Physiology of Seed Germination

In this article we will discuss about:- 1. Subject-Matter of Seed Germination 2. Factors Influencing in Seed Germination 3. Mobilization of Reserves during Seed Germination 4. How to Test Seed Viability?.

Subject-Matter of Seed Germination:

Seeds develop and mature within the fruits. Once the fruit attains maturity and ripens it is shed and the seeds inside it undergo period of dormancy. In some of the succulent fruits even though the seeds are provided with moisture they do not germinate. This is because of lack of other germination factors.

Dormancy is imposed by several inhibitors present in the seed coat or the seed itself. Sometimes seed coat is thick and highly impervious to water and oxygen. In the latter part of our discussion we shall discuss some of the factors which cause seed dormancy and also how this dormancy is overcome.

Seeds will only germinate if given appropriate environmental conditions including water, air, temperature, free from high salt concentrations, inhibitors and sometimes specific spectral quality of light.

Seeds are highly dehydrated and naturally require water before germination. The first phase of seed germination is water imbibition till critical level of water is attained. Once the imbibition is completed, seeds begin to germinate and seedling emerges out. Radicle or root penetrates the seed coat and is followed by shoot emergence.

This is phase of emergence. Clearly in this phase root and shoot systems develop. Thus germination is preceded by imbibition and followed by emergence. The period of the two vary in different species and may be spread over several days or weeks.

Factors Influencing in Seed Germination:

Several environmental factors influence seed germination and these are described below:

i. Temperature:

Dry seeds can withstand diverse temperatures but once water is imbibed and germination begins they are sensitive to high temperatures. For every seed minimal, maximal and optimal temperatures exist and can be conveniently worked out. The minimal and maximal temperatures vary for different species and no reasons can be ascribed to such a variability.

ii. Oxygen:

Oxygen is essential for seed germination. The initial phase of seed germination may involve anaerobic respiration but immediately it shifts to aerobic state. In a seed where testa is retained the oxygen consumption is much higher than in the seeds where testa has been removed. Several other gases like CO₂, CO, N₂, H₂S and ozone also affect germination by affecting several metabolic processes.

iii. Inhibitors:

Several different types of compounds are known to affect seed germination and these include phenols, cyanides, alkaloids, herbacides, fungicides, salts of some metals, diverse acids, etc.

iv. Phytochrome:

Some of the seeds are responsive to light. Light may trigger or inhibit seed germination. Lettuce seeds germinate rapidly when exposed to brief red light period.

v. Age:

The age of seeds is an important factor in germination.

Mobilization of Reserves during Seed Germination:

Seeds may be endospermic or non-endospermic depending upon the state whether endosperm is retained or consumed by the cotyledonary leaves of the embryo. Following seed germination several different types of metabolites e.g. starch, proteins, fats or other polysaccharides have to be hydrolysed and mobilized for the nutrition of the growing embryo and then seedling.

Clearly during early stages of seed germination hydrolytic enzymes are activated or synthesized. Seemingly gibberellins play a very vital role in their enhancement. In cereal grains the endosperm is starchy and is surrounded by a cellular tissue called aleurone layer. Several of the hydrolases are increased or secreted in this tissue.

β-amylase enzyme concerned with starch digestion is already present in the seed. However, α- amylase and protease appear soon after germination. Several investigators have shown that removal of embryo led to non-appearance of amylases and the addition of GA could replace the embryo removal effect.

It has been concluded that β-amylase was activated whereas α-amylase was synthesized de novo. Both the processes were mediated by gibberellin. Using ^l4C-amino acids it was shown that they were incorporated in α-amylase indicating its fresh synthesis.

Gibberellins seemingly act at the molecular level and derepress the genes which cause α-amylase synthesis. Further embryo provides the requisite GA needed to initiate the synthesis or activation of amylases.

On the contrary seeds which have fats as the stored material convert fats into sugars and the latter are translocated to the growing embryo (Fig. 24-1). In such seeds fats are converted to acetyl-CoA through β-oxidation pathway. Acetyl CoA enters glyoxysomes and undergo glyoxylate cycle. In this cycle two molecules of acetyl CoA are converted to one of succinate.

