Branching underground root systems are often more extensive

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1 35 Roots and Mineral Nutrition Storage roots. Carrots (Daucus carota) are biennials (plants that live for 2 years). During the first year s growth, food is stored in the fleshy root system; during the second year, the shoot elongates and produces flowers. Carrots and other root crops are important sources of human food. R. Calentine/ Visuals Unlimited 748 KEY CONCEPTS Roots anchor the plant in the soil, absorb water and minerals, conduct these materials to the rest of the plant body, and store carbon compounds and water. Primary tissues (epidermis, cortex, endodermis, pericycle, xylem, and phloem) of roots develop from root apical meristems. Secondary tissues (wood and bark) of woody roots develop from lateral meristems. The endodermis absorbs mineral ions from the soil solution by an active, energy-requiring process. Many roots form associations with soil bacteria and fungi. Soil, the layer of Earth s crust that has been modified by contact with weather, wind, water, and organisms, contains most of the essential elements required by plants. Branching underground root systems are often more extensive than a plant s aerial parts. The roots of a corn plant, for example, may grow to a depth of 2.5 m (about 8 ft) and spread outward 1.2 m (4 ft) from the stem. Desert-dwelling tamarisk (Tamarix) trees reportedly have roots that grow to a depth of 50 m (163 ft) to tap underground water. The extent of a plant s root depth and spread varies considerably among different species and even among different individuals in the same species. Because roots are usually underground and out of sight, people do not always appreciate the important functions they perform. First, as anyone who has ever pulled weeds can attest, roots anchor a plant securely in the soil. A plant needs a solid foundation from which to grow. Firm anchorage is essential to a plant s survival so that the stem remains upright, enabling leaves to absorb sunlight effectively. Second, roots absorb water and dissolved minerals (inorganic nutrients) such as nitrates, phosphates, and sulfates, which are necessary for synthesizing important organic molecules. These dissolved minerals are then transported throughout the plant in the xylem. Many roots perform the function of storage. Carrots (see photograph), sweet potatoes, cassava, and other root crops are important sources of human food. Surplus sugars produced in the leaves by photosynthesis are transported in the phloem to the roots for food storage (usually as starch or sucrose) until needed. Other plants, particularly those living in arid regions, possess storage roots adapted to store water. In certain species, roots are modified for functions other than anchorage, absorption, conduction, and storage. Roots specialized to perform uncommon functions are discussed later in the chapter.

2 ROOT STRUCTURE AND FUNCTION Learning Objectives 1 Distinguish between taproot and fibrous root systems. 2 Label cross sections of a primary eudicot root and a monocot root, and describe the functions of each tissue. 3 Trace the pathway of water and mineral ions from the soil through the various root tissues, and distinguish between the symplast and apoplast. 4 Discuss the structure of roots with secondary growth. 5 Describe at least three roots that are modified to perform uncommon functions. Two types of root systems, a taproot system and a fibrous root system, occur in plants ( Fig. 35-1). A taproot system consists of one main root that formed from the seedling s enlarging radicle, or embryonic root. Many lateral roots of various sizes branch out of a taproot. Taproots are characteristic of many eudicots and gymnosperms. A dandelion is a good example of a common herbaceous plant with a taproot system. A few trees, such as hickory, retain their taproots, which become quite massive as the plants age. Most trees, however, have taproots when young and later develop large, shallow, lateral roots from which other roots branch off and grow downward. A fibrous root system has several to many roots of similar size developing from the end of the stem, with lateral roots branching off these roots. Fibrous root systems form in plants that have a short-lived embryonic root. The roots originate first from the base of the embryonic root and later from stem tissue. The main (a) Figure 35-1 Animated Root systems (a) A taproot system develops from the embryonic root in the seed. (b) The roots of a fibrous root system are adventitious and develop from stem tissue. (b) roots of a fibrous root system do not arise from pre-existing roots but from the stem; such roots are called adventitious roots. Adventitious organs occur in an unusual location, such as roots that develop on a stem, or buds that develop