1. Plant Nutrition and Transport25
Plant Nutrition and Transport

2. Chapter 25 Plant Nutrition and Transport
Key Concepts
25.1 Plants Acquire Mineral Nutrients from the Soil
25.2 Soil Organisms Contribute to Plant Nutrition
25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion– Tension
25.4 Solutes Are Transported in the Phloem by Pressure Flow

3. Chapter 25 Opening Question
How can soil be managed for optimal plant growth?

4. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Plants are autotrophs.
They get carbon from atmospheric CO2; hydrogen and oxygen mostly from water.
Nitrogen comes, directly or indirectly, from the activities of bacteria.
Phosphorus, sulfur and other mineral nutrients come from the soil solution that surrounds plant roots.
LINKS Chapter 2 The major elements of life are described; Concepts 6.5 and 6.6 The chemical reactions of photosynthesis

5. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Essential elements—their absence severely disrupts plant growth and reproduction.
• Macronutrients—at least 1 g/kg of the plant’s dry matter: N, P, K, S, Ca, Mg
• Micronutrients—less than 0.1 g/kg of the plant’s dry matter: Fe, Cl, Mn, Zn, Cu, Ni, Mo
Characteristic symptoms can be used to diagnose deficiencies of essential elements, and they can be provided by fertilizer.
ANIMATED TUTORIAL 25.1 Nitrogen and Iron Deficiencies

6. Figure 25.1 Mineral Nutrient Deficiency Symptoms

7. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
The essential elements were identified by growing plants hydroponically—with roots in nutrient solutions instead of soil.

8. In-Text Art, Ch. 25, p. 522

9. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Identifying micronutrients is more difficult because of the small amounts involved.
Controlled laboratories with special air filters and the purest chemicals are required.
A seed may contain enough of a micronutrient to supply the plant throughout its lifetime.

10. Figure 25.2 Nickel Is an Essential Element for Plants (Part 1)

11. Figure 25.2 Nickel Is an Essential Element for Plants (Part 2)

12. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Soils provide:
• Anchorage for mechanical support
• Mineral nutrients and water from the soil solution
• O2 for root respiration from air spaces between soil particles
• The services of soil organisms: bacteria, fungi, protists, earthworms, arthropods

13. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Soils develop a soil profile of horizontal layers, called horizons.
A horizon: topsoil; contains most of the soil’s living and dead organic matter
B horizon: subsoil; accumulates materials from the topsoil and the parent rock
• C horizon: parent rock from which the soil arises

14. Figure A Soil Profile

15. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Soil fertility (ability to support plant growth) is determined by several factors:
Particle size affects leaching—mineral elements are washed from the A horizon and become unavailable to plants.
Clay particles bind water; larger sand particles have a lot of air space.
Loam—soil with sand, silt, and clay, and sufficient air, water, and nutrients for plants.

16. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Humus: organic matter in soil.
Also improves soil texture and provides air spaces that increase oxygen availability to plant roots.

17. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Negatively charged clay particles bind the cations of many important minerals (K+, Mg2+, and Ca2+).
This makes them unavailable for plants uptake.
Ion exchange releases mineral nutrient cations into the soil solution and makes them available.

18. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Ion exchange:
Root hair cells actively pump protons (H+) out of the cells, and cellular respiration releases CO2.
CO2 dissolves in soil water forms carbonic acid, which ionizes:
Clay has more affinity for H+ than cations, so, protons change places with the cations.

19. Figure Ion Exchange

20. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
There is no mechanism for binding and releasing negatively charged ions.
Important anions such as nitrate (NO3–) and sulfate (SO42–) may be leached from the A horizon.

21. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Crop harvesting and leaching can deplete soil nutrients. Three ways to replenish:
Shifting agriculture: when soil is depleted, people move to another location and natural processes replenish soil.
Organic fertilizers: humus is used as a food source by soil organisms, which release simpler molecules to the soil solution.

22. Concept 25.1 Plants Acquire Mineral Nutrients from the Soil
Chemical fertilizers: supply mineral nutrients directly in forms that are easily used.
Characterized by their “N-P-K” percentages
Produced in manufacturing processes that require a lot of energy.

23. Concept 25.2 Soil Organisms Contribute to Plant Nutrition
One gram of soil contains 6,000 to 50,000 bacterial species and up to 200 meters of fungal hyphae!
Many consume dead and living plant material.

24. Concept 25.2 Soil Organisms Contribute to Plant Nutrition
Some plants have symbiotic relationships with bacteria and fungi:
Mycorrhizal fungi
Nitrogen-fixing rhizobia bacteria

25. Concept 25.2 Soil Organisms Contribute to Plant Nutrition
Formation of arbuscular mycorrhizae:
Roots produce strigolactones that stimulate growth of fungal hyphae toward the root.
The fungi produce signals that stimulate expression of plant symbiosis-related genes.
Fungal hyphae grow into roots and form arbuscules, where nutrients are exchanged.