Succinate is converted to oxaloacetic acid (OAA) which gives rise to phosphoenolpyruvate (PEP). Through reversal of glycolysis PEP is converted into sugars. ATP and reducing power needed in reverse glycolysis are obtained from the oxidation in the glyoxylate cycle and during succinate conversion to OAA and also from β-oxidation of fats when NADH is formed.

Figure 24-2 shows diagrammatic representation of mobilization of different nutrients in a germinating seed.

Areas of new growth and translocation of sugars, amides, etc. is clearly shown from the seed to the new centres of growth. The point to be noted is that the nitrogen transported compounds are reassembled in the growing embryo using carbon skeleton obtained from transported sugars. Thus amino acids are constituted and these are used during protein synthesis in the growing embryo.

Seed has been shown to have diverse types of storage products like fats, starch or proteins. The operation of different pathways is clearly indicated. It may be observed that ultimate products of translocation are sucrose, amides, amino acids, etc.

In summary we may state that during seed germination, the following types of metabolic processes are noticed:

(i) Water imbibition,

(ii) Organelles hydration,

(iii) Subcellular organization of the embryo or endosperm,

(iv) Alterations in the activity of phytochrome (if operative)

(v) Enzymes activation,

(vi) Enzymes synthesis de novo,

(vii) Hydrolysis of metabolites e.g. fats, starch, proteins etc.

(viii) Formation of organic molecules and their translocation to the new centres of growth,

(ix) Synthesis of nucleic acids and proteins,

(x) Oxygen uptake and respiration,

(xi) Enlargement of cell and cell division,

(xii) Upsurge of phytohormones,

(xiii) Synthesis of membranes and other cellular constituents,

(xiv) Variation in CO₂ and O₂ levels.

In the following we shall briefly discuss the chemical changes during germination of maize (monocot) seeds:

Maize Seed:

Maize seed is a grain filled up with starch and some amount of protein as well. Endosperm is surrounded by aleurone layers. It has one cotyledon which is modified into scutellum. Scutellum cells secrete hydrolases which digest endosperm metabolites.

Once immersed in water, maize seeds imbibe water and increase in diameter. Imbibition phase is completed within 12-14 hours of soaking. This is followed by enlargement of radicle and coleorhiza.

Seed coat is ruptured by the coleorhiza within 20-24 hours of imbibition and soon after radicle emerges out of seed or grain. In maize given favourable conditions and environments, germination is accomplished within a day or so. Biochemically changes begin after 24 to 48 hours of radical emergence. There is change in dry weight indicating loss of some metabolites from the endosperm.

Both fats and starch are digested after 72 hours of germination. Nitrogenous substances change after radical growth. It has been shown that after water imbibition by the seeds there is high metabolic activities and then the second upsurge of activity takes place after 72 hours. In the first phase there is high nucleic acids, protein and enzymes synthesis and activity.

How to Test Seed Viability?

The percentage of viable seeds can be determined through several methods and some of these are briefly mentioned below:

(i) Direct germination. A desired sample of seeds is germinated and viability percentage computed.

(ii) Seeds are soaked in distilled water and the electric conductivity of surrounding medium is determined. If there is high proportion of non-viable seeds then the conductance will increase.

(iii) Seeds soaked in potassium permagnate dilute solution also provide some indication on viability. If the number of seeds in a given sample is large then the KMnO4 solution will rapidly decolorize. Non-viable seeds are permeable and release high amounts of electrolytes and reducing substances into the surrounding medium.

(iv) In seeds with prolonged dormancy embryo is removed from the cotyledons or endosperm and put on sterilized nutrient medium. The viability is known within a week or so.

(v) Some seeds are split open and immersed in some redox dye (TTC) or tested for peroxidase using histochemical staining reaction. The loss of peroxidase or some dehydrogenase reactions also indicate dead nature of the seeds.