Success of Seeds: 4 Main Factors

This article throws light upon the four main factors responsible for the success of seeds. The factors are: 1. Longevity of Seeds 2. Dormancy 3. Germination 4. Establishment of Seedlings.

Factor # 1. Longevity of Seeds:

It is well known that several ambiguous factors such as cool temperatures, low oxygen tension, and low moisture content, all of which tend to decrease metabolic activity, increase the length of time that seeds can be stored. This finding is surprising, and makes it difficult to explain why many seeds apparently retain their viability longer when buried in moist soil than when kept in dry storage. (Fig. 23.1).

The longevity is high at low water content (depending on the species, 5-8 per cent) and at high water content when the seed has fully imbibed. Ageing phenomena occurred at all levels of water content.

These ageing processes are assumed to result in cross-linkage of macro-molecules, which render enzymes and membranes non-functional, and in a gradually increasing number of somatic mutations, many of which may cause the production of defective proteins.

The functioning of macro-molecule and organelle repair mechanisms was seriously impaired during air-dry storage at intermediate relative humidities of the air. In practice, the degree of ageing can be related to the amounts and kinds of substances lost by leaching from the seeds during soaking.

When dried seeds were moistened again the initial leakage of electrolytes which normally decreased rapidly when the seeds reached a water content of about 15 per cent persisted due to the globular state of the plasma membrane of these dry seeds. This globular state is ineffective in maintaining gradients of ions and charge across the membrane.

After re-wetting, the normal non-leaky bilayer structure is restored, a process which would take more time in aged seeds. In imbibed seeds stored in the soil, leakage will not occur while the seed remains viable, provided the membranes remain in good condition.

Repeated alternation of drying and wetting, which often occurs in the upper soil layers, could ultimately lead to the complete exhaustion of accumulated solutes and could therefore prove very harmful.

The germination was more rapid when the seeds were re-dried after short periods of inhibition. A prolonged wet period before re-drying resulted in embryo damage and poor germination, the critical factor being whether or not active cell division had begun in the imbibed embryo. The duration of the periods of imbibition that seeds can withstand depends on the species and the stage of development of the embryo.

It may be concluded that seeds lying in or on the soil are subject to deterioration as a consequence of ageing. The rate of deterioration increases with increasing temperature and decreasing water content. Reiterated drying wetting cycles are especially harmful.

In fully imbibed seeds deterioration and ultimate death are postponed by the continuous repair of damaged structures. Although seeds have a fairly efficient repair mechanism, their ability to produce vigorous seedlings gradually declines during the ageing process.

Factor # 2. Dormancy:

The fate of a seed after dissemination depends largely on external conditions and its internal features. In many cases seeds are not able to germinate immediately after dissemination, even when conditions seem to be favourable. These seeds are “dormant”.

The function of dormancy in determining the timing of growth resumption whenever external conditions become suitable is quite clear, but the phenomenon itself is very complex. For instance, dormancy may depend on a variety of internal features.

The seed coat may be highly resistant to the diffusion of oxygen from the environment to the embryo. Furthermore, the seeds coatings may contain substances that inhibit embryo growth and have to be broken down or rinsed out before germination can start.

Dormancy may also be governed by an internal hormonal balance between growth inhibiting and growth-promoting substances in the embryo itself, i.e. a balance which is inadequate for growth initiation.

In some cases the degree of dormancy of the seeds depends on the conditions to which the mother plant was exposed during fruit development. Very often, the inability to germinate directly after dissemination disappears for no apparent reason in a few weeks (after ripening).

There is also a kind of dormancy that can be induced in normally non-dormant seeds by the application of special treatments such as high temperatures (thermo-dormancy) or osmotic stress.

