PPT 7 - Roots PDF

Chapter 7 Roots  Concepts (1 of 3) Most roots have three functions. − Anchoring the plant firmly to a substrate − Absorbing water and minerals − Producing hormones  Concepts (2 of 3) Specialization in one area can negatively impact the root’s ef...

Chapter 7 Roots  Concepts (1 of 3) Most roots have three functions. − Anchoring the plant firmly to a substrate − Absorbing water and minerals − Producing hormones  Concepts (2 of 3) Specialization in one area can negatively impact the root’s efficiency in another area. − Several types of roots may occur on one plant to minimize these problems.  Concepts (3 of 3) Other root functions − Fleshy taproots are the plant’s main site of carbohydrate storage during winter. Some roots spread horizontally and produce shoot buds that grow and act as new plants. In some palms, roots grow from the trunk and harden into sharp spines. Vines have modified roots that act as holdfasts on rock or brick. Parasitic flowering plants use modified roots to get nutrients and water from host plants.  External Structure of Roots (1 of 4) Most seed plants have: − A single large taproot that develops from the radicle (embryonic root). − Radicle may develop into thick taproot with thinner branch roots Taproot system  External Structure of Roots (2 of 4) Most monocots and some eudicots have a fibrous root system. − A mass of many similarly sized roots − Adventitious in nature Adventitious roots do not arise on pre-existing roots or from the radicle. − Increase the absorptive and transport capacities Fibrous root system  External Structure of Roots (3 of 4) Many eudicots undergo secondary growth. − This increases the amount of wood (xylem) in both the trunk and roots. − This permits an increase in the number of leaves and absorptive roots.  External Structure of Roots (4 of 4) Most monocots cannot undergo secondary growth. − After their stem is formed, their conducting capacity cannot be increased. − An ever-increasing taproot system would not work here. − Formation of adventitious roots can overcome this limitation.  Structure of Individual Roots (1 of 2) Roots grow from an apical meristem at the tip. The root apical meristem is protected by the root cap. Just behind the root cap and root apical meristem is a short zone of elongation. − These cells undergo division and expansion.  Structure of Individual Roots (2 of 2) The root hair zone is a region in which many of the epidermal cells extend out as narrow trichomes. − Increase the root’s surface area. New lateral roots emerge behind the root hair zone. − Lateral roots may occur in rows or appear to be randomly distributed on the parent root.  Internal Structure: Root Cap (1 of 2) The root cap layer closest to the root meristem is meristematic. − Forms files of cells that are pushed forward. − Cells on the edge grow toward the side and proliferate. − Thimble-shaped mass of parenchyma cells covering each root tip Maturing cells detect gravity using starch grains.  Internal Structure: Root Cap (2 of 2) Close to the edge of the cap, the cell’s dictyosomes secrete mucigel. − The middle lamella breaks down and releases cells. − They are usually crushed by expansion of the root. − Protects tissues from damage as root grows  Internal Structure: Apical Meristem Root apical meristem is the source of regular files of cells. − They extend into the regions of mature root tissues. − There is a mitotically inactive central region known as the quiescent center. − It can replace the apical meristem if it becomes damaged. Courtesy of Lewis J. Feldman, University of California, Berkeley  Root Structure Region of Cell Division - Composed of apical meristem in the center of root tip Subdivided into 3 meristematic areas: o Protoderm - Gives rise to epidermis o Ground meristem - Gives rise to cortex and pith o Procambium - Gives rise to primary xylem and primary phloem Root tip showing primary meristems Internal Structure: Zone of Elongation Zone of elongation is the region beyond the meristematic region. − Here, cells are enlarging. − Nuclei become less distinct. − Protoderm, provascular tissue, and ground tissue begin to differentiate. − No cells are yet mature. Internal Structure: Zone of Maturation (1 of 3) Zone of maturation/root hair zone is marked by the production of root hairs growing outward. − This increases absorption of water and minerals. − Adhere tightly to soil particles − Increase total absorptive surface of root Tissues are differentiated. − Cortex cells transfer minerals from the epidermis to the vascular tissue. Internal Structure: Zone of Maturation (2 of 3) The innermost layer of cortical cells differentiates into a cylinder called the endodermis. − The radial walls of the endodermis are encrusted with waterproof lignin and suberin. − These Casparian strips are involved in controlling the minerals that enter the xylem. Internal Structure: Zone of Maturation (3 of 3) Casparian strips are involved in controlling the minerals that enter the xylem. − Minerals can cross the endodermis only if endodermal protoplasts absorb them. − From the cortex apoplast or the symplast. They then secrete them into the vascular tissues. Internal Structure: Vascular System (1 of 3) A cylinder of tissue, interior to the cortex, forms the stele. It consists of both the xylem and the phloem in the center. Xylem is in the center of the stele. The phloem surrounds the xylem. No pith is present.  