Seed dormancy

• The mature seeds do not germinate under optimum conditions. The failure of viable seeds to germinate under suitable condition is termed as dormancy.

Types:

• Imposed dormancy: due to unfavorable condition.
• Innate dormancy; when active growth is arrested even on favorable condition.
• Secondary dormancy; many crops capable of germinating immediately after ripening but pass to dormant phase this type is called as secondary dormancy.

Categories of seed dormancy:

A) Seed coat:

1. Hard seededness: Seed fail to imbibe water due to impermeable seed coat.

2. Mechanical resistance: Rigid seed coverings resist embryo expansion even though the tissues may successfully take up water.

3. Chemical inhibitors: Seed coverings contain chemicals which inhibit germination

4. Morphologically undeveloped embryo (Rudimentary embryo): Some species produce seeds with extremely small and underdeveloped embryos in seeds that are mature enough to detach from the plant. Such seeds simply need much more time for the embryo to grow and develop within the seed prior to germination.

B) Internal dormancy:

1. Physiologically shallow dormancy: Present in some freshly harvested seed. Disappears with dry storage over a period of days or months.

2. Light dormancy: Some species have exact requirements of light quality, quantity, and duration to induce germination.

3. Temperature dormancy: Many seeds are specific in their temperature requirements. Some need alternating temperatures (day/night). Others germinate over a narrow range of temperatures and go dormant at temperatures above or below this range.

4. Physiologically deep dormancy: Seeds are unable to germinate until they undergo a cool-moist period over a few days or a few months.

5. Combined dormancy: More than one type of dormancy may occur within the same seed. Induced or induced or secondary dormancy: Viable, non-dormant seeds may become dormant when placed in environments which are unfavorable to germination.

C) Exogenous dormancy

Exogenous dormancy is caused by conditions outside the embryo and is often broken down into three subgroups:

Ø Physical dormancy: Occurs when seeds are impermeable to water or the exchange of gases. Legumes are typical examples of physically dormant seeds; they have low moisture content and are prevented from imbibing water by the seed coat.

Ø Mechanical dormancy: Mechanical dormancy occurs when seed coats or other coverings are too hard to allow the embryo to expand during germination. These endogenous facts include physiologically dormancy caused by low embryo growth potential.

Ø Chemical dormancy: Includes growth regulators etc., that are present in the coverings around the embryo. They may be leached out of the tissues by washing or soaking the seed, or deactivated by other means. Other chemicals that prevent germination are washed out of the seeds by rainwater or snow melt.

D) Endogenous dormancy

Endogenous dormancy is caused by conditions within the embryo itself, and it is also often broken down into three subgroups: physiological dormancy, morphological dormancy and combined dormancy, each of these groups may also have subgroups.

Ø Physiological dormancy: Physiological dormancy prevents embryo growth and seed germination until chemical changes occur. These chemicals include inhibitors that often retard embryo growth to the point where it is not strong enough to break through the seed coat or other tissues. Physiological dormancy is indicated when an increase in germination rate occurs after an application of gibberellic acid (GA3) or after Dry after ripening or dry storage. It is also indicated when dormant seed embryos are excised and produce healthy seedlings: or when up to 3 months of cold (0–10°C) or warm (=15°C) stratification increases germination: or when dry after-ripening shortens the cold stratification period required. In some seeds physiological dormancy is indicated when scarification increases germination.

Seeds with physiological dormancy most often do not germinate even after the seed coat or other structures that interfere with embryo growth are removed.

Ø Morphological dormancy: Embryo underdeveloped or undifferentiated. Some seeds have fully differentiated embryos that need to grow more before seed germination, or the embryos are not differentiated into different tissues at the time of fruit ripening.

ØCombinational dormancy: Combinational dormancy occurs in some seeds, where dormancy is caused by both exogenous (physical) and endogenous (physiological) conditions. some Iris species have both hard impermeable seeds coats and physiological dormancy.

Conditions that affect physiological dormancy of seeds

• Drying;
• Photodormancy
• Thermodormancy