26. Figure 25.5 Roots Send Signals for Colonization (Part 1)

27. Figure 25.5 Roots Send Signals for Colonization (Part 2)

28. Concept 25.2 Soil Organisms Contribute to Plant Nutrition
Formation of nitrogen-fixing nodules:
Legume plant roots release chemical signals that attract rhizobia.
They also trigger transcription of bacterial nod genes.
Bacteria produce Nod factors, which cause root cortex cells to divide and form the nodule.
Bacteria enter the nodule cells and differentiate into bacteroids that can fix nitrogen.

29. Figure 25.5 Roots Send Signals for Colonization (Part 3)

30. Concept 25.2 Soil Organisms Contribute to Plant Nutrition
Mycorrhizae expand root surface area 10- to 1,000-fold, increasing the amount of soil the plant can explore for nutrients.
In most cases, roots alone can not optimally support vascular plant growth.
The primary nutrient obtained by mycorrhizae is phosphorus. The fungus obtains carbohydrates from the plant’s photosynthesis.
APPLY THE CONCEPT Soil organisms contribute to plant nutrition

31. Concept 25.2 Soil Organisms Contribute to Plant Nutrition
Plants cannot use atmospheric N2.
But some bacteria have nitrogenase to convert N2 to NH3 (nitrogen fixation).
Fixation is a reduction reaction and requires lots of energy.
LINKS Chapter 46 The cycling of nitrogen through biological systems; Concept 19.3 Some of the prokaryotes that participate in that cycling

32. Figure 25.6 Nitrogenase Fixes Nitrogen

33. Concept 25.2 Soil Organisms Contribute to Plant Nutrition
Nitrogenase is inhibited by O2.
Free-living N-fixing bacteria are anaerobes.
Plant root nodule environment provides enough O2 for aerobic respiration but not so much to inhibit nitrogenase.
O2 level is regulated by leghemoglobin, an O2 carrier.

34. Concept 25.2 Soil Organisms Contribute to Plant Nutrition
Carnivorous plants obtain some nutrients by digesting arthropods.
The plants live in boggy, nutrient-poor soils. Digestion of arthropods provides nitrogen and phosphorus to the plant.
VIDEO 25.1 The bladderwort, Utricularia intermedia, traps mosquito larvae
VIDEO 25.2 The round-leaf sundew, Drosera sp., traps an insect
VIDEO 25.3 The Venus flytrap, Dionaea

35. Figure 25.7 Nutrients from Other Organisms (Part 1)

36. Concept 25.2 Soil Organisms Contribute to Plant Nutrition
Parasitic plants
Hemiparasites can photosynthesize, but get water and mineral nutrients from living plants (e.g., mistletoe).
Holoparasites—completely parasitic; no photosynthesis. Witchweed (Striga) is a serious parasite of crops in Africa.

37. Figure 25.7 Nutrients from Other Organisms (Part 2)

38. Root cells take up water by osmosis.
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Root cells take up water by osmosis.
Water potential (psi, Ψ): tendency of a solution to take up water from pure water across a membrane.
Water potential of pure water is zero.
The more negative the water potential, the greater the driving force for water movement across the membrane.

39. Water potential has two components:
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Water potential has two components:
Solute potential (Ψs): (usually negative) solutes reduce concentration of free water; the more solutes, the lower the water potential— increases the tendency of cell to take up water.
Pressure potential (Ψp): as plant cells take up water, they swell; but cell wall resists swelling and results in turgor pressure, which decreases tendency of cell to take up water.

40. Water potential is the sum of solute and pressure potentials:
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Water potential is the sum of solute and pressure potentials:
Water moves across a membrane toward a region of lower (more negative) water potential.

41. Figure 25.8 Water Potential, Solute Potential, and Pressure Potential (Part 1)

42. If pressure potential drops, the plant wilts.
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Water will enter plant cells by osmosis until pressure potential balances solute potential.
Physical structure of many plants is maintained by turgor pressure (positive pressure potential).
If pressure potential drops, the plant wilts.
INTERACTIVE TUTORIAL 25.1 Water Uptake in Plants

43. Figure 25.8 Water Potential, Solute Potential, and Pressure Potential (Part 2)

44. Figure A Wilted Plant

45. Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Membrane proteins move water and nutrient ions across root cell plasma membranes:
Aquaporins: water can diffuse through toward region of lower water potential.
LINK Chapter 5 Review structure and functions of biological membranes

46. Creates an electrical and proton gradient.
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Proton pump—uses energy from ATP to move protons out of the cell against a proton concentration gradient.
Creates an electrical and proton gradient.
Cations (e.g., K+) move into the cell by facilitated diffusion through specific membrane channels.
Proton gradient can drive active transport of anions (e.g., Cl–).
LINK Concept 34.2 Electrochemical gradients are discussed in more detail

47. Figure 25.10 Ion Transport into Plant Cells

48. Water and ions move through roots to the xylem by one of two pathways:
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Water and ions move through roots to the xylem by one of two pathways:
Apoplast—cell walls and intercellular spaces; a continuous meshwork; water and solutes never cross a membrane.
Symplast—continuous cytoplasm of living cells connected by plasmodesmata. Plasma membranes control movement.

