M07_DAVI4493_08_SE_C07.qxd 8/10/10 8:31 PM Page 200 7 Principles of Propagation from Seeds learning objectives INTRODUCTION • Describe the process of Seed germination, from an ecological standpoint, is the beginning of germination. the next sexual generation. It is the first adaptive step toward colonizing • Compare methods for an environmental niche. Therefore, plant species have developed a vari- measuring germination. ety of seed germination and dormancy strategies that make the study of • Define the environmental and seed germination one of the most fascinating areas of plant growth and disease factors influencing development. germination. From a human ecology standpoint, humankind’s recognition that • Describe the types of seed seeds were highly nutritious and could be selected and used to propagate dormancy and how dormancy crop plants was pivotal to establishing communities that were self-sustaining controls germination. for food. Seeds are the genetic repositories of thousands of years of selection for crop plants. From the standpoint of modern commercial crop production, more plants are propagated from seeds for food, fiber, and ornamental use than any other method of propagation. In Chapter 7, we will summarize the important physiological mechanisms responsible for seed germination and dormancy. A command of these basic principles allows growers to take full advantage of cultural practices to optimize plant production. THE GERMINATION PROCESS A seed is a ripened ovule. At the time seed The next sexual of separation from the parent plant, it generation for a plant. It consists of an embryo and stored food consists of an embryo, food supply, both of which are encased in a storage tissue, and a protective covering (Fig. 7–1). The protective covering. activation of the seed’s metabolic germination The machinery leading to the emergence of committed stage of plant a new seedling plant is known as development following germination. For germination to radicle emergence from the be initiated, three conditions seed coverings, which leads must be fulfilled (51, 128): to a seedling. 1. The seed must be viable; that is, the embryo must be alive and capable of germination. 2. The seed must be subjected to the appropriate envi- ronmental conditions: available water, a proper temperature range, a supply of oxygen, and, some- times, light. M07_DAVI4493_08_SE_C07.qxd 8/10/10 8:31 PM Page 201 principles of propagation from seeds chapter seven 201 Cotyledons Endosperm Figure 7–1 A seed consists of an embryo, a food supply (usually endosperm or cotyledon) and a protective covering (seed coat or pericarp). Intact seed Radicle on the left and half seed on the right, Seed coat exposing the embryo. 3. Any primary dormancy condition present within the detail in Chapters 4 and 6. The discussion of seed germina- seed (19, 56) must be overcome. Processes leading to tion in this chapter will focus on the basic process of seed removal of primary dormancy result from the interac- germination in orthodox seeds that complete maturation tion of the seed with its environment. If the seeds are drying and are dormant or nondormant after separation subjected to adverse environmental conditions, a from the mother plant. secondary dormancy can develop and further delay the period when germination takes place (133, 142). Phases of Early Germination Early seed germination begins with imbibition of water by the seed and follows a triphasic (three-stage) increase in Transition from Seed Development seed fresh weight due to increasing water uptake (Fig. 7–3, to Germination page 202); the three phases are described as follows: As reviewed in Chapter 4, many seeds lose water during 1. Imbibition is characterized by an initial rapid the maturation drying stage of seed development. These increase in water uptake. seeds are either dormant 2. The lag phase follows imbibition and is a period of dormancy The or nondormant at the time where there is active metabolic activity but condition where seeds time they are shed from little water uptake. will not germinate the plant. However, some water potential 3. Radicle protru- even when the environ- seeds either do not enter As it relates to seed sion results from ment is suitable for the maturation drying germination, is a meas- a second period germination. stage of seed develop- ure of the potential for of fresh weight gain ment and germinate a cell to take up water driven by additional prior to being shed from the plant (vivipary or precocious from its surrounding water uptake. germination) or can tolerate only a small degree of desicca- environment. Changes tion (recalcitrant seeds). Figure 7–2 illustrates the fate of These processes in the seed’s water various seeds as they approach the end of seed develop- rely on the water potential are the driving ment. Viviparous and recalcitrant seeds are discussed in potential of the cells force behind germination. Figure 7–2 The transition from seed