Biological Foundations 112, Lecture 13


Transport in Phloem and Plant Nutrition

Chapter 32, pages 705-710
Chapter 33, pages 711-717


I.   Transport in Phloem

      A.   Mass Flow Theory - mechanism of transport in the phloem

             1.   Contents of the sieve tube members makes up the continuous liquid system

             2.   Water and solutes move together as one mass in the sieve tubes along their length

             3.   Mechanism

                   a.   Photosynthetic products are produced in the leaf mesophyll

                   b.   Glucose diffuses to the bundle sheath along the vein

                   c.    Here a glucose and a fructose molecule are hooked together into sucrose 
                          (enzymatic action)

                   d.   Active transport dumps sucrose into the sieve tubes of the veins (source end)

                   e.   Phloem sap water potential is lowered below that of adjacent cells and the
                         xylem

                   f.   Water diffuses into phloem cells from adjacent cells and xylem

                   g.   Hydrostatic pressure is raised in the sieve tubes

                   h.   Hydrostatic pressure pushes phloem sap in mass away from the leaf and on  
                         down the stem

                   i.   At sites where sucrose is removed from the sieve tube (by active transport) the                                  water potential raises above the adjacent cells and the xylem

                   j.   Water diffuses into the adjacent cells and the xylem

                   k.   This lowers the pressure at this end (sink end)

                    l.   Thus the phloem sap moves from areas of high pressure to areas of low
                         pressure

      B.   Direction of flow

             1.   Depends on where the active sink sites are of the plant

             2.   Can go up or down depending upon where the sink is

             3.   Sucrose is the principal substance transported

             4.   Other substances transported also:

                   a.   Hormones

                   b.   Amino acids

                   c.   Inorganic ions

     C.   Mechanisms affecting rate of flow

            1.   Rates at which products are added and removed from the system

                  a.   Products can only be removed from the system as fast as they are put in

                  b.   Products can only be put into the system as fast as they are being removed

             2.   Callose

                   a.   Deposited and removed by enzymatic action on the sieve plates

                   b.   Thickness of callose affects aperture size of the holes in the sieve plate and  
                          therefore the flow of material through the sieve plate

                   c.   The callose thickness is dynamically controlled

                   d.   Enzymes are highly sensitive to mechanical stimuli (vibrations, crushing, etc.)

             3.   P-protein

                   a.   A spider-web network firmly anchored to the ends of the cell

                   b.   Under normal conditions the p-protein doesn't obstruct the flow

                   c.   Injury causing abrupt increases in flow, rips the network loose and it piles up
                         against the nearest sieve plate, blocking the flow and limiting the loss of  
                         phloem sap

II.  Plant Nutrition

      A.   Formation of soil

             1.   Steps of soil formation

                   a.   Comes from rock that is gradually broken down into smaller and smaller
                         particles

                   b.   Process is by both physical and chemical weathering

                   c.   All minerals that a plant absorbs (except nitrogen) comes from these rocks

                   d.   Different rocks have different mineral contents

                   e.   Various soils reflect the mineral contents of their parent rocks

             2.   Factors in soil formation

                   a.   Climate

                   b.   Living organisms that break down the rocks and humus in the soil

                   c.   Topography of the landscape

      B.   Composition of soil

             1.   Five different materials

                   a.   Inorganic minerals

                   b.   Organic matter

                   c.   Soil organisms

                   d.   Soil atmosphere

                   e.   Soil water

             2.   Inorganic minerals

                   a.   Composed of sand (0.02-2mm), silt (0.002-0.02 mm), and clay (less than
                         0.002 mm)

                   b.   Clay component tends to attract and bind positive ions, making the ions immune 
                         from leaching

                   c.   Different soils have different combinations of sand, silt, and clay

                         (1)  The more sandy the soil the less water it holds

                         (2)  More clay makes the soil thick and heavy and compacted

             3.   Organic matter

                    a.   The decaying or decayed remains of plants and animals in the soil

                    b.   Called humus

             4.   Soil organisms

                    a.   Microorganisms break down the organic matter

                    b.   Includes bacteria, algae, fungi, protozoans, worms, "bugs", small mammals, etc.

                    c.   Produces a complex community with millions of individuals

             5.    Soil atmosphere and water

                    a.   30-60% of soil volume is space between particles

                    b.   This space is filled with air and water

                    c.   Air -- O2 is lower and CO2 is higher in the soil

                    d.   The more water in the soil, the less air

                    e.   Air in soil is very important, the roots need to breathe

      C.   Plant Nutrients

             1.   Macronutrients

                   a.   Nutrients needed in fairly large quantities

                   b.   Nutrients in this category:  carbon, hydrogen, oxygen, nitrogen, phosphorus,
                         potassium, sulfur, calcium, and magnesium

                   c.   Remember the nutrients by:  C HOPKNS CaMg

                   d.   Pronounced as:  C Hopkns Cafe Mighty Good         

             2.   Micronutrients

                   a.   Nutrients needed in trace amounts

                   b.   Nutrients in this category:  iron, boron, manganese, copper, molybdenum,
                                                                 chlorine, and zinc

             3.   Fertilizers

                   a.   Two types of fertilizers:  organic and inorganic fertilizers

                   b.   Organic fertilizers

                         (1)  Organic substances such as manure, and the breakdown products of plant  
                               and animal remains

                         (2)  Increases the amount of organic material in soil

                         (3)  Releases soil nutrients slowly

                   c.   Inorganic fertilizers

                         (1)  Manufactured chemicals in the chemical form that the plant utilizes

                         (2)  More precise application of fertilizer because quantities are known

                         (3)  Variation of various amounts of fertilizer can cause different growth patterns

                                (a)  High nitrogen for lettuce

                                (b)  Low nitrogen for tomatoes

                         (4)   Most are combinations of three types of fertilizer:  N, P, and K

                         (5)   The three numbers on fertilizer bags are the relative amounts of nitrogen,  
                                 phosphorus, and potassium, in that order


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