1. Water In Plants Chapter 9 Copyright © McGraw-Hill Companies Permission
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2. Outline
Molecular Movement
Diffusion
Osmosis
Water Movement
Cohesion-Tension Theory
Regulation of Transpiration
Transport of Organic Solutes
Pressure-Flow Hypothesis
Mineral Requirements for Growth

3. Molecular Movement
Diffusion
Movement of molecules from a region of higher concentration to a region of lower concentration.
Move along a concentration gradient.
Move until equilibrium reached.

4. Diffusion

5. Osmosis
Osmosis is diffusion of water through a differentially permeable membrane from a region where the water is more concentrated to a region where it is less concentrated.
Water enters a cell by osmosis until the osmotic potential is balanced by the resistance to expansion of the cell wall.
Turgor Pressure
Pressure Potential

6. Osmosis
Water Potential of a plant is essentially its osmotic potential and pressure potential combined.
Water flows from the xylem to the leaves, evaporates within the leaf air spaces, and transpires through the stomata into the atmosphere.

7. In animal cells, the water potential is equal to the osmotic potential of the cytoplasm, but this is different in plant cells…
Plant cells have a cell wall, which exerts an inward pressure when the cell is turgid. This is known as the pressure potential.
The water potential of an plant cell is equal to the osmotic potential of the cytoplasm plus the cell wall pressure:
W.P.= O.P. + P.P.

8. Osmosis

9. A plant cell with water potential –50 is placed in a solution…

10. If the solution is hypotonic, net endosmosis occurs and the cell becomes fully turgid.
Water potential of cytoplasm = -50
Osmotic potential of solution = -20

11. If the solution is hypertonic, net exosmosis occurs and causes plasmolysis (the cell membrane pulls away from the cell wall. The cell wall stays intact).
Water potential of cytoplasm = -50
Osmotic potential of solution = -80

12. If the solution is isotonic, no net osmosis occurs
If the solution is isotonic, no net osmosis occurs. The cell is not plasmolysed, but it is not fully turgid either.
Water potential of cytoplasm = -50
Osmotic potential of solution = -50

14. Molecular Movement
Plasmolysis
Loss of water through osmosis is accompanied by shrinkage of protoplasm away from the cell wall.
Imbibition
Colloidal material and large molecules usually develop electrical charges when they are wet, and thus attract water molecules.

15. Plasmolysis

16. Molecular Movement
Active Transport
Plants absorb and retain solutes against a diffusion, or electrical, gradient through the expenditure of energy.
Involves proton pump.

17. Water and Its Movement Through The Plant
More than 90% of the water entering a plant passes into leaf air spaces and then evaporates through the stomata into the atmosphere (Transpiration).
Usually less than 5% of
water escapes through the
cuticle.
A mature corn plant
transpires about
15 liters/week
1 acre field: 1,325,000
liters/100 day growth season
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19. Water Transport Theories
1682 Nehemiah Grew : Xylem Pumping: Water also raise in dead stems.
Marcello Malphigi: Capillary action: Altough 1 mt can be reached, capillary must be open ended.
Stephen Hales: Root pressure: Only up to 30 gr/cm2 which is not enough to carry water up to 100 mt trees.

20. Cohesion-Tension Theory or Transpirational Pull Theory
Stephen Hales (1727) In his book: Vegetable Statistics proposed the principles.
However, Hales' ideas were not understood at the time, so his findings failed to influence the debate on water transport in plants in the 19th century.
At the begining of the 20th century cohesion theory of water movement in plants has been ascribed to Josef Böhm, Henry H. Dixon and John Joly and Eugen Askenasy

21. Cohesion - Tension Theory
When the negatively charged end of one water molecule comes close to the positively charged end of another water molecule, weak hydrogen bonds hold the molecules together.
Water molecules adhering to capillary walls, and each other, create a certain amount of tension.

22. Cohesion - Tension Theory
When water transpires, the cells involved develop a lower water potential than the adjacent cells.
Creates tension on water columns, drawing water from one molecule to another, throughout the entire span of xylem cells.
Theory proposes that water is pulled up xylem by the surface tension generated at the interface between the atmosphere and water inside the leaf.
A steep water potential gradient is created when the stomatal pore opens and the humid leaf interior is exposed to the dry air.
Water exits the leaf and menisci form at the air-water interface.

