1. Unit Plant Science

2. Problem Area Initiating Plant Growth

3. Lesson Osmotic Turgescence: The Forces of Plant Growth

4. Student Learning Objectives • 1. Explain why the seed coat ruptures during germination. • 2. Understand how soil condition affects seed germination and seedling establishment. • 3. Explain how seedbed preparation affects germination. • 4. Demonstrate an understanding of how germinating seeds work their way up through the soil.

5. Terms • Dicot • Hydraulic pressure • Monocot • Osmosis • Osmotic turgescence

6. What causes the rupture of the seed coat during germination? • A. The seed imbibes water in the early stages of germination. Until this point the water content of the seed is usually between 5–10%. During the first few hours of germination the water content of the seed will increase to 40–50% due to osmosis, the movement of water from areas of high concentration to areas of low concentration across a semi-permeable membrane.

7. This hydration of the cells causes an increase in the osmotic turgescence, the swelling of plant tissues, specifically cells, as a result of the hydraulic pressurescreated by the process of osmosis. • B. As the process of germination continues metabolic activity increases freeing the stored food to be used by the embryo. It is the embryonic growth, specifically the emergence of the root shoot that ruptures the seed coat.

11. How does soil condition affect seed germination and seedling establishment? • Soil moisture, temperature, and nutrient placement can each alter seed germination and seedling growth. • A. Soil moisture is of key importance for germination. Osmosis requires that there be a greater concentration of water surrounding the seed than is in the seed for imbibition to occur. Soil that is dry often causes producers to prolong planting or plant seeds deep enough to find subsurface moisture. At the same time soil voids must be present to allow for aeration. Oxygen is required for respiration.

12. B. Soil temperature affects the osmotic potential of the seed. As with most reactions, increased temperature causes more movement in the molecules within the cells. Therefore minimum soil temperatures must be reached to ensure active germination so the seed does not succumb to pathogens while waiting for the soil temperature to warm. • C. Nutrients are needed almost immediately once the radical emerges from the seed. The roots utilize the nutrients. Phosphorus and potassium remain relatively stable in the soil solution, while nitrogen has a tendency to leach. It is important that the proper amount of each nutrient is balanced to support seedling establishment.

13. How does seedbed preparation affect germination? • A. Seedbed preparation is key in maximizing seed to soil contact, and thus the osmotic potential. Good seed/soil contact also means that the soil has good moisture and that the soil is in close contact with the seeds allowing moisture in the soil to enter the seeds. Often slightly firming the soil after planting with a light roller presses the soil tighter around the seeds, improving the germination of seeds. • B. A loose soil also allows the seed an easier path, less resistance, to emerge through. This equates into higher germination percentages. • C. Seedbed preparation is more critical to smaller seeds than to larger seeds.

14. How do germinating seeds work their way up through the soil? • A. Monocots, plants having single cotyledons, begin growing only after the environmental factors have been met. The radical elongates, followed by the coleoptile with the embryonic plant enclosed. The last step includes the growth of the mesocotyl pushing through the soil surface. • B. When environmental factors are satisfied for dicots, plants having two cotyledons, the radical emerges and anchors into the soil. Then the hypocotyl elongates eventually breaking through the soil surface and pulling the cotyledons with it.

15. Review/Summary • What causes the rupture of the seed coat during germination? • How does soil condition affect seed germination and seedling establishment? • How does seedbed preparation affect germination? • How do germinating seeds work their way up through the soil?