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Higher plants comprise approximately 250,000 described species and represent a critical component of the planetary biomass. They contribute functions essential for life, of which the most important is photosynthesis, as it provides the means for conversion of incident solar radiation into biomass accumulation, as well as the oxygen required by aerobic life forms. Fixed carbon in the form of carbohydrate provides the basis of the food chain, and metabolic interconversions within plants provide a variety of essential dietary factors. Plants also provide biomass in the form of structural materials and are the source of many natural products with important biomedical properties. As a consequence, considerable scientific interest is invested in determining the molecular mechanisms underlying plant growth, development, metabolism, and responses to biotic and abiotic stresses. Investment has also been made in developing tools and resources for biological investigations using plants. Notable advances include the development of genetics, of means for transformation using defined DNA sequences, and most recently, of the entire nuclear genome sequences of two plant species (Arabidopsis thaliana and Oryza sativa). On the basis of information of this type and that from other sources, it is evident that higher plants share many features with other eukaryotic organisms. Shared features can be observed at many levels; for example, the overall method of construction of cells, in which a bilamellar plasma membrane separates the cytoplasm from the external milieu and provides primary homeostatic regulation. Eukaryotic cells of different kingdoms share organelles, as well as overall regulatory mechanisms. Shared, or highly similar, protein sequences are observed, and they perform similar functions as enzymes, regulatory molecules, or structural components . Higher land plants have evident differences from other eukaryotes. They contain unique classes of organelles primarily devoted to energy capture from sunlight (plastids and peroxisomes). Of these, chloroplasts contain highly fluorescent pigments devoted to photosynthesis, which, particularly chlorophyll, provide unique and powerful signals that can be employed for flow cytometric analysis. Higher plants are also essentially immobile in the sporophytic stage and hence must be capable of responding to changes in environmental conditions and to biotic attack.

Aside from grafting onto already established rootstock or the development of roots from a planted cane (vide supra), root systems develop from the radicle in the plant’s seed. Both as roots begin to form from the cane, and as the sprouting seed coat opens in response to soil temperature, moisture, and genetic programming left in place when the seed formed, the roots begin to grow and interact with the rhizosphere. Similarly, signals received by rootstock where grafting has been effected also occur. The roots begin to bring moisture and food to produce and support the stem and, eventually, the leaves, flowers, and fruit. Heavily fruited plants such as grapes require additional support for the stems. In the roots, epidermal (surface) cells elongate and develop into root hairs. Beneath the epidermal cells it appears that the phloem cells which bring the starch bodies (amyloplasts) to the root tips and help direct which way “down” is, develop first. Then xylem elements develop in order to move the minerals into the system. Most of the minerals are absorbed through channels developing in the walls of the growing undifferentiated cells (the meristems). Because of concentration gradients (i.e., there is less on one side of a cell membrane than on the other), some minerals appear to be actively transported into the cells of the xylem (presumably through similar channels) in response to signals emanating from the plant. From the xylem cells, the minerals and water move upward into the apical meristem and get distributed to other regions. Interestingly, although most of the cells are derived from the same group of meristems which thus might be considered true stem cells, it is genetic programming which permits that differentiation. Thus, the derivatives of the meristems undergo transformation and develop into various cell types that perform the different functions (Figure 6.1). Relatively recently there has been an increased interest in what has been the largely unexplored biology of roots.

Chapter 3 gives examples of how grapevines, being woody perennials, have the potential to develop extensive, deep root systems when soil conditions are favorable. One of the most important factors governing root growth is a soil’s structure, the essential attributes of which are • Spaces (collectively called the pore space or porosity) through which roots grow, gases diffuse, and water flows • Storage of water and natural drainage following rain or irrigation • Stable aggregation • Strength that not only enables moist soil to bear the weight of machinery and resist compaction but also influences the ease with which roots can push through the soil The key attributes of porosity, aeration and drainage, water storage, aggregation, and soil strength are discussed in turn. Various forces exerted by growing roots, burrowing animals and insects, the movement of water and its change of state (e.g., from liquid to ice) together organize the primary soil particles—clay, silt, and sand—into larger units called aggregates. Between and within these aggregates exists a network of spaces called pores. Total soil porosity is defined by the ratio . . . Porosity = Volume of pores/Volume of soil . . . A soil’s A horizon, containing organic matter, typically has a porosity between 0.5 and 0.6 cubic meter per cubic meter (m3/m3)—also expressed as 50% to 60%. In subsoils, where there is little organic matter and usually more clay, the porosity is typically 40% to 50%. Box 4.1 describes a simple way of estimating a soil’s porosity. Total porosity is important because it determines how much of the soil volume water, air, and roots can occupy. Equally important are the shape and size of the pores. The pores created by burrowing earthworms, plant roots, and fungal hyphae are roughly cylindrical, whereas those created by alternate wetting and drying appear as cracks. Overall, however, we express pore size in terms of diameter (equivalent to a width for cracks). Table 4.1 gives a classification of pore size based on pore function.