23. Regulation of Transpiration:
Stomatal Conductance
The blue light at dawn is the signal that is recognized by a receptor on the guard cell.
The receptor signals the H+-ATPases on the guard cell’s plasma membrane to start pumping protons (H+) out of the guard cell. This loss of positive charge creates a negative charge in the cell.
Potassium ions (K+) enter the guard cell through channels in the membrane, moving toward its more negative interior.
As the potassium ions accumulate in the guard cell, the osmotic pressure is lowered.
A lower osmotic pressure attracts water to enter the cell.
As water enters the guard cell, its hydrostatic pressure increases.
The pressure causes the shape of the guard cells to change and a pore is formed, allowing gas exchange.

24. Regulation of Transpiration:
Factors
Stomata of most plants are open during the day and closed at night.
Stomata of many desert plants open only at night.
Conserves water, but makes carbon dioxide inaccessible during the day.
Humidity plays an inverse role in transpiration rates.
High humidity reduces transpiration, while low humidity accelerates it.

25. GUTTATION
If a cool night follows a warm, humid day, water droplets may be produced through hydathodes at the tips of veins of some plants.
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26. Measurements of water potential

27. Thermocouple
psychrometer

28. The pressure chamber

29. A pressure chamber measures the tension (Yp) in xylem
Air pressure in the chamber “squeezes” on the leaf tissues, forcing water out of leaves, into xylem, and out of the cut stem

30. To understand how to interpret measurements from pressure chambers you need to know:
FIRST:
a. Water in the xylem is under tension. The pressure potential is negative.
b. Solute potential in the xylem is usually close to zero
c. So – the total water potential of the xylem is approximately equal to the pressure potential:
Yw (xylem)  Yp (xylem)

31. Yw is the same everywhere
SECOND:
In equilibrium conditions, the total water potential of xylem (dead cells) is equal to the water potential of living cells surrounding it
Yw is the same everywhere

32. If the solute potential (Ys) of the xylem is
close to zero, then the pressure chamber
measures xylem water potential, which is in equilibrium with the water potential of leaf mesophyll cells:
Pchamber:
= - Yp (xylem)
- Yw (xylem)
- Yw (leaf mesophyll)

33. Transport of Organic Solutes in Solution
One of most important functions of water in the plant involves the translocation of food substances in solution by the phloem.
Most of our knowledge on this subject came from studying aphids feeding on phloem.

34. Pressure-Flow Hypothesis
Organic solutes flow from a source where water enters by osmosis.
Organic solutes are moved along concentration gradients between sources and sinks.
High sugar concentration in phloem sap at source leads to movement of water from xylem into phloem increasing turgor pressure.
Sugars are transported from source to sink along this turgor pressure
gradient in phloem.
Low sugar concentration in phloem sap at sink reduces turgor pressure and water moves from phloem sap to xylem.
Water flows in continuous loop driven by water potential
gradients between xylem and phloem.
Sugars move in one direction by bulk flow along turgor pressure gradient in phloem.

36. Sugar Loading and Unloading
Sucrose is transported into phloem cells at source against a concentration gradient and requires energy.
Two membrane proteins, a proton pump and a proton–sucrose cotransporter, are involved in phloem loading.

37. Plant Nutrition
Essential plant nutrient
An element functions in the metabolism of a plant and the plant cannot complete its life cycle without the element.

38. Macronutrients and Micronutrients
Macronutrients are used by plants in greater amounts.
Nitrogen, potassium, calcium, phosphorus, magnesium, and sulfur.
Micronutrients are needed by the plants in very small amounts.

39. Mineral Requirements for Growth
Essential Elements

40. Plant Nutrition Essential plant nutrients
Table 1-8 Textbook
Essential plant nutrients
Essential Element
Available Form(s)
Relative
Concentration (ppm)
Hydrogen
H2O
60,000,000
Oxygen
CO2 and H2O
30,000,000
Carbon
CO2
Nitrogen
NO3- and NH4+
1,000,000
Potassium K+
400,000
Calcium
Ca2+
200,000
Magnesium
Mg2+
100,000
Phosphorus
H2PO4- and HPO42-
30,000
Sulfur
SO42-
Chlorine
Cl-
3,000
Iron
Fe2+ and Fe3+
2,000
Boron
H3BO3
Manganese
Mn2+
1,000
Zinc
Zn2+
300
Copper
Cu+ and Cu2+
100
Molybdenum
MoO42- 1
C. H O P K i N S CaFe Mighty good CuZn, Burley Mnager, Motley Clerk

