34Roots and Mineral Nutrition

Lecture Outline

  1. Root structure and function
  2. There are two basic types of root systems
  3. A taproot system has one main root; many lateral roots of smaller size emanate from the main root

a)The taproot develops from the embryonic radicle

b)Taproots are characteristic of gymnosperms and dicots

c)Some mature trees retain their taproots, but most develop shallow lateral roots from which other roots branch and grow downward

  1. Fibrous root systems have multiple roots of similar size, which grow from the bottom end of the stem

a)Fibrous root systems are characteristic of monocots

b)Fibrous roots are called adventitious because they emerge from the stem, not from preexisting roots

  1. Both root systems function to obtain water and nutrients from the soil
  1. Roots have root caps and root hairs
  2. The root cap is a protective layer covering the apical meristem of the root

a)The parenchyma cells of the root cap are sloughed off as the root pushes through the soil

b)The root cap aids in downward growth of the root

  1. Root hairs are extensions of epidermal cells just behind the root cap

a)Root hairs form in the area of cell maturation and die at the mature end of the root hair zone

b)Root hairs act to increase the functional surface area of the roots

  1. Roots lack nodes and internodes and do not usually produce leaves or buds
  1. The arrangement of vascular tissues distinguishes the roots of herbaceous dicots and monocots
  2. In most herbaceous dicot roots, the central core of vascular tissue lacks pith

a)The epidermis covers the root

(1)In the area of root hairs, a cuticle is not formed, as it would prevent the inflow of water

(2)Water moves into the root by moving along cell walls because cellulose is hydrophilic

b)The cortex is composed of loosely packed parenchyma cells

(1)The parenchyma cells store starches

(2)The cortex lacks collenchyma, as the soil itself supports the roots; some may have sclerenchyma in older roots

(3)The intercellular spaces allow for movement of water, as well as diffusion of gases

(4)The inner layer of the cortex is the endodermis, important in regulating the passage of materials into the interior

(a)Endodermal cells are closely packed and have a Casparian strip on the radial and transverse walls

(b)The Casparian strips are impregnated with suberin

(c)Solutions follow apoplastic pathways via the interconnected cell walls by adsorbing to cellulose

(d)The symplastic pathway for solutions is from cell to cell, passing through the plasmodesmata

(e)At the Casparian strip, water enters the central portion of the root by osmosis

(f)At this point, minerals enter by passing through carrier proteins in the plasma membrane

(g)Active transport moves materials from a low concentration to a higher concentration

(h)This requires a substantial outlay of ATP

c)The pericycle is inside the endodermis and is composed of meristematic parenchymal cells

(1)The pericycle gives rise to lateral roots (branch roots)

(2)The lateral roots must grow out through the surrounding layers of the root

(3)The pericycle also forms lateral meristems involved in growth of woody roots

d)The stele is the centrally located vascular tissue

(1)Xylem extends laterally in 2 to 4 or more xylem arms

(2)Phloem is located between the xylem arms

(3)Water moves into the xylem from the endodermal cells

(4)Phloem carries dissolved sugars to the roots where they may be stored as starch

(5)The vascular cambium is located between the phloem and xylem and gives rise to secondary tissues

  1. Xylem does not form the central tissue in some monocot roots

a)Phloem and xylem are located in alternating bundles around the centrally located pith

b)No vascular cambium exists as no secondary growth occurs in most monocots

c)Some long-lived monocots may have thickened roots due to expanded primary growth

  1. Woody plants have roots with secondary growth
  2. The secondary growth is a result of meristematic activity in the vascular cambium and cork cambium
  3. Roots may have both bark and woody tissue
  4. At the onset of secondary growth, the vascular cambium extends out to the pericycle forming a complete loop of tissue

a)The cambium produces secondary xylem (wood) on the inside and secondary phloem (inner bark) to the outside

b)As the root grows, the primary tissues are crushed

c)The cork cambium produces both cork cells and cork parenchyma cells that compose the periderm