on roots. Onions, crabgrass, and other monocots have fibrous root systems. Taproot and fibrous root systems are adapted to obtain water in different sections of the soil. Taproot systems often extend down into the soil to obtain water located deep underground, whereas fibrous root systems, which are located relatively close to the soil surface, are adapted to obtain rainwater from a larger area as it drains into the soil. Roots have root caps and root hairs Because of the need to adapt to the soil environment instead of the atmospheric environment, roots have several structures, such as root caps and root hairs, that shoots lack. Although stems and leaves have various types of hairs, they are distinct from root hairs in structure and function. Each root tip is covered by a root cap, a protective, thimblelike layer many cells thick that covers the delicate root apical meristem ( Fig. 35-2a; also see Fig. 32-7). As the root grows, pushing its way through the soil, cells of the root cap are sloughed off by the frictional resistance of the soil particles and replaced by new cells formed by the root apical meristem. The root cap cells secrete lubricating polysaccharides that reduce friction as the root passes through the soil. The root cap also appears to be involved in orienting the root so that it grows downward (see discussion of gravitropism in Chapter 37). When a root cap is removed, the root apical meristem grows a new cap. However, until the root cap has regenerated, the root grows randomly rather than in the direction of gravity. Root hairs are short-lived tubular extensions of epidermal cells located just behind the growing root tip. Root hairs continually form in the area of cell maturation closest to the root tip to replace those that are dying off at the more mature end of the root-hair zone ( Fig. 35-2b; also see Fig. 32-7). Each root hair is short (typically less than 1 cm, or 0.4 in, in length), but they are quite numerous. Root hairs greatly increase the absorptive capacity of roots by increasing their surface area in contact with moist soil. Soil particles are coated with a microscopically thin layer of water in which minerals are dissolved. The root hairs establish an intimate contact with soil particles, which allows efficient absorption of water and minerals. Unlike stems, roots lack nodes and internodes and do not usually produce leaves or buds. Although herbaceous roots have certain primary tissues (such as epidermis, xylem, phloem, cortex, and pith) found in herbaceous stems, these tissues are arranged quite differently. The arrangement of vascular tissues distinguishes the roots of herbaceous eudicots and monocots Although considerable variation exists in herbaceous eudicot and monocot roots, they all have an outer protective covering (epidermis), a cortex for storage of starch and other organic mol- Roots and Mineral Nutrition 749

3 Root apical meristem (area of cell division) Runk /Rannels/Grant Heilman Photography Root cap Dennis Drenner Root hairs Soil air Soil water Soil particles 250 µm Epidermis (a) Root cap. LM of an oak (Quercus sp.) root tip showing its root cap. The root apical meristem is protected by the root cap. (b) Root hairs. Each delicate hair is an extension of a single cell of the root epidermis. Root hairs increase the surface area of the root in contact with the soil. This radish (Raphanus sativus) seedling is approximately 5 cm (2 in) long. Figure 35-2 Animated Structures unique to roots ecules, and vascular tissues for conduction. Let us first consider the structure of herbaceous eudicot roots. In most herbaceous eudicot roots, the central core of vascular tissue lacks pith The buttercup root is a representative eudicot root with primary growth ( Fig. 35-3). Like other parts of this herbaceous eudicot, a single layer of protective tissue, the epidermis, covers its roots. The root hairs are a modification of the root epidermis that enables it to absorb more water from the soil. The root epidermis does not secrete a thick, waxy cuticle in the region of root hairs, because this layer would impede the absorption of water from the soil. Both the lack of a cuticle and the presence of root hairs increase absorption. Beginning with the root hairs, most of the water that enters the root moves along the cell walls rather than enters the cells. One of the major components of cell walls is cellulose, which absorbs water as a sponge does. An example of the absorptive properties of cellulose is found in cotton balls, which are almost pure cellulose. The cortex, which is composed primarily of loosely packed parenchyma cells, composes the bulk of a herbaceous eudicot root. Roots usually lack supporting collenchyma cells, probably because the soil supports the root, although roots as they age may develop some sclerenchyma (another supporting tissue; see