seeds may be treated in this way before planting. Pelleting makes planting easier and spacing more precise. New methods of preparing seeds for planting are being investigated. For uniform germination and establishment of a crop, planting of pregerminated seeds is being investigated. Recently, sprouted seeds have been suspended in a protective gel or drilled with some water, a method called fluid drilling. 9.12 ENVIRONMENTAL CONDITIONS FOR SEED GERMINATION Germination involves physiological and biochemical processes. The belowground environmental conditions (Chapter 4) must be adequate for seeds to germinate. The critical factors are as follows: 1. Moisture. Seeds must imbibe moisture to a certain degree for the germination process to be initiated. Moisture is needed to initiate the enzymatic breakdown of food reserves. The critical degree of imbibition differs among species. For example, whereas soybean needs to imbibe about 50 percent of its weight before germination, sorghum requires only about one-third of its weight. Horticultural plants that germinate best under conditions of high moisture include beet, celery, and lettuce. When raising seedlings in flats, a glass plate may be used to cover the flat to prevent moisture loss through evaporation (Figure 9–4). Similarly, the flat may be placed in a plastic bag to accomplish the same purpose (Figure 9–5). Although moisture is critical to germination, excessive moisture encourages rotting and other diseases. Sheet of glass FIGURE 9–4 Seed germination: (a) using a flat covered with a sheet of glass to retain humidity; (b) using a plastic cover over a flat. (Source: For (b) George Acquaah) (a) Flat containing recently planted seed (b) 9.12 Environmental Conditions for Seed Germination 299
FIGURE 9–5 Seed germination in a flat enclosed in a plastic sack to retain humidity. Plastic bag 2. Temperature. Temperature regulates seed germination. In seeds that require cold temperature treatment to break dormancy, abscisic acid or other inhibitors are broken down under low temperatures (as obtained in winter). When warm spring temperatures arise, levels of endogenous gibberellins increase, resulting in germination. The rates of biochemical reactions are controlled by temperature. When soils are too cold, growth processes slow down or cease altogether. Generally, a warm seedbed is desirable for seed germination, but warm-season crops (such as bean and squash) do better at warmer temperatures (15 to 25°C or 59 to 77°F) and cool-season crops (such as cole crops) do well at cooler temperatures (less than 10°C or 50°F). 3. Light. Most seeds do not need light to germinate. A number of horticultural species including some herbaceous garden flowers, some vegetables (e.g., lettuce and celery), and grasses require light to germinate. The species are therefore planted shallowly in the soil. Light inhibits germination in some species (such as onion), which must be planted deeper in the soil or covered with a dark cloth or other material in the nursery. Geraniums require darkness to germinate. 4. Air. In most species, germination is an aerobic process. The low oxygen concentration of soil air is inhibitory to most species. The seedbed must be well drained for good aeration. 5. Disease free. As indicated previously, soilless media are sterile, but field soil may contain pathogens that can overwhelm a developing embryo or young seedling. One of the most common diseases of seedlings is damping-off, a fungal attack caused especially by Pythium ultimum and Rhizoctonia solani. These fungi are active at warm temperatures (20 to 30°C or 68 to 86°F) and thus are less of a problem when germination occurs under cooler conditions. It is important to note that these factors work together for good germination. The conditions must occur in a good balance. If, for example, moisture is present in adequate amounts but temperature is excessive, seeds might be heated in the soil and rot. 9.13 SEED GERMINATION AND EMERGENCE Seed Germination A sequence of events in a viable seed starting with water imbibition and leading to embryo growth and development. Seed germination is a complex process involving metabolic, respiratory, and hormonal activities. The dry seed first imbibes water to initiate enzymatic breakdown of stored metabolites. These metabolites provide the source of nutrition for the developing embryo and are located in the cotyledons of dicots and endosperms of monocots. As these stored foods (proteins, fats, and oils) are metabolized, respiration occurs to synthesize chemical energy, or ATP. Also DNA and RNA are synthesized, the RNA being required for the production of certain hydrolytic enzymes such as amylases, proteases, and lipases. The net result of these biochemical and enzymatic processes is the production of new cells and formation of new tissue, leading to growth and development of the embryo into a seedling. 300 Chapter 9 Sexual Propagation
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