  1. Some roots are specialized for unusual functions
  2. Adventitious roots arise at the nodes of stems
  3. Prop roots, such as those seen in corn, support the plant

a)Red mangroves and banyan trees produce prop roots

  1. Some tropical rainforest trees produce buttress roots
  2. Pneumatophores are specialized roots for gas exchange

a)Pneumatophores are seen in black mangrove, white mangrove, and bald cypress

  1. Epiphytes have specialized aerial roots adapted for anchorage, photosynthesis, or absorption of nutrients from the host plant
  2. Corms and bulbs have contractile roots that pull the corm or bulb deeper into the soil
  1. Roots associations with other organisms
  2. Mycorrhizae are symbiotic relationships between fungi and plant roots
  3. Root nodules are swellings in the roots of plants that contain nitrogen-fixing bacteria
  4. These bacteria belong to the genus Rhizobia
  5. The plant and the Rhizobia communicate via signaling molecules
  6. Root grafts may occur between the roots of plants of two different species
  7. Nutrients and hormones may be passed between the vascular tissues of the two different plants
  1. The soil environment
  2. Soil is produced from parent rock by weathering
  3. The thickness of soil varies
  4. Soils are changed by a variety of biological and chemical factors
  5. Soil is composed of inorganic minerals, organic matter, organisms, air, and water
  6. The components of soil all interact and cycle from the soil to the organisms of the soil

a)Soil contains different-sized particles of sand, silt, and clay

b)Sand particles are the largest, and visible to the naked eye; clay particles can only be seen with an electron microscope

c)Clay particles give a great surface area to the soil and are negatively electrically charged

d)Loam is a soil with an optimal combination of different soil particles—40% each sand and silt, and 20% clay

  1. A soil’s organic matter consists of the wastes and remains of soil organisms

a)Decomposers release the organic compounds of nonliving organic material

b)The partially decayed organic material in the soil is humus

  1. About 50% of soil volume is composed of pore spaces

a)Pore spaces are filled with soil air and soil water, both important to plants

b)Soil air has high concentrations of carbon dioxide and lower concentrations of oxygen than the atmosphere

c)Soil water has low concentrations of dissolved minerals

d)Soil water moves through the soil and removes minerals by leaching

  1. The organisms living in the soil form a complex community
  2. Bacteria number in the hundreds of millions per gram of soil

a)Worms and ants are important in soil processing

  1. Soil pH affects soil characteristics and plant growth
  2. Soil pH ranges from 4-8
  3. At extreme pH values, solubilities and leaching rates of minerals are affected
  4. The optimal soil pH for plant growth is pH 6 to 7
  5. Acid precipitation is one of several factors responsible for forest decline
  6. Soil provides most of the minerals found in plants
  7. Nineteen elements are essential for plant growth

a)Nine elements are macronutrients (C, H, O, N, P, K, S, Ca, and Mg)

(1)Four of these come from water or the atmosphere (C,H, O, N)

b)Seven elements are micronutrients (Fe, B, Mn, Cu, Mb, Cl, and Zn)

  1. How do biologists determine whether an element is essential?

a)Hydroponics allows determination of essential elements

  1. Soil can be damaged by human mismanagement
  2. Nutrient mineral depletion often occurs in soils that are farmed

a)Agriculture prevents the return of nutrients from decomposing plants

b)Nitrogen, phosphorus, and potassium typically become limiting

c)Organic or inorganic fertilizers are added to restore fertility

(1)The numbers on bags of inorganic fertilizers indicate the proportions of nitrogen to phosphorus to potassium

  1. Soil erosion is the loss of soil from the land

a)Erosion is a natural loss of soil from land, which is accelerated by human activities such as agriculture, clear cutting, and construction

  1. Salt accumulates in soil that is improperly irrigated

a)As irrigation water evaporates, the salts remain behind

b)The process is called salinization

Research and Discussion Topics

  • Examine the real "trouble spots" of soil erosion on an international level. What soil management practices are being implemented to reduce this loss?
  • Research the development of resistance in weeds to commercial pesticides. What about the BT-corn and the monarch butterfly controversy in 2002? Is the scientific evidence sound?