Growth and Development of Horticultural Crops (1+1)

Morphological and biochemical changes accompanying seed development

• The morphological aspects of embryo development (embryogenesis) following pollination, fertilization, and zygote formation have been described for many plant species. Biochemical changes accompanying embryogenesis and seed development are characterized by vigorous anabolic processes, resulting in the formation of new cells, tissues and organs rich in proteins, nucleic acids, carbohydrates and fats. The early stages of seed development, Phase I, Figure 16-2, involve pollination, fertilization, and zygote formation, processes that contributes very little to dry weight formation but must involve intense metabolic activity. Much more is known of the biochemistry of embryogenesis, initiated by cell division of the zygote. The embryo increase in dry weight as new cells are formed and cellular constituents synthesized. This is a period of intense metabolic activity with a high demand for low molecular weight precursors, such as sucrose, amino acids, fatty acids, nucleosides, organic acids, water, and inorganic ions. The bulk of these materials are supplied by the parent plant through vascular connections, but some also comes from the dissolution of cellular material in the ovule and embryo sac. Phase I comes to an end when the embryonic plant is fully differentiated and cell division ceases.
  
• Full-term embryos, excised and nourished by a suitable array of organic molecules and inorganic ions, will continue to develop and form mature plants. The young embryos generally require growth substances in addition to organic and inorganic nutrients. The developing seed is in direct vascular contact with the parent plant. If environmental factors, such as low or high temperature, reduced light, moisture stress, or mineral deficiency, alter the metabolism of the parent plant, the pattern of development during embryogenesis may be altered.
  
• Precocious germination is prevented by the action of an inhibitor from ovule tissue. The inhibitor, abscisic acid, may move into the ovule through vascular connections from the parent plant or it may be synthesized in the ovule. Abscisic acid prevents premature or precocious germination. Later in seed development, when vascular connections between the ovules and parent plant are broken by desiccation, low seed water content prevents premature germination.
  
• Phase I comes to an end when the embryonic plant is fully formed and cell division ceases. Seed dry weight continues to increase rapidly during Phase II, however, because of the synthesis and deposition of seed storage materials-starch, protein, fats, phytin, etc- in endosperm or cotyledonary tissues. In monocots such as maize and rice, the endosperm cells lose their nuclear material and fill up with starch and phytin. A specialized tissue, the aleurone layer, forms to the outside of the endosperm, next to the developing seed coat. The aleurone layer may be several cells thick and is composed of dense living cells filled with protein. In dicots, such as pea, the cotyledonary cells retain their nuclear contents and become packed with starch, protein lipids, and phytin.
  
• Phase II is the period of maximum seed dry weight increase. The storage materials are synthesized from small precursor molecules from the parent plant. The synthesis and deposition of storage molecules in developing seeds constitute a major sink for carbohydrate and nitrogenous components made by the parent plant. Sucrose, the major product of leaf photosynthesis, supplies carbon skeletons for starch and fats. Moreover, sucrose is a source of carbon skeletons for nitrogenous constituents – amino acids, amides, nucleotides. During seed filling, the demand for carbonaceous and nitrogenous molecules is high and may not be met by current CO2, NO3, or N2 assimilation. In such cases, reserve materials in the parent plant may be mobilized and transported to developing seeds. To obtain maximum seed yields, especially in food plants such as maize, peas, soybeans, and beans, it is essential that the leaves and other assimilatory organs of the parent plant be kept active as long as possible. In soybeans, it has been observed that leaf nitrogenous compounds have been hydrolyzed and transported to developing seeds under conditions when the roots cannot supply enough nitrogenous material to support seed filling. The loss of leaf nitrogen leads to premature leaf senescence and the loss of photosynthetic surfaces for carbon assimilation.
  
• Phase II comes to an end as the seed begins to lose water. The synthesis of storage molecules involves the elimination of water molecules, but there appears to be an accelerated process of water loss, possibly through an alternation of membrane structure. Vascular connections between the developing seed and parent plant are broken so that no water or solutes can move into the seed. Moisture content during seed filling may be in the range of 50 to 60%, but after the desiccation process is under way, water content drops to 10 to 15% at maturity. Water loss is not uniform in all parts of the seed. The embryonic axis, composed of nonvacuolated parentchyma cells, contains relatively little free water, but the structural components are hydrated. Cells in endosperm and cotyledonary tissues, however, contain low amounts of water. Also, the tissues surrounding the seed that develop into seed coats undergo desiccation and sclerification, forming a hard protective structure.
  
• During Phase III, the desiccation process continues, attaining moisture levels of between 5 and 15% (total seed). With desiccation, the subcellular organelles in cotyledonary cells seem to lose their structural integrity. In addition, organized ribosome’s (polysomes) essential for protein synthesis break up into single ribosome’s. The entire picture is one of very low metabolic activity, and if seed moisture remains low, further development of the embryonic axis into a mature plant does not occur. The seed is said to be dormant.
  
• To obtain seeds of high vitality and vigor, it is important that seed moisture levels be brought to at least 10 to 12% during maturation. In some instances, it is necessary to dry seeds by artificial heat if they are harvested early. Several hybrid maize varieties, for instance, have been developed for regions with short growing seasons. If the growing season is terminated early by frost and cold weather, the normal seed desiccation process may not bring the seed moisture levels sufficiently low for safe storage. The grain is harvested and then put through a slow drying process to attain moisture levels of around 10% or lower. This process is costly and adds to the expense of producing the crop. If the grain is not dried, it may spoil in storage by fermentive respiratory processes or by fungal and bacterial growth. In the dry state seeds can withstand environmental conditions generally unfavorable to growth: Low temperature, drought, excessive water, fire, and toxic materials in the soil.
  
• Commercial seed producers, especially flower seed growers, follow rather stringent procedures of seed drying, storage, and packaging so as to provide the home gardener with a quality product – seeds that will readily germinate to produce a vigorous plant.