The Effects of Leaf Extract on Seed Germination By McKenna Manning & Ainsley Chapman

The seed is the flowering plant’s most basic unit of reproduction, which in response to the environmental cues it receives has evolved to assure the plants greatest chance of survival.

The project which we plan to undertake will involve five common plant species of the lower Sonoran Desert and are all located on the Phoenix College grounds, including: brittlebush, creosote bush, bursage as well as ironwood and palo verde trees.

But first, a brief review:

Anatomy of the Seed

Seeds vary according to their plant species, via natural selection and various other evolutionary forces they have evolved a number of different anatomies. As an example some are small and easily carried in the wind to either high or distant places. These seeds generally contain very little endosperm and therefore have little reserve energy to support a sprout during germination.

On the other hand, seeds that typically germinate underground tend to contain high reserves of endosperm. This is because the deeper a seed is buried the more energy is required to produce a sprout that will break the surface and begin the process of photosynthesis – thereby creating its own food.

Dormancy and Germination       

Dormancy is a state in which the seed will not germinate. This does not mean that the seed is defective or not viable; it simply means that the seed will not germinate until dormancy has been removed or alleviated. Dormancy does not have a simple temperature, time or environmental cause it is the result of, “blocks within the seed that prevent germination, as distinguished from the absence of factors required to evoke germination”(Vleeshouwers, 1995). During this time the seed metabolism is slowed and very little energy is required to stay alive.

Germination is an awaking from dormancy. It is the process, under the right conditions, when a seed begins the process of plant growth. The process of germination is typically divided into three main phases:

1.     Activation Phase

This phase begins with imbibition or the uptake of water by the seed. The most dramatic physical change in the seed is an increase in volume. The seed respiration increases and a number of enzymes are manufactured.   The enzymes will break down the endosperm into simple compounds such as sugar which will be used for growth. By the end of this phase the embryonic cells have begun to elongate and the radicle has started to lengthen.

2.     Digestion and Translocation Phase

In this phase there is a dramatic increase in the metabolic activity. Protein synthesis begins as the stored energy in the endosperm becomes metabolized.  The synthesis of enzymes to soften the cell was begins and there is a further increase in cell volume and elongation. Nutrients are transported to areas of growth, such as the embryo axis and roots.

3.     Seedling growth

This is the final stage of the germination process, when the radicle emerges from the seed coat and the embryo obtains access to water and nutrients in the environment via its’ developing root system. In this stage the primary activity is rapid cell elongation and cell division.  

Our next post will explore the factors that influence germination.

Vleeshouwers, L. M. (1995). Redefining seed dormancy: an attempt to integrate physiology and ecology. The Journal of Ecology, 83, 1031-1037. Retrieved from http://www.jstor.org.ezproxy1.lib.asu.edu/stable/2261184