Light is a fundamental environmental cue that dictates the transition from a dormant seed to an actively growing seedling. This process, known as germination, requires the seed to resume metabolic activity and growth. Alongside water availability and temperature, light acts as a signal, informing the seed about its surroundings and whether conditions are favorable for survival. Seeds that require light to germinate are typically small and lack the stored energy to push through deep soil. Sensing light ensures they are near the surface or in a gap in the existing vegetation canopy.
The Phytochrome Sensor System
Light is not used for energy production during germination, but rather as a specific piece of information. The seed detects this signal using specialized photoreceptor proteins, primarily phytochrome. This molecule exists in two interconvertible forms that act like a biological switch to sense the light spectrum.
The default, inactive form is called Pr (phytochrome red), which absorbs red light (near 660 nanometers). When Pr absorbs red light, it rapidly converts into the biologically active form, Pfr (phytochrome far-red). Pfr acts as the active signal that initiates germination. If Pfr absorbs far-red light (around 730 nanometers), it switches back to the inactive Pr form, reversing the germination signal.
This molecular switch allows the seed to assess its environment. The active Pfr form triggers downstream hormonal changes within the seed. Germination is controlled by the balance of two major hormones: gibberellic acid (GA) and abscisic acid (ABA).
Pfr promotes germination by increasing GA levels, which supports growth, and simultaneously decreasing ABA levels, which enforces dormancy. The degradation of specific transcription factors, such as PIF1, by active Pfr is a key step in altering the metabolism of these hormones. This mechanism ensures the seed only commits to germination when the light signal confirms a viable surface environment.
Classifying Seeds by Light Requirement
Seeds are classified into three categories based on their germination behavior, reflecting evolutionary strategies for survival. The term photoblastic refers to seeds sensitive to light.
Positive Photoblastic Seeds
Seeds that require light exposure to germinate are known as positive photoblastic. This strategy is common among species with small seeds, such as lettuce, some grasses, and parsley. Germination is inhibited if these seeds are buried too deeply, ensuring the seedling sprouts only near the soil surface or in a sunlight gap.
Negative Photoblastic Seeds
Conversely, negative photoblastic seeds are actively inhibited by light and prefer total darkness beneath the soil surface. Examples include some varieties of onion and Amaranthus. This adaptation often ensures seeds are buried to access moisture or avoid predation.
Neutral or Aphotoblastic Seeds
The final group consists of neutral or aphotoblastic seeds, which germinate regardless of light exposure. Many large-seeded crops, including beans, corn, and peas, fall into this category. These seeds have sufficient nutrient reserves to establish a seedling even if buried, making the light signal less relevant.
The Influence of Light Quality and Intensity
The phytochrome system highlights that light quality is important for triggering germination. Light quality refers to the spectral composition, particularly the ratio between red (R) light and far-red (FR) light. The R:FR ratio is the primary environmental factor determining the final state of the phytochrome molecule inside the seed.
Full sunlight in open areas has a high R:FR ratio, meaning there is an abundance of red light that converts Pr to the active Pfr form. This high ratio signals to positive photoblastic seeds that the environment is suitable for growth, promoting germination. When light passes through a dense leaf canopy, chlorophyll absorbs most of the red light, while far-red light is transmitted or reflected.
Under a canopy, the R:FR ratio drops significantly, leading to the conversion of Pfr back to the inactive Pr form. This low ratio signals that the seed is shaded by competing plants, inhibiting germination until a canopy gap appears.
Light intensity, or irradiance, also plays a role, though it is secondary to quality. Even if the R:FR ratio is favorable, the overall light intensity must be above a threshold to ensure enough phytochrome converts to the active Pfr form. Low light levels may not provide a strong enough signal to overcome hormonal resistance to germination.
Applying Light Knowledge to Successful Planting
Understanding seed light requirements is essential for successful planting, primarily through controlling depth. For positive photoblastic seeds, such as various herbs and flowers, the goal is to ensure necessary light exposure. These seeds must be sown very shallowly, either directly on the soil surface or covered with only a thin layer of planting medium.
Burying a positive photoblastic seed more than a few millimeters deep prevents red light from reaching the phytochrome sensor, maintaining dormancy. Conversely, negative photoblastic seeds, like certain vegetable varieties, must be planted deep enough to guarantee total darkness. The soil acts as the necessary light block, allowing germination without inhibitory illumination.
For all light-sensitive seeds, caution is advised after sowing. Even brief exposure to white light, such as checking dark-germinators with a flashlight, can reset the phytochrome switch to the inhibitory state. Using a green light, which is largely ineffective at switching phytochrome, is a common practice in research to avoid prematurely triggering or inhibiting germination.