Roots and Soils PDF

Chapter 6: Roots and Soils Outline ❖ Function of Roots ❖ Root Development ❖ Root Structure ❖ Specialized Roots ❖ Mycorrhizae ❖ Root Nodules ❖ Human Relevance of Roots Function of Roots ❖ Anchor plants into soil ❖ Absorption of water and minerals ❖ Store food or water ❖ Other specialized functions Ro...

Chapter 6: Roots and Soils Outline ❖ Function of Roots ❖ Root Development ❖ Root Structure ❖ Specialized Roots ❖ Mycorrhizae ❖ Root Nodules ❖ Human Relevance of Roots Function of Roots ❖ Anchor plants into soil ❖ Absorption of water and minerals ❖ Store food or water ❖ Other specialized functions Root Development ❖ Upon seed germination, Embryo’s radicle grows out to become the first root. Radicle may develop into: – thick taproot with thinner branch roots o Typically Dicotyledonous plants (dicots) Root Development Or, after radicle formation, – Adventitious roots may arise that can develop into a fibrous root system. o Adventitious roots: Form from stems or leaves o Fibrous roots: large number of fine roots of similar diameter « Monocotyledonous plants (monocots) and some dicots Adventitious Fibrous Root Structure ❖ 4 regions: Root cap Region of cell division Region of cell elongation Region of maturation 1. 2. 3. 4. Root Cap - Thimble-shaped, parenchyma cells covering each root tip – Protects tissues from damage as root grows o Secretes lubricating mucilage – Longitudinal section through root tip Functions in gravitropism (gravity perception) Root Structure ❖ Region of Cell Division Composed of apical meristem in the center of root tip Subdivided into 3 meristematic areas: – Protoderm – Gives rise to epidermis – Ground meristem - Gives rise to cortex and pith – Procambium - Gives rise to primary xylem and primary phloem Root tip showing primary meristems Root Structure ❖ Region of Elongation - Cells become several times their original length. Vacuoles merge into central vacuole ❖ Region of Maturation - Cells differentiate into various distinctive cell types. Root hairs form. – Epidermal cell extensions with thin cuticle – Absorb water and minerals – Adhere tightly to soil particles – Increase total absorptive surface of root Root hair zone of radish seedling Root Structure: Region of Maturation ❖ Most of cells of vascular cylinder are primary xylem or primary phloem. In dicot or conifer roots - Solid core of xylem, with “arms” in cross section In monocots, xylem surrounds pith. Phloem in patches between xylem arms Vascular cambium forms secondary phloem to the outside and secondary xylem to the inside. Root Steles (cross-sections) Eudicot Monocot Root Structure: Region of Maturation ❖ Cortex - Parenchyma cells btwn epidermis and vascular cylinder Mostly stores food Root Structure: Region of Maturation Endodermis - Inner boundary of cortex, single-layered cylinder of compact cells – Casparian strips: cell walls impregnated w/ suberin & lignin on radial & transverse walls o Forces water and dissolved substances entering and leaving the central core to pass through endodermis, specifically the passage cells. « Regulates types of minerals absorbed Root Structure: Flow of Water and Minerals 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 ❖ Growth Determinate growth - Growth that stops after an organ is fully expanded or after a plant has reached a certain size Indeterminate growth - New tissues are added indefinitely, season after season Specialized Roots ❖ Food Storage Roots Starch and other carbohydrates Sweet Potatoes ❖ Water Storage Roots Pumpkin family, especially in arid regions ❖ Propagative Roots Manroot, water storage root Adventitious buds on roots - Develop into suckers (aerial stems) – Fruit Trees Specialized Roots ❖ Pneumatophores In plants with roots growing in water Spongy roots extending above the water for gas exchange btwn atmosphere and subsurface roots ❖ Specialized Roots Bald Cypress ‘knees’: once thought to be pneumatophores Specialized Roots: Aerial Roots ❖ Orchids - Velamen roots, with epidermis several layers thick to reduce water loss Orchid aerial (velamen) roots Specialized Roots: Aerial Roots ❖ Ivies (English ivy, Virginia creeper) - Aerial roots aid plants in climbing Specialized Roots: Aerial Roots ❖ Corn, Hala, and mangrove - Prop roots support plants in high wind Specialized Roots ❖ Contractile Roots Pull plant deeper into the soil – ❖ Lilly bulbs, dandelions Buttress Roots Stability in shallow soil – ❖ Tropical Trees Parasitic Roots Most have no chlorophyll and dependent on chlorophyll-bearing plants for nutrition Buttress roots of tropical fig tree Specialized Roots: Contractile Roots ❖ Root Symbioses - Mycorrhizae Associations Mycorrhizae - Fungi that form a mutualistic association with plant roots Fungi facilitate absorption and concentration of nutrients, especially phosphorus for roots. Plant furnishes sugars and amino acids to fungus. – Plants with mycorrhizae develop few root hairs compared with those growing without an associated fungus. Particularly susceptible to acid rain Root Symbioses - Mycorrhizae Associations ❖ Two Types: Endomycorrhizae Fungi – AM Fungi – Penetrate only the cell walls Ectomycorrhizae Fungi – Form a mantle on the root exterior and grow between the cell walls Mycorrhizae Mycorrhizae Associations Truffles Root Nodules and Nitrogen Fixing Bacteria ❖ Plants cannot convert free nitrogen to usable form ❖ A few species of bacteria produce enzymes that converts nitrogen nitrates and other nitrogenous substances readily absorbed by roots. Root nodules contain large numbers of nitrogen-fixing bacteria. Legume Family (Fabaceae) Root nodules on roots Human Relevance of Roots ❖ Sources of food Carrots, sugar beets, turnips, horseradishes, cassava (tapioca), yams, sweet potatoes ❖ Spices Sassafras, sarsaparilla, licorice ❖ Dyes ❖ Drugs Aconite, ipecac, gentian, reserpine ❖ Insecticide Rotenone Chapter 6: Plant Organs – Roots Learning Objectives 1. Describe the functions of roots and describe two features of roots that shoots lack. 2. Contrast the structure of a primary eudicot root and a monocot root and describe the functions of each tissue. 3. Trace the pathway of water from the soil through the various root tissues. 4. Describe several roots that are modified to perform unusual functions. 5. Discuss the significance of roots to humans.