Seed germination is the biological process where a dormant seed embryo resumes growth and develops into a seedling. A seed is a protective, self-contained package containing an undeveloped plant embryo, stored food, and a protective seed coat. The seed remains in a low-metabolic state until environmental cues signal that conditions are suitable for the young plant to emerge. This growth phase depends entirely on the successful combination of three fundamental external requirements.
The Three Essential Requirements for Germination
The first requirement for a seed to resume growth is water. The seed rapidly absorbs water from the environment through imbibition, causing it to swell and soften its protective coat. This hydration rehydrates the embryo’s tissues and activates stored enzymes within the seed. These activated enzymes begin breaking down the seed’s stored food reserves into usable forms for the growing embryo.
Once the seed’s tissues are hydrated, the second requirement, oxygen, becomes necessary. Oxygen is used for aerobic respiration, which efficiently converts mobilized food reserves into chemical energy (ATP). This rapid energy production fuels the intense cellular activities of germination, including cell division and the elongation of the embryonic root (radicle). Limited oxygen forces the seed to rely on less efficient anaerobic respiration, often resulting in poor or failed germination.
The third necessary condition is a suitable temperature range. Temperature governs the rate of chemical reactions within the seed by controlling enzyme activity. Every species has a minimum, optimum, and maximum temperature range for germination. Moving outside this range slows or halts the process entirely. Low temperatures drastically slow enzyme activity, while excessively high temperatures can permanently denature the enzymes, preventing germination.
Distinguishing Germination from Seed Dormancy
A seed waiting only for the three necessary conditions (water, oxygen, and suitable temperature) is considered quiescent. Quiescence is quickly reversed once favorable environmental conditions return. Seed dormancy, by contrast, is a biological state where a viable seed is prevented from germinating even when all three external conditions are present.
Dormancy is an evolutionary survival mechanism that prevents a seed from sprouting at an inappropriate time, such as during a temporary warm spell. This state is often maintained by internal chemical inhibitors, like the plant hormone abscisic acid (ABA), which must be overcome. Some seeds exhibit physical dormancy due to hard seed coats that are impermeable to water or oxygen.
To break true dormancy, the seed requires a specific environmental trigger beyond the three basic requirements. These triggers mimic natural events. Scarification involves physically weakening the seed coat to allow water uptake. Stratification requires a period of cold, moist conditions to chemically break down internal inhibitors. These mechanisms ensure germination only happens when the prospects for seedling survival are high.
Conditions That Are Often Misunderstood
Two factors often mistakenly believed to be universally necessary for germination are light and external nutrients. Light requirements vary significantly between species, classifying seeds as either photoblastic or neutral. Some very small seeds require light to germinate (positive photoblastic) because it signals they are near the soil surface.
Conversely, many larger seeds are inhibited by light (negative photoblastic) or are entirely neutral to it. Light becomes a universal requirement only after germination is complete, when the seedling must begin photosynthesis for sustained growth. Similarly, external nutrients, such as those found in soil or fertilizer, are not required for the initial germination process. The embryo relies entirely on the stored food reserves within the cotyledons or endosperm until its own root and leaf systems are functional. Nutrients are only necessary for the growth and development of the seedling after it has successfully emerged from the seed coat.