1. Chapter 5 Roots and Soils Lecture Outline
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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 radicle grows out and develops into first root.
Radicle may develop into thick taproot with thinner branch roots.
Dicotyledonous plants (dicots)
Taproot system

5. 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

6. 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)

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. Mycorrhizae

20. 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

21. 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

22. 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.

23. 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

24. 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

25. 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.

26. 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

27. 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.

28. 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+

29. 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.

30. 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.

31. 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

32. 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

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