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Roots and Soils

Roots and Soils. Chapter 5. 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

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Roots and Soils

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  1. Roots and Soils Chapter 5

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

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

  4. Root Development • Upon germination, embryo’s radiclegrows out and develops into first root • May develop into thick taproot with thinner branch roots • Dicotyledonous plants (dicots) Taproot system

  5. Root Development • Or, after radicle formation, adventitious roots develop into fibrous root system • Adventitious Roots - develop from stem or leaf • Fibrous Roots - large number of fine roots of similar diameter • Monocotyledonous plants (monocots) and some dicots Fibrous root system

  6. Root Structure • 4 regions: • Root cap • Region of cell division • Region of cell elongation • Region of maturation Longitudinal section through root tip

  7. Root Structure • 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 • Functions in gravitropism (gravity perception) Longitudinal section through root tip

  8. Root Structure • Region of Cell Division - composed of apical meristem in 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 1° xylem and 1°phloem Root tip showing primary meristems

  9. Root Structure • Region of Elongation - cells become several times original length • Vacuoles merge • Region of Maturation - cells differentiate into various distinctive cell types • Root hairs Root hair zone of radish seedling

  10. Root Structure • Root Hairs • 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

  11. Root StructureRegion of Maturation • Cortex - parenchyma cells between epidermis and vascular cylinder • Mostly stores food Cross section of dicot root

  12. Root StructureRegion of Maturation • Endodermis - inner boundary of cortex, consisting of 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 central core to pass through endodermis • Regulates types of minerals absorbed • Eventually inner cell walls become thickened with suberin, except for passage cells

  13. Root StructureRegion of Maturation Enlargement of vascular cylinder of dicot root

  14. Root StructureRegion 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 vascular cambium Lateral root formation

  15. Root StructureRegion of Maturation • Most cells of vascular cylinder 1° xylem or 1° 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 2° phloem to outside and 2°xylem to inside Vascular cylinder of dicot root

  16. Root Structure • Growth • Determinate Growth - growth that stops after organ fully expanded or after plant has reached certain size • Indeterminate Growth - new tissues added indefinitely, season after season

  17. Specialized Roots • Food 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

  18. Specialized Roots • Pneumatophores • Plants with roots growing in water • Spongy roots that extend above water’s surface and enhance gas exchange between atmosphere and subsurface roots Mangrove pneumatophores

  19. 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

  20. Specialized Roots • Contractile Roots • Pull plant deeper into soil • Lily 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

  21. Mycorrhizae • Mycorrhizae - fungi that form mutualistic association with plant roots • Mutualistic association: Both fungus and root benefit and 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

  22. Mycorrhizae

  23. Root Nodules • A few species of bacteria produce enzymes that can convert nitrogen from 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

  24. Soils • Soils divided into horizons: • Topsoil • A Horizon - dark loam, more organic material than lower layers • E Horizon - light loam • B Horizon - subsoil • More clay, lighter in color • C Horizon - parent material Soil profile

  25. SoilsParent Material • Parent Material - rock not 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

  26. SoilsClimate • Climatevaries throughout globe, as does its role in weathering of rocks • Deserts - little weathering by rain, and soils poorly developed • Areas of moderate rainfall - well-developed soils • Areas of high rainfall - excessive water flow through soil leaches out important minerals.

  27. SoilsLiving Organisms and Organic Composition • Many kinds of organisms, roots and other plant parts in soil • 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 increasing rate at which minerals dissolve • Small animals alter soil by activities and wastes • Humus - partially decomposed organic matter, gives soil dark color

  28. SoilsTopography • 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

  29. SoilsSoil 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 -micelles • Negatively charged and attract, exchange or retain positively charged ions, such as Mg++ and K+

  30. 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 - granular with pore spaces occupying between 40-60% of total soil volume • Particle size more important than total volume

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

  32. SoilsWater 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 permanent wilting point

  33. SoilsSoil pH • Affects nutrient availability • Alkalinitycauses 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 • Acidityinhibits growth of nitrogen-fixing bacteria • Counteract by adding calcium or magnesium compounds = liming

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

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