Seed germination

Germination is the growth of an embryonic plant contained within a seed, it results in the formation of the seedling. The seed of a higher plant is a small package produced in a fruit or cone after the union of male and female sex cells. Most seeds go through a period of quiescences where there is no active growth, during this time the seed can be safely transported to a new location and/or survive adverse climate conditions until it is favorable for growth. The seed contains an embryo and in most plants stored food reserves wrapped in a seed coat. Under favorable conditions, the seed begins to germinate, and the embryonic tissues resume growth, developing towards a seedling.

Requirements for seed germinationThe 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 - It 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 the soil is waterlogged or the seed is buried within the soil, it might be cut off from the necessary oxygen it needs. Oxygen is used in aerobic respiration, the main source of the seedling's energy until it has leaves, which can photosynthesize its energy requirements. Some seeds have impermeable seed coats that prevent oxygen from entering the seeds, causing seed dormancy. Impermeable seed coats to oxygen or water, are types of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange or water uptake between the seed and its 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, while most germinate when the soil is warm (60-75F) and some when the soil is even warmer (76-90F). 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.

Dormancy- Many live seeds have dormancy, meaning they will not germinate even if the environment has sufficient water and warmth for the seed to germinate. Dormancy factors include conditions affecting many different parts of the seed, from the embryo to the seed coat. Dormancy is broken or ended by a number of different conditions, and is caused be internal or external and sometimes both factors. Environmental factors like light, temperature, fire, ingestion by animals, are conditions that can end seed dormancy. Internally seeds may be dormant because of plant hormones, which affect cell growth and prevents germination. One plant hormone that is a common dormancy inducing chemical is absciscic acid, while the production and application of the hormone gibberellin can break dormancy and induces seed germination. The use of hormones to end seed dormancy is used in brewing, where barley is treated with gibberellin to ensure uniform seed germination in the production of barley malt.

Photoperiodism-Many flowering plants use a photoreceptor protein, such as phytochrome or cryptochrome, to sense seasonal changes in night length, or photoperiod, which they take as signals to flower. In a further subdivision, obligate photoperiodic plants absolutely require a long or short enough night before flowering, whereas

facultative photoperiodic plants are more likely to flower under the appropriate light conditions, but will eventually flower regardless of night length.Photoperiodic flowering plants are classified as long-day plants or short-day plants, though the regulatory mechanism is actually governed by hours of darkness, not the length of the day.Modern biologists believe that it is the coincidence of the active forms of phytochrome or cryptochrome, created by light during the daytime, with the rhythms of the circadian clock that allows plants to measure the length of the night. Other than flowering, photoperiodism in plants includes the growth of stems or roots during certain seasons, or the loss of leaves.Long-day plantsA long-day plant requires fewer than a certain number of hours of darkness in each 24-hour period to induce flowering. These plants typically flower in the late spring or early summer as days are getting longer.

Some long-day obligate plants are:Carnation (Dianthus)Henbane (Hyoscyamus)Oat (Avena)Ryegrass (Lolium)Clover (Trifolium)Bellflower (botany) (Campanula carpatica)

Some long-day facultative plants are:Pea (Pisum sativum)Barley (Hordeum vulgare)Lettuce (Lactuca sativa)Wheat (Triticum aestivum, spring wheat cultivars)Turnip (Brassica rapa)

Short-day plantsShort-day plants flower when the night is longer than a critical length. They cannot flower under the long days of summer. In general, these plants flower in late summer or fall, as days are getting shorter. Short-day plants will not flower if a pulse of artificial light is shone on the plant for several minutes during the middle of the night; they require a consolidated period of darkness before floral development can begin. Natural nighttime light, such as moonlight or lightning, is not of sufficient brightness or duration to interrupt flowering.

Some short-day obligate plants are:ChrysanthemumCoffeePoinsettia

StrawberryTobacco, var. Maryland MammouthCommon duckweed, (Lemna minor)Cocklebur (Xanthium)Maize - tropical cultivars only

Some short-day facultative plants are:Hemp (Cannabis)Cotton (Gossypium)RiceSugar cane

Day-neutral plantsDay-neutral plants, such as cucumbers, roses and tomatoes, do not initiate flowering based on photoperiodism at all; they flower regardless of the night length. They may initiate flowering after attaining a certain overall developmental stage or age, or in response to alternative environmental stimuli, such as vernalization (a period of low temperature), rather than in response to photoperiod.