Chapter 32). The primary function of the root cortex is storage. A microscopic examination of the parenchyma cells that form the cortex often reveals numerous amyloplasts (see Figs. 35-3b and 3-9a), which store starch. Starch, an insoluble carbohydrate composed of glucose subunits, is the most common form of stored energy in plants. When used at a later time, these reserves provide energy for such activities as growth and cell replacement following an injury. The large intercellular (between-cell) spaces, a common feature of the root cortex, provide a pathway for water uptake and allow for aeration of the root. The oxygen that root cells need for aerobic respiration diffuses from air spaces in the soil into the intercellular spaces of the cortex and from there into the cells of the root. The water and dissolved minerals that enter the root cortex from the epidermis move in solution along two pathways: the symplast and apoplast ( Fig. 35-4). The symplast is the continuum of living cytoplasm, which is connected from one cell to the next by cytoplasmic bridges called plasmodesmata (see Fig. 5-27). Some dissolved mineral ions move from the epidermis through the cortex via the symplast. The apoplast consists of the interconnected porous cell walls of a plant, along which water and mineral ions move freely. The water and mineral ions can diffuse across the cortex without ever crossing a plasma membrane to enter a living cell. The inner layer of the cortex, the endodermis, regulates the movement of water and minerals that enter the xylem in the root s center. Structurally, the endodermis differs from the rest of the cortex. Endodermal cells fit snugly against each other, and each has a special bandlike region, called a Casparian strip ( Fig. 35-5), on its radial (side) and transverse (upper and lower) walls. Casparian strips contain suberin, a fatty material that is water proof. (Recall from Chapter 34 that suberin is also the waterproof material in cork cell walls.) Until the endodermis is reached, most of the water and dissolved minerals have traveled along the apoplast and therefore 750 Chapter 35

4 Most crops, unless they have been genetically selected to tolerate high salt, are not productive in saline soil. Research on the molecular aspects of salt tolerance suggests that mutations in membrane transport proteins confer salt tolerance. Biologists genetically engineered a salt-tolerant variety of Arabidopsis to overexpress a single gene that codes for a sodium transport protein in the vacuolar membrane. These plants can store large quantities of sodium in their vacuoles, thereby tolerating saline soils. Review What are the four components of soil, and how is each important to plants? What is cation exchange? How do weathering processes convert rock into soil? What are macronutrients and micronutrients? How are minerals lost from the soil? SUMMARY WITH KEY TERMS Learning Objectives 1 Distinguish between taproot and fibrous root systems (page 749). A taproot system has one main root (formed from the radicle), from which many lateral roots extend. A fibrous root system has several to many adventitious roots of the same size developing from the end of the stem. Lateral roots branch from these adventitious roots. Learn more about root systems by clicking on the figure in ThomsonNOW. 2 Label cross sections of a primary eudicot root and a monocot root, and describe the functions of each tissue (page 749). Primary roots have an epidermis, ground tissues (cortex and, in certain roots, pith), and vascular tissues (xylem and phloem). Each root tip is covered by a root cap, a protective layer that covers the delicate root apical meristem and may orient the root so that it grows downward. Epidermis protects the root. Root hairs, short-lived extensions of epidermal cells, aid in absorption of water and dissolved minerals. Cortex consists of parenchyma cells that often store starch. Endodermis, the innermost layer of the cortex, regulates the movement of water and minerals into the root xylem. Cells of the endodermis have a Casparian strip around their radial and transverse walls that is impermeable to water and dissolved minerals. Minerals are actively transported through carrier proteins in the plasma membranes of endodermal cells. Pericycle, xylem, and phloem collectively make up the root s stele, or vascular cylinder. Pericycle gives rise to lateral roots and lateral meristems. Xylem conducts water and dissolved minerals; phloem conducts dissolved sugar. Xylem of a herbaceous eudicot root forms a solid core in the center of the root. The center of a monocot root often consists of pith surrounded by a ring of alternating bundles of xylem and phloem. Monocot roots lack a vascular cambium and do not have secondary growth. 