Factors affecting seed germination

1. External factor

a. Temperature:

• Generally, germination is most satisfactory around 25-300C.
• The minimum and maximum temperature just permit germination while the optimum temperature permits the highest percentage of germination in the shortest period.

b. Moisture:

• Water is essential for enzyme activation, thus permitting breakdown, translocation, and use of reserve food materials.
• Field capacity moisture is about optimum for germination in soil and extreme moisture may inhibit germination.

c. Air (O2 and CO2):

• It is observed that O2 is required for the germination of most species; CO2 concentration higher than 0.03% retard germination and N gas has no influence.
• If the O2 concentration is reduced substantially below that of air, the germination of most seeds is retarded.

d. Light:

• Tobacco seeds germinate better in light than in darkness.
• Freshly harvested lettuce seed would also germinate in light but after storage germinability will be manifested in red light only.
• Datura, tomato, and onion would germinate at a faster rate in the dark than in the light.
• Light intensities of 100-200 FC from indirect light in the average seed laboratory are probably adequate for the germination of most species.

2. Internal or seed factors

a. Reserved food materials:

• Seeds contain stored food materials like CHO, proteins, and lipids.
• If these are not accumulated in appropriate amounts in the seed, they may not germinate.

b. Resting period

• After seed maturity, a rest period is necessary to develop the embryo after harvesting. This period is known as after ripening period or resting period.

c. Viability of seeSeedsed retain viability for a certain period, after which the embryo becomes dead.

• Storage conditions and circumstances in which the seed matures often determine the period of viability.

d. Presence of poisons and inhibitors:

• Hydrogen cyanide will poison and kill the growing embryo.
• Extracts from fruits, leaves, twigs, and roots also have been found to inhibit seed germination.
• For example, the seeds of a tomato will not germinate as long as they are enclosed within the fruit, but if they are removed and thoroughly washed free of fruit tissues, they will germinate.
• Plant extracts consist of a variety of complex organic compounds like alkaloids, essential oils, and abscisic acid that may inhibit seed germination.

3. Agronomic factors

• Cloudy days during grain filling produce chaffy non-viable rice seeds.
• Inadequate plant protection during fruit ripening may cause total loss of seed germination.
• Mechanically injured seeds due to rough handling during production, harvesting, and packaging may cause about 20-30% loss in germination.