Unit A 5 2 Plant Science Problem Area

Unit A 5 -2 Plant Science

Problem Area 5 Initiating Plant Growth

Lesson 2 The Role of the Embryo in Germination

Student Learning Objectives n n n 1. Explain the chain of events that occur during the germination process. 2. Describe the source and function of amylase during seed germination. 3. Explain how seed condition at harvest affects germination potential and eventual use of the grain for processing.

Terms n n n aleurone layer amylase diffusion enzyme gibberellin hydrolysis reaction n n metabolism micropyle osmosis protease water-soluble

What steps are involved in the germination process? n Germination occurs in three stages: water intake, increase in metabolic activity, and embryo enlargement. n A. Imbibition is the intake of water. It is the first step in the germination process. When the seed takes up water through the micropyle, a small opening in the seed coat, cells begin to enlarge thereby creating greater turgidity in the seed. Water content rises from 5– 10% to 30– 40% as cells are restored to full turgor. The intake of water is also associated with an increase in respiration.

n B. Increased metabolic rate is the second stage in germination. This increase in metabolism, a sum of all chemical processes that occur in organisms, requires that the large starch and protein molecules stored in the endosperm or cotyledons must be broken down for use by the embryo. n C. The enlargement of the embryo is the final step. Once the starch and protein molecules are broken down they are then used to accelerate the development of the embryo. As this occurs the embryo increases in size causing the seed coat to rupture. The radicle emerges from the seed followed by the emergence of the shoot from the soil.

Where is amylase produced and what is its function during seed germination? n Amylase is an enzyme that breaks down starch into sugars. n A. Enzymes are proteins substances that affect the rate at which chemical reactions can occur. During the reaction, however, the enzyme is not consumed. n B. After water has been absorbed into the seed through the micropyle, the water spreads throughout the seed through the process of diffusion, moving from areas of high concentration to areas of low concentration. n C. The presence of water in the embryo triggers the production of gibberellin, a plant growth hormone.

n D. The gibberellin is absorbed into the water that passes out of the embryo by osmosis, the movement of water from areas of high concentration to areas of low concentration across a semi-permeable membrane, and diffuses throughout the rest of the seed where it comes in contact with the aleurone layer, a group of cells in the endosperm responsible for the production of amylase and protease. n E. The amylase diffuses through the endosperm wherein the presence of water in soluble starch molecules are digested into glucose and maltose both of which are water-soluble, able to be dissolved by water. The protease diffuses and converts the stored protein into amino acids. The process of starch digestion is a hydrolysis reaction, the addition of water to a substance being degraded in a chemical reaction. n F. The glucose and maltose are then moved to the embryo where cellular respiration and mitosis can continue.

How does the condition of the seed at harvest affect germination potential and the use of grain for processing? n A. Broken or cracked seed allows for the introduction of pathogens in the seed. These pathogens may use the stored food of the endosperm, or may attack the embryo. Fungi in particular cause seed rot in germinating seed.

n n B. Seed condition can be compromised while still in the field or during storage, as well. Occasionally wheat seed may begin germinating before the seed is harvested. If it is improperly stored it can begin germinating there before processing into flour. Because of the production of amylase early in the process of germination, the hydrolysis of the starch yields a wheat flour that has a high concentration of sugars, this causes poor dough handling properties and poor bread quality.

Review/Summary What steps are involved in the germination process? n Where is amylase produced and what is its function during seed germination? n How does the condition of the seed at harvest affect germination potential and the use of grain for processing? n