49. Figure 25.11 Apoplast and Symplast (Part 1)

50. Figure 25.11 Apoplast and Symplast (Part 2)

51. Water in apoplast can travel to endodermis.
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Water in apoplast can travel to endodermis.
Casparian strip forces water and ions to enter cytoplasm of endodermal cells.
Mineral ions are actively transported into the apoplast of the stele. As water potential becomes more negative, water moves by osmosis.
Water and minerals end up in the xylem.

52. How does xylem move water to the tops of trees?
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
How does xylem move water to the tops of trees?
Transpiration–cohesion–tension theory:
Transpiration—evaporation of water from cells in the leaves
• Cohesion of water molecules in the xylem sap due to hydrogen bonding
• Tension on the xylem sap resulting from transpiration
LINK Chapter 2 Review the properties of water

53. Figure 25.12 The Transpiration–Cohesion–Tension Mechanism

54. Transpiration—water diffuses out of leaf through stomata.
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Transpiration—water diffuses out of leaf through stomata.
Water evaporates from moist walls of mesophyll cells, which generates tension on water in the xylem.
Cohesion between water molecules forms a continuous water column from roots to leaves.
Water enters the root and moves into the xylem by osmosis.
APPLY THE CONCEPT Water and solutes are transported in the xylem by transpiration–cohesion–tension

55. Transpiration also cools the leaves.
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Transpiration also cools the leaves.
Evaporation of water from mesophyll cells consumes heat, decreasing the leaf temperature.
ANIMATED TUTORIAL 25.2 Xylem Transport

56. Transpiration also results in water loss.
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Transpiration also results in water loss.
Leaf cuticle reduces water loss, but also prevents gas exchange.
Stomata, or pores on the undersides of leaves, regulate gas exchange—they can open or close by the action of the guard cells.
VIDEO 25.4 Cell walls and stomatal complexes in Tradescantia
VIDEO 25.5 Stomatal complexes forming

57. During day, stomata open to allow CO2 to enter.
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
During day, stomata open to allow CO2 to enter.
At night, stomata close to conserve water (may also close in daytime if water loss is too great).

58. Guard cells respond quickly to light.
Concept 25.3 Water and Solutes Are Transported in the Xylem by Transpiration–Cohesion–Tension
Guard cells respond quickly to light.
Light absorbed by guard cell pigments activates a proton pump. H+ is actively transported out of cells.
Resulting electrochemical gradient drives K+ and Cl– into the guard cells, increasing solute concentration, and water enters by osmosis.
Increased turgor pressure in cells stretches them and open the stomata.

59. Figure 25.13 Stomata Regulate Gas Exchange and Transpiration (Part 1)

60. Figure 25.13 Stomata Regulate Gas Exchange and Transpiration (Part 2)

61. Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow
Translocation: movement of solutes through plant in the phloem, from sources to sinks.
Source: organ that produces or stores photosynthate—leaves, storage root, etc.
Sink: consumes photosynthate for growth and storage—roots, flowers, fruit.

62. Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow
The role of phloem was investigated in the 1600s:

63. Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow
Phloem contents can be analyzed using aphids—insects that drill into sieve tube elements with a stylet.
LINK Concept 24.1 The anatomy of sieve tubes

64. Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow
Pressure potential in the sieve tube forces phloem sap through the stylet and into the aphid’s digestive tract.
The aphids can be frozen and removed, leaving the stylet in place and sap will continue to flow out the stylet.

65. Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow
Observations from aphid experiments:
Most of phloem is sucrose.
Flow rate can be very high.
Different sieve tubes conduct in different directions.
Movement of phloem sap requires living cells.

66. Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow
Pressure flow model:
At a source, sucrose is actively transported into companion cells; it flows through plasmodesmata into sieve tube elements.
Higher sucrose concentration than surrounding cells—water enters sieve tube elements from xylem by osmosis.
This increases turgor pressure which pushes sieve tube contents towards the sink.
ANIMATED TUTORIAL 25.3 The Pressure Flow Model

67. Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow
At the sink, sucrose moves out and water moves back to the xylem.
The gradient of solute potential and pressure potential needed for movement of phloem sap (translocation) is maintained.

68. Figure 25.14 The Pressure Flow Model

69. Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow
Loading—transport of solutes from sources into sieve tubes.
Unloading—transport of solutes from sieve tubes into sinks.
Both require energy; requires living cells.

70. Concept 25.4 Solutes Are Transported in the Phloem by Pressure Flow
Solutes can move from mesophyll cells to phloem by apoplastic or symplastic pathways.
If solutes enter the apoplast, specific molecules are actively transported into cells of the phloem and reenter the symplast.
In a symplastic pathway, solutes remain in the symplast at all times. No membranes are crossed; no membrane transport involved.

71. Answer to Opening Question
Sustainable farming methods maintain the structure, nutrient content, and water-holding capacity of soil.
Crop rotation: different crops have different nutrient requirements.
Conservation tillage: contour plowing reduces erosion runoff; crop residues are left on the soil or plowed back in.

72. Figure 25.15 Conservation Tillage