Author: Dr. Riad Z. Baalbaki
A clear definition of germination is a prerequisite to correctly interpreting reported tests results. Many definitions of seed germination are in current usage, and it is important to understand the meaning of each. The so-called “physiological definition” defines germination as the emergence of the radicle through the seed coat. More precisely, the sequence of interrelated catabolic and anabolic events, triggered by seed hydration under a favorable germination environment, that lead to initiation of embryo growth in a previously quiescent seed. In contrast, a seed analyst would define germination as emergence and development from the seed embryo of those essential structures which are indicative of the ability to produce a normal plant under favorable conditions (AOSA, 2019).
Official seed testing rules use the latter definition, and percentage germination results reflect only the number of normal seedlings. A somewhat hybrid application of both definitions is often used when evaluating seed germination of native species with no official testing rules.
In that case, a seed whose radicle has emerged is considered as germinated and removed from the test before significant seedling development, but seedlings with abnormal development apparent at this early stage are not included in percentage germination results. For most species, this means that absence of or abnormal radicle development are the only seedling evaluation criteria (Kew reference; this should be a link to Kew’s Germination testing: procedures and evaluation; Kew Technical Information Sheet 13a http://brahmsonline.kew.org/Content/Projects/msbp/resources/Training/13a-Germination-testing-procedures.pdf). When considering germinating test results, knowledge of what definition was used is essential for correctly interpreting and replicating such results.
It should be noted that the term “germination” can apply to a single seed as well as a seed lot. When describing germination of a single seed, reference is made to physical, physiological, biochemical and molecular events taking place within that seed. In contrast, seed testing laboratories approach germination as a seed lot measure, specifically, a determination of a seed lot’s capacity to germinate.
The Germination Process: Seed-Based Events during Germination
Seed-based events can best be understood by superimposing their timing with the known triphasic pattern of water uptake by seeds. The first phase, or Phase I, involves passive water uptake and physical process of water absorption from a wet environment. In Phase II, there is no net water uptake. This is a period where all the water that can be absorbed by physical forces has been taken up; the seed is undergoing changes but as yet has no active mechanism to take up water. Phase III is the phase of active water uptake by the newly emerged radicle. This phase, strictly speaking, is post-germination. Each of these phases is associated with unique seed events. For seeds of poorly studied species, such as many wild and native species, understanding the timing and impact of these events is essential for correct interpretation of seed behavior and test results.
The Standard Germination Test: Applicability to Native Species
Regardless of the testing rules used (AOSA, ISTA, FSA or CMP), the aim of germination testing is to estimate the maximum number of seeds that will produce normal seedlings under optimum conditions. The ability to produce normal seedlings in the field, and the adjustment of planting densities based on test results, is the first requirement for producing uniform stands at the time of harvest. While uniform stands for cultivated agricultural, vegetable and flower species are an important concern, the same may not be true for native and wild species, many of which are never harvested. How, then, should natives and wild species be tested for germination? Under all existing rules, germination tests have three components: standardization, absolute requirement for normal development of essential structures, and testing under empirically proven favorable conditions. What components of a germination test should apply to testing natives?
For species without rules, a satisfactory level of standardization can be achieved by limiting the choice of media to one or two, listing a single temperature regime, prescribing light during testing (with exceptions), and recommending 4 weeks per test, with the option of terminating early (see following section). Germination can be determined based on radicle emergence, but seedlings with abnormal development apparent at this early stage would be considered as abnormal. All of the above should be considered as preliminary, and validation tests are needed for developing rules for each species.
Of special importance is the need to address the issue of dormancy. Should dormancy breaking technique be used? Scarification might be necessary since many hard seeds might not be viable. Dormancy breaking treatments must be clearly defined for each type of dormancy. There are two common classifications of dormancy types. The first is by Baskin and Baskin (2004), and the other by Bonner (1984). Refer to Baskin’s Seed Dormancy Classification of varying plant families.
Seedling Evaluation (For species without rules)
Following the analysts’ definition of germination, seedlings with intact or slightly defective essential structures, indicative of the ability to produce a normal plant under favorable conditions, are to be classified as normal. Seedling abnormalities can be due to many causes, and specific criteria for evaluating seedlings are included for each family (see section xxx for germination rules and seedling evaluation criteria). However, the following criteria should be regarded as benchmarks for seedling classifications, especially when evaluation criteria are being developed for previously uncategorized species. Significant defects of essential structures can be separated into either root or shoot problems. In general, injury to photosynthetic tissue will have a significant impact on early seedling growth; before development of photosynthetic tissue, loss of food reserves will have a profound negative impact on seedling survival. In the absence of root development, no sustained seedling growth is possible, and late development of roots can decrease the survival chances of a seedling under field conditions. It should also be noted that abnormal growth patterns can be the result of seed position on/in substrate or the nature of substrate.
Criteria for Seedling Evaluation
As mentioned above, while specific evaluation criteria are included for each family, a list of general criteria is needed for developing appropriate evaluation standards relevant to each family/species. For the criteria listed below, an answer of ‘Yes’ implies that the seedling in question has a low chance of survival in the field, and should be classified as abnormal. While not every criterion is relevant to all growth habits and plant types, two fundamental questions apply to all species: was enough functional root mass produced, and is the seedling likely to become autotrophic?
Was enough functional root mass produced:
- Weak, stubby or missing primary root with weak secondary or adventitious roots?
- Main root split extending into the hypocotyl, with no secondary roots?
- Less than one strong seminal root?
- Most root mass decayed as a result of primary infection?
- Glassy or spindly roots?
- Root shows negative geotropism?
Is seedling likely to become autotrophic?
- Is photosynthetic apparatus significantly damaged or impaired?
- Does seed/seedling have enough reserves that can be utilized to reach the autotrophic stage?
- Is less than half of the original cotyledon tissue remaining attached?
- Is less than half of the original cotyledon tissue free of necrosis or decay?
- Is the epicotyl missing or damaged?
- Are there deep open cracks in the hypocotyl extending into the conducting tissue?
- Are less than half of the first leaves attached and free of decay?
- Is this an albino seedling?
- Is the seedling decayed as a result of primary infection?
- Is seedling glassy or spindly?
- Is the endosperm detached and seedling growth retarded?
- Is the coleoptile missing?
- Is point of attachment of cotyledons to seedling axis decayed?