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The germination of seeds is dependent on both internal and external conditions. The most important external factors include: temperature, water, oxygen and sometimes light or darkness. Different plants have seeds that require distinctive variables for successful germination. Often this depends on the individual seed variety and is closely linked to the ecological conditions of the plants' natural habitat. For some seeds, future germination responses are affected by environmental conditions during seed formation; most often these responses are types of seed dormancy.
Water - is required for germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the seeds dry weight, before cellular metabolism and growth can resume. Most seeds respond best when there is enough water to moisten the seeds but not soak them. The uptake of water by seeds is called imbibition which leads to the swelling and the breaking of the seed coat. When seeds are formed, most plants store food, such as starch, proteins, or oils, to provide nourishment to the growing embryo inside the seed. When the seed imbibes water, hydrolytic enzymes are activated that break down these stored food resources in to metabolically useful chemicals, allowing the cells of the embryo to divide and grow, so the seedling can emerge from the seed. Once the seedling starts growing and the food reserves are exhausted, it requires a continuous supply of water, nutrients and light for photosynthesis, which now provides the energy needed for continued growth.
Oxygen - is required by the germinating seed for metabolism: If a seed is in soil with too much water or is buried too deeply, it can be cut off from the necessary oxygen. Oxygen is used in aerobic respiration, the main source of the seedling's energy until it grows leaves. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment.
Temperature - affects cellular metabolic and growth rates. Different seeds germinate over a wide range of temperatures, with many preferring temperatures slightly higher than room-temperature while others germinate just above freezing and others responding to alternation in temperature between warm to cool. Some seeds germinate when the soil is cool 28-40F (-2 - 4C), while most germinate when the soil is warm 60-75F (16 - 24C) and some when the soil is even warmer 76-90F (24 - 32C). Often, seeds have a set of temperature ranges where they will germinate and will not do so above or below this range. In addition, some seeds may require exposure to cold temperature (vernalization) to break dormancy before they can germinate. As long as the seed is in its dormant state, it will not germinate even if conditions are favorable. Seeds that are dependent on temperature to end dormancy, have a type of physiological dormancy. For example, seeds requiring the cold of winter are inhibited from germinating until they experience cooler temperatures. For most seeds that require cold for germination 4C is cool enough to end dormancy, but some groups especially with in the family Ranunculaceae and others, need less than -5C. Some seeds will only germinate when temperatures reach hundreds of degrees, as during a forest fire. Without fire, they are unable to crack their seed coats; this is a type of physical dormancy.
Light or darkness - can be a type of environmental trigger for germination in seeds and is a type of physiological dormancy. Most seeds are not affected by light or darkness, but many seeds, including species found in forest settings will not germinate until an opening in the canopy allows them to receive sufficient light for the growing seedling.
Stratification mimics natural processes that weaken the seed coat before germination. In nature, some seeds require particular conditions to germinate, such as the heat of a fire (e.g., many Australian native plants), or soaking in a body of water for a long period of time. Others have to be passed through an animal's digestive tract to weaken the seed coat and enable germination.
Sunflower seedlings, just three days after germination
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