a definitive approach to dormancy classification since the subject is continuously under scientific scrutiny.
Table 1. A proposed classification system for seed dormancy, attempting to locate different types of dormancy mechanisms (modified from Nikolaeva, 19773).
|Endogenous: Related to the seed embryo and/or endosperm
||Physiological inhibiting mechanisms (PIM) of the embryo that differs in depth.
||– Deep (strong PIM)- Intermediate
– Non-deep (weak PIM)
||Embryo underdeveloped or undifferentiated.
||PD united with under development of embryo.
||– Morpho-physiological- Intermediate simple
– Deep simple
– Deep simple epicotyl
– Deep simple double
– Intermediate complex
– Deep complex
|Exogenous: Related to properties of the external seed covering structures (pericarp/seed coat)
||Tissues impermeable to water.
||Tissues contain chemical germination inhibitors.
||Tissues restricting embryo expansion and development.
Dormancy classification can be subjective
It is important to exercise caution when trying to diagnose dormancy type. The dormancy classification that we assign to a species can be influenced by our choice of experiments and seed treatments.
For example, seeds that germinate following removal of a small proportion of the seed coat could be diagnosed with either physical, mechanical or physiological dormancy (to understand why, see further explanations of these dormancy types).
Similarly, morphological dormancy may be overlooked if, during your investigations, you fail to check whether or not the seed embryo is growing inside the seed (see Morphological dormancy).
- Simpson GM. 1990. Terminology and definitions of dormancy. In: Seed Dormancy in Grasses. Cambridge, England, Cambridge University Press: 43-59.
- Nikolaeva MG. 1969. Physiology of Deep Dormancy in Seeds. Izdatel’stvo “Nauka”, Leningrad. Translation from Rusian by Z. Shapiro. National Science Foundation, Washington D.C.
- Nikolaeva MG. 1977. Factors controlling the seed dormancy pattern. In: The physiology and biochemistry of seed dormancy and germination. Khan AA, ed. New York, North-Holland Publishing Company: 51-74.