41. Plant Nutrition Nitrogen (N) 1 - 5% N 50-500 lb/A
adsorbed as both nitrate (NO3-) and ammonium (NH4+)
component of amino acids and proteins
component of nucleic acids (DNA and RNA)
component of chlorophyll
many enzymes contain N
continuously reused as proteins are broken down and resynthesized
mobile in the plant

42. Plant Nutrition Phosphorus (P) 0.1 -0.5% P 30 – 175 lb/A (P2O5)
adsorbed as H2PO4- and HPO42-
important in energy storage and transfer (ADP and ATP)
component of nucleic acids (DNA and RNA)
component of phosphoproteins and phospholipids
many enzymes contain P
important in root growth and seed (grain) production
mobile in the plant

43. Plant Nutrition Potassium (K) 0.5 – 6 % 50 – 500 lb/A adsorbed as K+
important in plant water uptake and balance through effect on osmotic potential
cation balance for anion transport
cofactor for many enzymes
used in many process such as synthesis of proteins, ATP and in photosynthesis
however not a constituent of any compounds
mobile in the plant

44. Plant Nutrition Calcium (Ca) 0.2 – 1% 10 -175 lb/A adsorbed as Ca2+
component of cell membranes and cell walls (calcium pectate)
important for nutrient uptake
important for cell elongation and division
cation balance for anion transport
immobile in the plant

45. Plant Nutrition Magnesium (Mg) 0.1 – 0.4 % 10 – 175 lb/A
adsorbed as Mg2+
component of chlorophyll
activates many enzymes
component of ribosomes thus important for protein synthesis
mobile in the plant

46. Plant Nutrition Sulfur (S) 0.1-0.5 %S 10 - 80 lb/A adsorbed as SO42-
component of amino acids (cysteine and methionine) and thus proteins
important in synthesis of vitamins, hormones, and other plant metabolites
component of glycocides which give odor to onions, mustard, etc.
Immobile in the plant

47. Plant Nutrition Boron (B) Monocots 6 - 18 ppm Dicots 20 – 60 ppm
adsorbed as boric acid (H3BO3) small amounts as dissociated ionic borates
involved in transport of sugars across cell membranes and in carbohydrate metabolism
important in cell development and elongation
important in nodulation in legumes
immobile in the plant

48. Plant Nutrition Iron (Fe) 10 – 1000 ppm
adsorbed as Fe2+ and Fe3+ may also be adsorbed as organically complexed Fe (chelates)
involved in redox reactions in cells
involved in photosynthesis
involved in chlorophyll and protein synthesis
important in respiratory enzymes
immobile in the plant

49. Plant Nutrition Manganese (Mn) 20 – 500 ppm adsorbed as Mn2+
involved in redox reactions in cells
involved in photosynthesis (formation of O2)
can substitute for magnesium in activating many enzymes
immobile in the plant

50. Plant Nutrition Copper (Cu) 5 – 20 ppm adsorbed as Cu2+
involved in redox reactions in cells
activates many enzymes
involved in cell wall formation
immobile in plants

51. Plant Nutrition Zinc (Zn) 25 – 150 ppm adsorbed as Zn2+
involved in enzyme synthesis and activation
component of auxin (growth regulator)
immobile in plants

52. Plant Nutrition Molybdenum (Mo) < 1 ppm adsorbed as MoO4 2-
component of enzymes systems important in the reduction of NO3 to NH4 and in N2 fixation by legumes
involved in Fe adsorption and transport
immobile in the plant

53. Plant Nutrition Chlorine (Cl) 0.2 – 2 % adsorbed as Cl-
involved in photosynthesis
plays role with K in the water balance of the plant
not a constituent of any compounds
may be important in disease resistance
mobile in the plant

54. Review
Molecular Movement
Diffusion
Osmosis
Water Movement
Cohesion-Tension Theory
Regulation of Transpiration
Transport of Organic Solutes
Pressure-Flow Hypothesis
Mineral Requirements for Growth

55. Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display