How do differents factors affect seed germination?

The essential factors for seed germination are
i) Moisture or water
ii) Oxygen
iii) Temperature
iv) Light
Water A dormant seed contains 10-15% of water and is generally dehydrated.
So the dormant seed has to absorb water to become active and exhibit germination.
Water makes the seed coat soft, causes it to rupture after swelling and start germination.
Water is also needed to bring in the dissolved oxygen for use by the growing embryo.
OxygenIn the dormant condition the seeds respiratory rate is very low and so oxygen is required in very small quantities.
But for germination, oxygen is needed in large quantities.
The seeds obtain this oxygen from the air contained in the soil.
Seeds sown deeply in soil, fail to germinate because of lack of oxygen.
Ploughing aerates the soil and helps in good germination.
TemperatureGermination can take place over a wide range of temperature
(5 - 40o C), but the optimum for most of the crops is between 25o C to 30o C.
LightLight has varied effects on germinating seeds of different plants. Some seeds need light for germination,
while in some seeds germination is hindered by light.

1. Abiotic factors that control or affect seed germination
2. Temperature = Low or cold temp. is not favorable for seed germination . They prefer higher temp.To put it differently -' The rate of seed germination is directly proportionate to the rise in temp ( Up to a limit ! you don't expect seed germination at 100 degree Celsius! )
3. Moisture or water = Dry seeds do not germinate . They must first imbibe water to trigger off the process of germination
4. soil = It is not required initially , but as the seedlings grow they require mineral elements for further growth . this requirement can be fulfilled only by soil.
5. Light = for germination only light is not required but later on it plays a dominant role . Biotic factors that control seed germination
6. Viability of the seeds = After the seeds are produced they remain viable ( have potential to germinate ) up to certain period that varies from plant to plant or seed to seed .( It is like expiry date of a medicine! ).Lotus seeds have viability period of 800 years Once that period expires , the seed germination is difficult . They are as good as dead
7. Dormancy period = Many seeds do not germinate immediately after they are produced . The reason is they require a resting period during which they remain dormant and once that period is over they are ready to germinate