Chapter 5 Roots and Soils Lecture Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Outline Function of Roots Root Development Root Structure Specialized Roots Mycorrhizae Root Nodules Human Relevance of Roots Soils

Function of Roots Anchor plants into soil Absorption of water and minerals Store food or water Other specialized functions

Root Development Upon germination, embryo’s radicle grows out and develops into first root. Radicle may develop into thick taproot with thinner branch roots. Dicotyledonous plants (dicots) Taproot system

Root Development Fibrous root system Or, after radicle formation, adventitious roots may arise that develop into a fibrous root system. Adventitious roots do not develop from another root, but instead from a stem or leaf. Fibrous roots - Large number of fine roots of similar diameter Monocotyledonous plants (monocots) and some dicots

Longitudinal section through root tip Root Structure 4 regions: Root cap Region of cell division Region of cell elongation Region of maturation Root Cap - Thimble-shaped mass of parenchyma cells covering each root tip Protects tissues from damage as root grows Secretes mucilage that acts as lubricant Longitudinal section through root tip Functions in gravitropism (gravity perception)

Root tip showing primary meristems 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 hair zone of radish seedling Root Structure Region of Elongation - Cells become several times their original length. Vacuoles merge Region of Maturation - Cells differentiate into various distinctive cell types. Root hairs form. Root hair zone of radish seedling Epidermal cell extensions with thin cuticle Absorb water and minerals Adhere tightly to soil particles Increase total absorptive surface of root

Root Structure Region of Maturation Cortex - Parenchyma cells between epidermis and vascular cylinder Mostly stores food Cross section of dicot root

Root Structure Region of Maturation Endodermis - Inner boundary of cortex, consisting of a single-layered cylinder of compact cells Cell walls with suberin bands called casparian strips on radial and tangential walls Forces water and dissolved substances entering and leaving the central core to pass through endodermis Regulates types of minerals absorbed Eventually inner cell walls become thickened with suberin, except for passage cells. Enlargement of vascular cylinder of dicot root

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 Lateral root formation Continues to divide, even after mature Forms lateral (branch) roots and part of the vascular cambium

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. Vascular cylinder of dicot root

Root Structure 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 Water Storage Roots Starch and other carbohydrates Sweet Potatoes Water Storage Roots Pumpkin family, especially in arid regions Manroot, water storage root Propagative Roots Adventitious buds on roots - Develop into suckers (aerial stems) Fruit Trees

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

Specialized Roots Aerial Roots Orchids - Velamen roots, with epidermis several layers thick to reduce water loss Corn - Prop roots support plants in high wind Ivies (English ivy, Virginia creeper) - Aerial roots aid plants in climbing Orchid aerial (velamen) roots

Buttress roots of tropical fig tree Specialized Roots Contractile Roots Pull plant deeper into the soil Lilly bulbs, dandelions Buttress Roots Stability in shallow soil Tropical Trees Buttress roots of tropical fig tree Parasitic Roots No chlorophyll and dependent on chlorophyll-bearing plants for nutrition

Mycorrhizae Mycorrhizae - Fungi that form a mutualistic association with plant roots Mutualistic association: Both fungus and root benefit and are dependent upon association for normal development Fungi facilitate absorption of water and nutrients, especially phosphorus for roots. Plant furnishes sugars and amino acids to fungus. Particularly susceptible to acid rain

Mycorrhizae

Root Nodules A few species of bacteria produce enzymes that can convert nitrogen from the atmosphere into 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

Soils Soil is formed through the interaction of climate, parent material, topography, vegetation, living organisms and time. Solid portion of soil consists of minerals and organic matter. Pore spaces between solid particles filled with air or water.

Soils Soils divided into horizons: Topsoil B Horizon - Subsoil Soil profile Soils divided into horizons: Topsoil A horizon - Dark loam, with more organic material than lower layers E horizon - Light loam B Horizon - Subsoil More clay, lighter in color C Horizon - Parent material

Soils Parent Material Parent material - Rock that has not been broken down into smaller particles Rock types: Igneous – Volcanic Sedimentary - Deposited by glaciers, water or wind Metamorphic - Changes in igneous or sedimentary rocks from pressure or heat

Soils Climate Climate varies throughout the globe, as does its role in weathering of rocks Deserts - Little weathering by rain, and soils poorly developed In areas of moderate rainfall - Well-developed soils Areas of high rainfall - Excessive water flow through soil leaches out important minerals.

Soils Living Organisms and Organic Composition In soil there are many kinds of organisms, roots and other plant parts. Bacteria and fungi decompose organic material from dead leaves, plants and animals. Roots and other living organisms produce carbon dioxide, which combines with water and forms acid that increases the rate at which minerals dissolve. Small animals alter soil by their activities and by their wastes. Humus - Partially decomposed organic matter, gives soil a dark color

Soils Topography Topography - Surface features Steep areas: Soil may erode via wind, water or ice. Flat, poorly drained areas: Pools and ponds may appear. Development of soil arrested. Ideal topography permits drainage without erosion.

Soils Soil Texture and Mineral Composition Soil Texture - Relative proportion of sand, silt and clay in soil Sand - Many small particles bound together chemically Silt - Particles too small to be seen without microscope Clay - Only seen with electron microscope Individual clay particles called micelles Negatively charged and attract, exchange or retain positively charged ions, such as Mg++ and K+

Soils Best agricultural soils - loams composed of 40% silt, 40% sand and 20% clay Coarse soils drain water too quickly. Clay soils allow little water to pass. Soil Structure - Arrangement of soil particles into aggregates Productive agricultural soils are granular with pore spaces occupying between 40-60% of the total soil volume. Particle size is more important than total volume.

Soils Water in the Soil Hygroscopic Water - Physically bound to soil particles and unavailable to plants Gravitational Water - Drains out of pore spaces after a rain Capillary Water - Water held against the force of gravity in soil pores Determined by structure and organic matter, by density and type of vegetation, and by the location of underground water tables Plants mostly dependent upon this type.

Soils Water in the Soil Field capacity - Water remaining in soil after water drains away by gravity Determined by texture, structure and organic content of soil Permanent Wilting Point - Rate of water absorption insufficient for plant needs Plant permanently wilts. Available Water - Soil water between field capacity and the permanent wilting point

Soils Soil pH Affects nutrient availability Alkalinity causes some minerals, such as copper, iron and manganese to become less available. Counteract by adding sulfur, which is converted to sulfuric acid by bacteria, or by adding nitrogenous fertilizers Acidity inhibits growth of nitrogen-fixing bacteria. Counteract by adding calcium or magnesium compounds = liming

Review Function of Roots Root Development Root Structure Specialized Roots Mycorrhizae Root Nodules Human Relevance of Roots Soils