The breaking of dormancy requires a specific sequence of external factors. The dormancy- breaking agents into a number of categories, and found a very close resemblance between the stimulation of germination and the possible stimulation of the PP pathway, which for the relevant ecological factors can be summarized as follows:

Nitrate:

Nitrate is known to break seed dormancy in a large number of species and stimulates hydrogen acceptance by intermediates of the PP pathway; it shares this property with agents such as nitrite, oxygen, and methylene blue, which have been shown to stimulate germination in certain cases;

Temperature:

Temperature has a number of quite different effects on dormancy, depending on whether the seed is “dry” or “wet” freshly shed seeds that have been kept dry show a rapid loss of dormancy at high temperatures (after ripening); when kept imbibed immediately after harvesting they germinate only within a small temperature range that is sometimes high(35-45°C) and in other cases low (3-7°C) another well-known way of breaking dormancy is to keep the imbibed seeds for a certain time at low temperatures (3-5°C) (stratification treatment); in some cases fluctuating temperatures are required to stimulate germination.

Light:

Light has effects on dormancy breaking, invariably via the phytochrome system; light- sensitive seeds respond to red light and their germination is promoted, whereas far red counteracts this stimulation; there does not seem to be enough evidence yet to prove that the PP pathway is involved, but there are indications that this may be the case, e.g. the changes in respiration seen after exposure to light and the involvement of phytochrome in the stimulation of gibberellic acid synthesis.

Factor # 3. Germination:

Viable, non-dormant seeds germinate if the environment is suitable. The essential environmental factors are an adequate supply of water, a suitable temperature, an adequate supply of oxygen, and in some species either the presence or absence of light. According to most definitions, a seed may be considered to have germinated when the radicle breaks through the seed coat.

Seedlings germinating on the ground become fixed in the soil by subsequent root growth. It is obviously important for this process to occur quickly, particularly at sites where conditions are subject to rapid changes. Seeds germinating within the soil must complete emergence before their reserves are exhausted. In any case, the germination process represents a risky period in the life-cycle of plants in the field.

Hence a seed’s germination rate should be high in order to ensure rapid attachment to the soil and to diminish the risk. A hundred per cent germination can be reached within one or two days or may take much longer even though the germination rate of each individual seed is still fast.

The risks are mainly determined by changes in water availability, since the most sensitive tissue, the growing root-tip, is confined to the upper soil layer which is bound to follow changing weather conditions quite rapidly. In bare soil changes in temperature and water content are much more pronounced than when there is a cover, particularly at high levels of irradiance.

Young root parts of various species do not differ according to their function, which means that differences in sensitivity between species are mainly determined by either the rate of root differentiation or the capability of the root to resume growth upon alleviation of stress. These pre-dominantly morphogenetic properties determine the changes of survival in environments with changing degrees of adversity.

Both the temperature and the water content of the soil are important in determining the rate of germination and the germination percentage ultimately reached. At constant temperatures the proportion of seeds that germinate tends to increase with rising temperature up to an optimum and then decreases at higher temperatures.

High levels of water supply may eliminate some of the seeds, thus reducing the percentage germinated although still favouring the rate of germination of the surviving seeds. The most likely assumption is that an excessive water supply affects the emergence of seedlings by reducing oxygen availability.

Due to subsequent crusting of the soil under field conditions, this situation may even continue for a certain time after a return to a lower level.

Factor # 4. Establishment of Seedlings:

Soil factors affect seedling growth via their influence on root growth and root activities. The latter can be divided into two categories, viz. absorption of water and minerals, on the one hand, and on the other, the synthesis of substances which are essential for shoot performance and are not (or insufficiently) synthesized in the shoot itself.

In the past, most attention was paid to the meaning of the absorptive capacities of the roots for the performance of the whole plant, but more recently the emphasis has shifted to the role of root-borne growth essentials (e.g. hormones) in determining such processes like shoot growth, green leaf area duration, chlorophyll formation, and even the rates of photosynthesis and transpiration.

The relative importance of these control systems may alter from case to case. As a whole, however, all these activities will depend on the supply of energy from the shoot. Since the basic problem of the young seedling is the energy supply, its success will depend on illumination conditions. If it is situated in a standing crop that absorbs most of the light, the energy supply will be poor.

Consequently, the over-all performance of the seedling will be bad and root growth will be more restricted than shoot growth. However, seedlings can perhaps survive for a rather long time in such a situation, because, to a certain extent, respiration losses are coupled to the available reserve of carbohydrate.