Factors affecting seed germination

A. External factor

1. 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 of time.

2. 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.

3. Air (O2 and CO2):

• It is observed that O2 is required for 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, germination of most seeds is retarded.

4. 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 dark than in light.
• Light intensities of 100-200 FC from indirect light in the average seed laboratory are probably adequate for germination of most species.

B. Internal or seed factors

1. Reserved food materials:

• Seed contain stored food materials like CHO, proteins and lipids.
• If these are not accumulated in appropriate amount in the seed, it may not germinate.

2. 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.

3. Viability of seed

• Seed retain viability for a certain period of time, after which the embryo becomes dead.
• Storage conditions and circumstances in which the seed mature often determine the period of viability.

4. Presence of poisons and inhibitors:

• Hydrogen cyanide will poison and kill growing embryo.
• Extracts from fruits, leaves, twigs and root also have been found to inhibit seed germination.
• For example, the seeds of 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.

C. 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.