Unit Plant Science

Problem Area Initiating Plant Growth

Lesson Environmental Factors Affecting Seed Germination

Student Learning Objectives 1. Explain the role light plays in the germination of some plants. 2. Explain the importance of temperature in germination. 3. Understand the effects oxygen has on germination. 4. Explain the role of water in germination. 5. Explain the role of planting depth on germination.

Terms Abscisic acid Aeration Field capacity Growth inhibitors Permanent wilting point Phytochrome Stratification

Why do some plants require exposure to light in order to germinate? A. Generally seeds require darkness to germinate. However, lettuce, tobacco, tomato and many grasses need to light exposure to germinate. B. These seeds require the red portion of the light spectrum, while far red light inhibits germination. Many small seeds with low amounts of storage reserves (such as lettuce) show such a red light requirement.

C. These seeds must not be buried below the soil so deeply that light cannot penetrate. Although research suggests that even a few minutes of exposure will allow the germination to occur. D. Phytochrome is a plant pigment found in cytoplasm that senses the presence of red light. The red-absorbing form of phytochrome changes to the far-red absorbing form when it absorbs red light (660 nm) and back again when it absorb far-red light (730 nm). Thus the presence of the far-red form of phytochrome ends the inhibition of germination in these seeds.

Why is temperature an important factor in germination? A. As with most reactions germination generally occurs faster when at warmer temperatures. However there is sometimes a need for cool temperatures to break dormancy. B. Stratification is one strategy that is employed in woody species in particular. It requires a moist, cool period that degrades growth inhibitors that prevent germination. Once the inhibitors are degraded and all other conditions are met then germination will occur.

C. The temperature for germination to occur is quite different than the temperature requirement to break dormancy. Generally the temperature for germination correlates to the temperature needed for successful plant growth. D. Seeds of cool season crops germinate best at temperatures between 32 and 50 degrees Fahrenheit. (Examples: celery, lettuce, peas.) Warm season crops germinate best at temperatures between 59 to 79 degrees Fahrenheit.

Is oxygen needed for seeds to germinate? Another requirement for germination is aeration (oxygen). A. Respiration rates for germinating seeds are very high, therefore adequate oxygen is necessary. The germination percent of most seeds will be retarded if the oxygen percent goes below 20 percent. (Normal air is 20 percent oxygen.) B. Seedbeds that are over-watered or poorly drained will cause the oxygen supply to become limited, so the germination percent will diminish.

Why is water necessary for germination? Water is essential to activate the processes in the seed that result in embryo growth. A. Water is always needed to allow vigorous metabolism to begin. It is also sometimes needed to leach away a germination inhibitor within the seed. The inhibitor is often abscisic acid (ABA). B. Most seeds germinate satisfactory at moisture levels between field capacity and permanent wilting point. Field capacity is the ideal soil moisture level where water available for plants is at its maximum. At the permanent wilting point water is so tightly bound to soil particles it is unavailable to be absorbed.

Does planting depth effect germination? Proper planting depth is a direct correlation to seed size. A. The general rule of thumb is larger seeds can be planted more deeply than small seeds. This is due to the energy needed to emerge. Larger seeds have greater food reserves from which to draw energy for respiration and growth. They are able to emerge from greater depths. B. Soil types also affect the planting depths. The surface of sandy soils tends to dry out quickly, so seeds planted in these soils should be planted deeper, than in loam soils.

Review/Summary Why do some plants require exposure to light in order to germinate? Why is temperature an important factor in germination? Is oxygen needed for seeds to germinate? Why is water necessary for germination? Does planting depth effect germination?