- - AGRICULTURE CORE CURRICULUM - - (CLF6000) Advanced Core Cluster: ORNAMENTAL HORTICULTURE (CLF6200) Unit Title: PLANT PHYSIOLOGY AND GROWTH ____________________________________________________________________________ (CLF6204) Topic: SEEDLING GROWTH Time Taught in Year(s) 1 hour 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (C-13) - List the requirements for seed germination and indicate factors that might reduce germination rates (e.g., temperature, water, oxygen, seed dormancy, and disease). (C-14) - Trace the stages of development (using bean, pea, onion, and corn plants) from germination to the stage of first true leaves. Compare and contrast the types of growth. (C-16) - Discuss and give examples of the effects of photoperiodism on plant growth. Special Materials and Equipment: Seeds of beans, alfalfa and corn; petri dishes, blotters; jars; Captan (fungicide); and pasteurized and untreated soil. References: Hartmann, H. T., Flocker, W. J., & Kofranek, A. M. (1981). PLANT SCIENCE: GROWTH, DEVELOPMENT, AND UTILIZATION OF CULTIVATED PLANTS. Englewood Cliffs, NJ: Prentice-Hall. Evaluation: Quiz by instructor TOPIC PRESENTATION: SEEDLING GROWTH A. Germination versus dormancy 1. A mature seed contains an inactive, non-growing embryo that may be either dormant or merely quiescent. 2. When the embryo resumes its growth and breaks through the seed coat, the seed is said to germinate. 3. If the embryo is merely quiescent, the seed will germinate as soon as it receives adequate water, warmth, and oxygen. 4. If the embryo is truly dormant, additional signals are needed to relieve dormancy before the embryo can resume its growth. B. Several kinds of signals control dormancy. 1. Some dormant embryos are immature and must undergo further development before they can resume growth. Here, time is the critical requirement. 2. Other seeds need influences from the environment to break out of dormancy. a. These requirements cause germination to occur when conditions are best for the survival of the young plant. b. Some seeds cannot germinate until the seed coat is weakened. 1) The seed coat may prevent water or oxygen from entering. 2) In nature, such seeds must be carried by running water, leading to abrasion of the seed coat. This assures that water will be present for growth. 3) The seed coats can be artificially weakened by treatments with strong acid, alkali, or coffee, or by scraping the seeds with sandpaper, file, etc. These treatments are called scarification. c. Some seeds contain inhibitors that prevent germination. 1) The hormone abscisic acid is one such inhibitor. It can also be artificially added to prolong dormancy. 2) The inhibitor content may slowly decline with age, or it may be washed out by exposure to running water. In nature, a need for water to remove inhibitors assures that adequate water for growth will be present. 3. Some seeds require light or darkness before they will germinate. a. Small wild seeds usually require light. This delays germination until competing plants are removed. 1) Weeds persist in cultivated fields because removal of existing weeds pulls dormant seeds to the surface where they get the light needed for germination. 2) For this reason, weeds should be removed before they make seeds. b) Some large seeds are inhibited by light. This delays germination until the seeds are buried, increasing the chance that roots will reach water before the shoots reach dry air. 4. Some seeds require prolonged chilling before they will germinate. a. Usually several weeks at temperatures just above freezing are needed. In nurseries, such treatments are called stratification because the seeds are spread between layers (strata) of moist sand before being chilled. b. This requirement assures that seeds released in autumn will not germinate until winter is past. C. Horticultural practices influence germination and seedling growth. 1. Remember that a seed must first be viable (alive) before it can germinate. a. When selecting seed for planting be sure that it is pure of the chosen variety. b. It should have a high germination percent for that variety. c. Seeds should be large and plump. d. There should be absolutely no noxious or objectionable weed seeds present. e. Seed should be of uniform size and shape. f. There should be no insects, insect eggs, or pathogens on the seed. g. It should be free of chaff, rocks, or other foreign material. 2. Certain environmental conditions are required for seed germination and successful seedling growth. a. Adequate moisture is one of those requirements. 1) Water is essential to activate the processes in the seed that result in embryo growth. 2) Most seeds germinate satisfactorily at moisture levels between field capacity and permanent wilting percent. b. Temperature can also influence the rate and percentage of germination. 1) Seeds of cool season plants germinate best at temperatures between 32 and 50 degrees Fahrenheit (for example: celery, lettuce, peas). 2) Warm season plants germinate best at temperatures between 59 and 79 degrees Fahrenheit. c. Another requirement for germination is aeration (oxygen). 1) The respiration rate for germinating seeds is very high, so adequate oxygen is a requirement. 2) The germination percentage of most seeds will be lowered if there is less than 20 percent oxygen. (Normal air is 20 percent oxygen.) 3) Seedbeds that are overwatered or poorly drained will limit the oxygen, so the germination percentage will diminish. d. Light is also essential for the germination of some kinds of seeds. 1) Lettuce, celery, and some grasses are examples. 2) Shallow planting of these seeds is necessary to allow proper germination. e. An environment free of pathogenic organisms is also a requirement for high rates of germination. 1) Damping-off is a diseased condition of seedlings or cuttings caused by certain parasitic fungi and marked by wilting and rotting; seedlings die during or shortly after germination. 2) The best methods of control are fumigation or pasteurization of the germination media. 3) Surface treatment of seeds with fungicides before planting and good sanitation practices will help minimize problems with fungi. f. Salts in the germination media, irrigation water, or fertilizer can injure or kill seedlings as they germinate. 1) This problem can be prevented by using soil mixtures and water low in salts. 2) Irrigating less frequently but more copiously will also help leach the salts away from the seedlings. __________________________________________________________ ACTIVITY: 1. Provide samples of several different seeds. 2. Assign groups of students to the following experiments: a. Germinate seeds with varying amounts of water in petri dishes. Use blotters or shredded paper towels for the media. b. Germinate seeds in petri dishes. Place under varying temperatures. (Examples - one in room, one in refrigerator, one outside.) c. Germinate seeds in an airtight container. Dampen a blotter at bottom of container before adding seeds and sealing. d. Germinate (alfalfa, bean or corn) seeds in varying amounts of light. e. Germinate seeds in: 1) media that has been pasteurized 2) media that has been treated with fungicide (Captan works well) 3) untreated media (garden soil) 3. Have students observe all experiments, discuss, and record results. __________________________________________________________ 12/14/90 PJK/sg #%&C