Post on 30-Dec-2015
transcript
Chapter 5
Lecture Outline
Roots and Soils
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
• 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
Fibrous root system
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 growso Secretes mucilage that
acts as lubricant
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
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 rootRoot hair zone of radish seedling
Root StructureRegion of Maturation
Cortex - Parenchyma cells between epidermis and vascular cylinder
• Mostly stores food
Cross section of dicot root
Root StructureRegion 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 wallso Forces water and dissolved substances entering and
leaving the central core to pass through endodermiso 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 StructureRegion of Maturation
Vascular cylinder - Core of tissues inside endodermis
• Pericycle - Outer boundary of vascular cylinder
– Continues to divide, even after mature
– Forms lateral (branch) roots and part of the vascular cambium
Region of endodermis and pericycle in dicot root
Lateral root formation
Root StructureRegion 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• Starch and other
carbohydrates• Sweet Potatoes
Water Storage Roots• Pumpkin family,
especially in arid regions
Propagative Roots• Adventitious buds on roots - Develop into suckers
(aerial stems)– Fruit Trees
Manroot, water storage root
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
Specialized Roots
Contractile Roots
• Pull plant deeper into the soil– Lilly bulbs, dandelions
Buttress Roots
• Stability in shallow soil– Tropical Trees
Parasitic Roots
• No chlorophyll and dependent on chlorophyll-bearing plants for nutrition
Buttress roots of tropical fig tree
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
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
– 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
Soil profile
SoilsParent 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
SoilsClimate
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.
SoilsLiving 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
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.
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 called micelles
o 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.
SoilsWater 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.
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 the permanent wilting point
SoilsSoil 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