Physiological Seed Dormancy


Physiological Seed Dormancy: Inhibitors and Promotors

Dormancy due to inhibitors is based upon the fact that germination and growth promoting enzymes and hormones can be inhibited, thus preventing germination. Inhibitors, such as absissic acid (ABO) may be at sufficient level to counteract growth promoting enzymes, such as gibberellins (GA).

Usually it is the balance or ratio between inhibitors and promotors that needs to be tipped in the favor of those that will allow germination to proceed. These inhibitors are found in the endosperm, cotyledons, or other food storage tissue. Sometimes these chemicals are found in the outer coverings of the seed or fruit.

Many of these chemicals are water soluble and can be leached from the seed, thus shifting the balance towards the growth promoting chemicals and allowing it to germinate. Others must be degraded into other forms or chemicals to reduce their concentration. With inhibitors that are found within the embryonic axis, it is temperature (and sometimes light) that generally controls this shift.

Temperature may also favor the production of growth promoting hormones and enzymes in the embryonic axis. Cool temperatures generally shift the balance of promotors and inhibitors towards promoting germination.

Physiological Seed Dormancy: The Role of Temperature Control

As noted earlier when discussing membrane permeability, cool temperatures make certain membranes allow oxygen to be uptaken into the seed. At the same time, cool temperatures may lower the demand for oxygen in the respiration process, thus making it more readily available for other uses.

Oxygen then can be used to oxidate and degrade germination inhibitors while at the same time be used to activate germination promotors. This shifts the balance towards promoting the seed to germinate if the conditions are right.

There are other processes within the seed that are affected by temperature. Cool temperatures trigger shifts in metabolic pathways when the seed is in moist, imbibed conditions. Nucleic acid synthesis is aided by these cool-moist conditions that are found naturally in the spring after snow melt.

Cool temperatures also aid in the digestion of some food reserve components, thus allowing for an increase in germination energy. One other, but very important factor affected by cool temperatures is that they aid in the softening of the endosperm structure. This is particularly true for the area of the endosperm that surrounds the radicle of the embryo. This softening diminishes the physical barrier that impedes the protrusion of the radicle during the germination process.

Physiological Seed Dormancy: Light and its Role

Light has an important role in the dormancy of some seed species. This light sensitivity may also be connected to the temperature events that we saw above. And, there has been shown that light may also have and interaction with KNO3 and other chemicals that have been shown to promote germination in some species.

It isn't just light but the quality of light reaching the seed. In the diurnal (night to day) shift, the quality of light changes in the red spectrum. During the day, red light in the spectrum promotes shifts within the seed that allow it to germinate.

During the night, the main light comes from the sun reflecting off the moon. This light is strong in the far-red spectrum and light in that spectrum inhibits the processes that favor germination.

Many seed species with small seeds need light to germinate. When buried deeply in the soil, they lack the light and go dormant. This can be seen in your garden where you think you have gotten rid of the weeds and then you disturb the soil. This disturbance brings seeds to the surface where they are stimulated by the light and begin to germinate.

To continue the discussion on seed dormancy go to: Classification of Dormancy Types.

If you wish to go back to the previous page, go to: Dormancy Mechanisms.