3 Trace the pathway of water and mineral ions from the soil through the various root tissues, and distinguish between the symplast and apoplast (page 749). As water and dissolved mineral ions move from the soil into the root, they pass through the following tissues: root hair/epidermis cortex endodermis pericycle root xylem. Water and dissolved minerals move through the epidermis and cortex along one of two pathways, the apoplast (along the interconnected porous cell walls) or the symplast (from one cell s cytoplasm to the next through plasmodesmata). Explore the pathways of water and dissolved minerals in the root by clicking on the figures in ThomsonNOW. 4 Discuss the structure of roots with secondary growth (page 749). Roots of gymnosperms and woody eudicots develop secondary tissues (wood and bark). The production of secondary tissues is the result of the activity of two lateral meristems, the vascular cambium and cork cambium. The vascular cambium produces secondary xylem (wood) and secondary phloem (inner bark). The cork cambium produces periderm (outer bark). 5 Describe at least three roots that are modified to perform uncommon functions (page 749). Prop roots develop from branches or from a vertical stem and grow downward into the soil to help support certain plants in an upright position. Buttress roots are swollen bases or braces that support certain tropical rainforest trees that have shallow root systems. Pneumatophores are aerial breathing roots that may assist in getting oxygen to submerged roots. Certain epiphytes have roots modified to photosynthesize, absorb moisture, or, if parasitic, penetrate host tissues. 6 List and describe two mutualistic relationships between roots and other organisms (page 756). Mycorrhizae are mutually beneficial associations between roots and soil fungi. Root nodules are swellings that develop on roots of leguminous plants and house millions of rhizobia (nitrogenfixing bacteria). 7 Describe the roles of weathering, organisms, climate, and topography in soil formation (page 757). Factors influencing soil formation include parent material (type of rock), climate, organisms, the passage of time, and topography. Most soils are formed from parent material that is broken into smaller and smaller particles by weathering processes. Climate and organisms work together in weathering rock. Soil organisms such as plants, algae, fungi, worms, insects, spiders, and bacteria are important not only in forming soil but also in cycling minerals. Topography, a region s surface features, affects soil formation. Steep slopes have little or no soil on them, whereas moderate slopes often have deep soils. 8 List the four components of soil, and give the ecological significance of each (page 757). Soil is composed of inorganic minerals, organic matter, air, and water. Inorganic minerals provide anchorage and essential minerals for plants. Organic matter increases 764 Chapter 35

5 the soil s water-holding capacity and, as it decomposes, releases essential minerals into the soil. Soil air provides oxygen for soil organisms to use during aerobic respiration. Soil water provides water and dissolved minerals to plants and other organisms. 9 Describe how roots absorb positively charged mineral ions by the process of cation exchange (page 757). Cations, positively charged mineral ions, are attracted and reversibly bound to clay particles, which have predominantly negative charges on their outer surfaces. In cation exchange, roots secrete protons (H ), which are exchanged for other positively charged mineral ions, freeing them into the soil water to be absorbed by roots. 10 Distinguish between macronutrients and micronutrients (page 757). Plants require 19 essential elements for normal growth. Ten elements are macronutrients: carbon, hydrogen, oxygen, nitrogen, potassium, calcium, magnesium, phosphorus, sulfur, and silicon. Macronutrients are required in fairly large quantities. Nine elements are micronutrients: chlorine, iron, boron, manganese, sodium, zinc, copper, nickel, and molybdenum. Micronutrients are needed in trace amounts. 11 Explain the impacts of mineral depletion and soil erosion on plant growth (page 757). Mineral depletion may occur in soils that are farmed because the natural pattern of nutrient cycling is disrupted when crops are harvested (and not allowed to decompose into the soil). Soil erosion is the removal of soil from the land by the actions of agents such as water and wind. Erosion causes a loss in soil fertility, because essential minerals and organic matter that are a part of the soil are removed. TEST YOUR UNDERSTANDING 1. One main root, formed from the enlarging embryonic root, with many smaller lateral roots branching off it is a(an) (a) fibrous root system (b) adventitious root system (c) taproot system (d) storage root system (e) prop root system 2. Roots produced at unusual places on the plant are (a) fibrous (b) adventitious (c) taproots (d) rhizobial (e) mycorrhizae 3. Root hairs (a) cover and protect the delicate root apical meristem (b) increase the absorptive capacity of roots (c) secrete a waxy cuticle (d) orient the root so it grows downward (e) store excess sugars produced in the leaves 4. Certain plants adapted to flooded soil produce aerial breathing roots known as (a) fibrous roots (b) pneumatophores (c) mycorrhizae (d) nodules (e) prop roots 5. Unlike stems, roots produce (a) nodes and internodes (b) root caps and internodes (c) axillary buds and root hairs (d) terminal buds and axillary buds (e) root caps and root hairs 6. The waterproof region around the radial and transverse walls of endodermal cells is the (a) Casparian strip (b) pericycle (c) apoplast (d) symplast (e) pneumatophore 7. The apoplast is (a) a layer of cells that surround the vascular region in roots (b) the layer of cells just inside the endodermis (c) a system of interconnected plant cell walls through which water moves (d) the central cylinder of the root that comprises the vascular tissues (e) a continuum of cytoplasm of many cells, all connected by plasmodesmata 8. Plants obtain positively charged mineral ions from clay particles in the soil by cation exchange, in which (a) roots passively absorb the positively charged mineral ions (b) mineral ions flow freely along porous cell walls (c) roots secrete protons, which free other positively charged mineral ions to be absorbed by roots (d) the Casparian strip effectively blocks the passage of water and mineral ions along the endodermal cell wall (e) a well-developed system of internal air spaces in the root allows both gas exchange and cation exchange 9. The cell layer from which lateral roots originate is the (a) epidermis (b) cortex (c) endodermis (d) pericycle (e) vascular cambium 10. The center of a herbaceous eudicot root is composed of, whereas the center of a monocot root is composed of. (a) pith; cortex (b) xylem; phloem (c) phloem; xylem (d) xylem; pith (e) pith; xylem 11. Mutually beneficial associations between certain soil fungi and the roots of most plant species are called (a) mycorrhizae (b) pneumatophores (c) nodules (d) rhizobia (e) humus 12. Which of the following statements about soil is true? (a) pore spaces are always filled with about 50% air and 50% water (b) a single teaspoon of fertile agricultural soil may contain up to several hundred living microorganisms (c) the texture of a soil is determined by the soil s ph (d) a soil s organic matter includes litter, droppings, and the dead remains of plants, animals, and microorganisms (e) soil formation is unaffected by a region s climate or topography 13. The technique of growing plants in aerated water containing dissolved mineral salts is known as (a) hydration (b) hydroponics (c) hydrophilic (d) hydrostatic (e) hydrolysis 14. Carbon, hydrogen, oxygen, nitrogen, potassium, calcium, magnesium, phosphorus, sulfur, and silicon are collectively known as (a) micronutrients (b) microvilli (c) micronuclei (d) macronuclei (e) macronutrients 15. In roots of woody plants, (a) xylem does not form the central tissue of the root (b) the cortex composes the bulk of the root (c) the vascular cambium gives rise to secondary xylem and secondary phloem (d) the pericycle gives rise to the apical meristem (e) secondary growth occurs despite the lack of a vascular cambium 16. Corn, sorghum, red mangrove, and banyan are plants that have (a) prop roots (b) buttress roots (c) pneumatophores (d) storage roots (e) root nodules Roots and Mineral Nutrition 765

6 CRITICAL THINKING 1. A mesquite root is found penetrating a mine shaft about 46 m (150 ft) below the surface of the soil. How could you determine when the root first grew into the shaft? (Hint: Mesquite is a woody plant.) 2. How would you distinguish between a root hair and a small lateral root? 3. You are given a plant part that was found growing in the soil and are asked to determine whether it is a root or an underground stem. How would you identify the structure without a microscope? With a microscope? 4. How would you design an experiment to determine whether gold is essential for plant growth? What would you use for an experimental control? 5. Why does overwatering a plant often kill it? 6. Explain why, once secondary growth has occurred, that portion of the root is no longer involved in absorption. Where does absorption of water and dissolved minerals occur in plants that have roots with secondary growth? 7. Evolution Link. A barrel cactus that is 60 cm tall and 30 cm in diameter has roots more than 3 m long. However, all the plant s roots are found in the soil at a depth of 5 to 15 cm. What possible adaptive value does such a shallow root system confer on a desert plant? Additional questions are available in ThomsonNOW at login 766 Chapter 35