The process of seed germination is the moment when a dormant seed embryo develops into an active, independent seedling. This transformation is triggered only when a specific set of environmental conditions is met. Three requirements are universally necessary for a seed to successfully break dormancy and begin to grow.
These needs are water, which initiates metabolic processes; a proper temperature range, which activates growth enzymes; and oxygen, which fuels the high rate of respiration. While these three factors must always be present, the requirement for light is far more variable. Light acts as a specialized environmental cue for certain species, determining whether a seed will sprout or remain dormant.
Light Requirements of Most Seeds
The vast majority of seeds are non-photoblastic, meaning they are indifferent to the presence or absence of light for germination. These seeds germinate just as readily in complete darkness as they do under bright light, provided all other conditions are met. This indifference is a biological advantage for species whose seeds are naturally buried by animals, wind, or soil.
These seeds contain substantial internal food reserves that provide enough energy for the seedling to push its way through the soil until it reaches sunlight. If they were dependent on light, they could not germinate from a buried position, which would limit the plant’s ability to propagate. Common examples of light-indifferent seeds include many popular vegetables and grains, such as corn, beans, peas, and squash.
Seeds That Must Have Light
While most seeds are light-indifferent, many species have evolved a specific response to light, known as photoblasty. This mechanism ensures the seed only germinates when conditions favor survival. These exceptions are divided into two groups: those that require light (positively photoblastic) and those inhibited by light (negatively photoblastic).
Positively photoblastic seeds, such as lettuce, petunia, and many herbs, are typically small and have minimal internal food reserves. Germination must occur on or very near the soil surface, as they lack the energy to push through even a thin layer of soil. The light requirement guarantees the seed is not buried too deeply, preventing the seedling from exhausting its limited resources before it can begin photosynthesis.
Negatively photoblastic seeds, including species like onion, some grasses, and certain varieties of Amaranthus, require darkness to sprout successfully. For these, the presence of light inhibits germination. This response often occurs in seeds dispersed in open, sunny environments where surface germination could lead to desiccation. Requiring darkness ensures the seed is safely covered by soil where moisture levels are more stable.
The Internal System for Light Detection
The mechanism by which seeds sense light is managed by a pigment protein called phytochrome. Phytochrome acts as a biological switch, existing in two interconvertible forms: Pr and Pfr. The Pr form is the inactive state and absorbs red light, which is abundant in direct sunlight. When Pr absorbs red light, it converts into the Pfr form, the biologically active state that promotes germination.
Conversely, the Pfr form absorbs far-red light, a wavelength prevalent in light filtered through a leaf canopy or deep soil. Absorption of far-red light converts Pfr back to the inactive Pr form, which inhibits germination.
This interconversion allows the seed to assess its immediate environment. If a seed is near the surface in full sun, the high proportion of red light shifts the balance toward the active Pfr form, signaling conditions are right to sprout. If the seed is buried or beneath dense leaves, the light contains more far-red wavelengths, maintaining the inactive Pr form and keeping the seed dormant.
The seed measures the ratio of red to far-red light to decide its fate. A high ratio indicates open ground and a clear path to sunlight, triggering the internal cascade that breaks dormancy. This system prevents a small seedling from wasting resources on a futile attempt to grow in deep shade or from an inescapable depth.
Practical Rules for Planting Depth
Understanding a seed’s light response translates directly into successful planting techniques. For most light-indifferent seeds, plant them at a depth roughly two to three times their diameter. This ensures they remain consistently moist and anchored, yet are shallow enough for the emerging seedling to push through the soil easily.
For positively photoblastic seeds, the approach changes completely. Since they require light, these small seeds should be surface-sown and only lightly pressed onto the soil. A common technique is to use a fine material like horticultural vermiculite or a thin dusting of soil to cover them. This covering helps maintain moisture around the seed, which is prone to drying out on the surface, while allowing light to penetrate and activate the phytochrome system.
Conversely, negatively photoblastic seeds, such as onions, must be covered completely to ensure germination in darkness. Following the general depth rule is usually sufficient to exclude light and create the necessary dark environment. Applying these rules based on the seed’s biology increases the likelihood of a successful start for the new plant.