Internal Structure: Vascular System (2 of 3) Within the xylem, the inner wide cells are metaxylem and outer narrow cells are protoxylem. Within phloem strands the protophloem occurs on the outer side and the metaphloem occurs on the inner side.  Root Structure Region of Maturation Vascular cylinder - Core of tissues inside endodermis Pericycle - Outer boundary of vascular cylinder Region of endodermis and pericycle in dicot root – Continues to divide, even after mature – Forms lateral (branch) roots and part of the vascular cambium Lateral root formation Internal Structure: Vascular System (3 of 3) The arrangement of tissue within the stele differs for monocots. − Large vessels and separate bundles of phloem. − A pith is present.  Root Structure (1 of 2) Mature portions of the root − Root hairs function only for several days. − In the endodermis, more suberin and lignin are laid over the radial and tangential surfaces. − Endodermal cells that are slow to mature and have only the Casparian strip are called passage cells. − The mature, watertight endodermis keeps water in the vascular cylinder of a mature root.  Root Structure (2 of 2) Shortly after the root hairs die, underlying cortex and epidermis often die. − They are shed from the root. The endodermis becomes the root surface until a bark can form. The large fibrous roots of many monocots are: − Strictly annual − Completely replaced by more adventitious roots  Other Types of Roots and Modifications (1 of 4) Storage roots provide long-term storage for carbohydrates that accumulate during summer photosynthesis. − Biennials and perennials store nutrients during the winter. − Roots are less visible as food for foragers. − Roots’ environment is more stable than aboveground parts. − Sweet Potatoes  Specialized Roots Pneumatophores In plants with roots growing in water Spongy roots that extend above the water’s surface and enhance gas exchange between atmosphere and subsurface roots Mangrove pneumatophores  Other Types of Roots and Modifications (2 of 4) Prop roots are adventitious roots that can grow extensively through the air. − Additional nutrients and water to the stem. − Stabilize stems. − Brace them against wind and water currents. (c) Dhoxax/iStock Buttress Roots Stability in shallow soil Tropical Trees Buttress roots of tropical fig tree  Other Types of Roots and Modifications (3 of 4) Aerial roots of orchids have a specialized epidermis called a velamen. − Many orchids are epiphytic. − They grow on trees, with roots dangling in the air. − The waterproof velamen prevents water loss if the air becomes dry.  Other Types of Roots and Modifications (4 of 4) Contractile roots are important for stability and depth control. − Prop roots can undergo considerable contraction. − Root contraction may be important in anchoring newly germinated seeds. − Bulbs and corms use contractile roots to change their depth in the soil. Courtesy of Judy Jernstedt, University of California, Davis  Mycorrhizae (1 of 2) Mycorrhizae are symbiotic associations between the roots of seed plants and soil fungi. − Fungi gain carbohydrates from the roots. − Fungal hyphae added in phosphorus uptake. There are two types of associations. − Ectomycorrhizal relationship: Fungal cells penetrate between the outermost root cortex cells but never penetrate the cell wall. Typically woody species. Courtesy of R. L. Peterson and D. Hern, University of Guelph  Mycorrhizae (2 of 2) − Endomycorrhizal associations: Fungal cells penetrate the root cortex and cell walls but not the plasma membrane. o The fungus is unable to live without these sugars. o Often the plant becomes stunted without the fungus, probably due to phosphorus deficiency.  Root Nodules and Nitrogen Fixation (1 of 2) The scarcity of nitrogenous compounds in the soil is a growth-limiting factor. − In a small number of plants, especially legumes, a symbiotic relationship has © inga spence / Alamy Stock Photo evolved with nitrogen-fixing bacteria of the genus Rhizobium.  Root Nodules and Nitrogen Fixation (2 of 2) Rhizobium infects the host root hair. Triggers the cells of the cortex to divide and produce a root nodule. − Rhizobium gets an oxygen-free atmosphere and sugars. − Legume gets nitrogenous compounds. Courtesy of E. H. Newcomb, University of of Wisconsin Department of Botany  Haustorial Roots (1 of 2) Haustoria are highly modified roots of parasitic plants. Haustorial roots are modified to: − Attach the parasite to the host. − Penetrate the host vascular tissue.  Haustorial Roots (2 of 2) The haustoria penetrates the epidermis and cortex then continues until it makes contact with the host’s xylem. − A continuous vessel from host to parasite forms, constructed of cells of both. − Parasites that only attack the xylem photosynthesize. − Others that do not photosynthesize also attack the phloem. Courtesy of C.L. Calvin, Rancho Santa Ana Bot. Garden  Human Relevance of Roots Sources of food Carrots, sugar beets, turnips, horseradishes, cassava (tapioca), yams, sweet potatoes Spices Sassafras, sarsaparilla, licorice Dyes mahogany (red) talisay (black and yellow) narra (brown) Drugs Aconite, ipecac, gentian, reserpine Insecticide Rotenone