development to seed germination. Seeds may end seed development and display viviparous, recalcitrant, or orthodox seed behavior. Viviparous and recalcitrant seeds germinate before completing the maturation drying stage of development. Orthodox seeds continue to dry to about 10 percent moisture and can be either nondormant (sometimes termed quiescent) or dormant. M07_DAVI4493_08_SE_C07.qxd 8/10/10 8:32 PM Page 202 202 part two seed propagation BOX 7.1 GETTING MORE IN DEPTH ON THE SUBJECT WATER POTENTIAL AND SEED GERMINATION Water potential as it impacts water movement in plants is On the other hand, pressure potential is an opposing described as force and is expressed as a positive value. The pressure Water potential (ψcell) = Matric potential (ψm) + potential is the turgor force due to water in the cell press- Osmotic potential (ψπ) + Pressure potential (ψp) ing against the cell wall. It is also an expression of the Matric potential is the major force responsible for ability of the cell wall to expand. Cell wall loosening in water uptake during imbibition. Matric forces are due to the radicle is determined by the physical properties of the the hydration of dry components of the seed including cell cell wall and the counterpressure exerted by the seed walls and macromolecules like starch and proteins. Water tissues covering the radicle (Fig. 7–4). A combination of uptake due to matric forces during imbibition is usually increasing osmotic potential (more negative) and/or rapid, as might be expected because the seed is very dry change in the pressure potential can result in cell (less than 10 percent moisture) at the end of seed devel- enlargement and initiate radicle protrusion. This is opment; see Chapter 4. termed growth potential (21). Thus, changes in osmotic Osmotic potential and pressure potential determine potential of radicle cells, and cell wall loosening in radi- water uptake during the radicle protrusion phase of seed cle or seed covering cells, are essential components germination. The initial stage of radicle protrusion is due controlling radicle growth and germination. An under- to enlargement of the cells in the radicle corresponding to standing of this concept is essential to understanding increased water uptake. Osmotic potential is a measure of aspects of seed dormancy, effects of hormones on ger- the osmotically active solutes in a cell, including molecules mination, and treatments like seed priming. like organic or amino acids, sugars, and inorganic ions. Growth Potential The relative force generated by the Osmotic potential is expressed as a negative value. As the radicle during germination. Conceptually, a seed germi- number of osmotically active solutes increases in a cell, nates when the radicle force is sufficient to penetrate the the osmotic potential becomes more negative (i.e., from seed coverings. This is accomplished by an increase in -0.5 MPa to -1.0 MPa). This can result in more water mov- radicle growth potential and/or weakening of the seed ing into the cell. [Note: Water potential is expressed coverings. as either megapascals (MPa) or bars. One MPa is equal to 10 bars.] Figure 7–4 Schematic representation of water uptake in a cell. The opposing forces of osmotic potential (ψπ) and pressure potential (ψp) determine water uptake by the cell and the cell’s ability to expand. in the seed and embryo (see text box on water potential potential in dry seeds of near imbibition The and Chapter 3 for additional information). –100 to –350 MPa (216, 217). initial stage of Imbibition is a physical process water uptake in Water Uptake by Imbibition (Phase I) Most seeds are related to matric forces that dry seeds. dry (less than 10 percent moisture) after completing occurs in dry seeds with water- seed development. This results in a very low water permeable seed coats whether they are alive or dead, M07_DAVI4493_08_SE_C07.qxd 8/10/10 8:32 PM Page 203 principles of propagation from seeds chapter seven 203 Figure 7–3 There are three phases to germination that can be described by the seed’s increase in fresh weight (water uptake). These include the imbibition, lag, and radicle emergence phases. dormant or nondormant. There are two stages to imbi- cellular membranes to function normally until they are bition (Fig. 7–5) (186, 218). Initially, water uptake is fully hydrated (25, 168). However, there are some very rapid over the first 10 to 30 minutes. This is fol- seeds, like members of the cucumber family, which lowed by a slower wetting stage that is linear for up to have a perisperm envelope that surrounds the embryo an hour for small seeds or several hours (5 to 10) for and inhibits ion leakage (241). large seeds. Water uptake eventually ends as the seed The quantity of leaked solutes is diagnostic for enters the lag phase of germination. seed quality and is the basis for the electrolyte leakage The seed does not wet uniformly during imbibi- assay for seed vigor testing (see page 184).