Chapter 34

1. Chapter 34 Roots and Mineral Nutrition

2. Taproot system has • One main root formed from the radicle • Multiple lateral roots extending from the main root

3. Fibrous root system has • Multiple adventitious roots of the same size (developing from the end of the stem) • Multiple lateral roots branching off from the adventitious roots

4. Taproot system Fibrous root system

5. LM of a lateral root Lateral roots originate at the pericycle

6. Primary roots have • An epidermis • Ground tissues • Cortex and sometimes pith • Vascular tissues • Xylem and phloem

7. Ranunculus root Cortex comprises the bulk of herbaceous dicot roots; note X-shaped xylem

8. Closeup of the root’s stele

9. Primary roots, cont. • Each root tip is covered by a root cap whose function is to • Protect the delicate root apical meristem • Orient the root so that it grows downward

10. LM of a Quercus sp. root tip showing its root cap. The root apical meristem is protected by the root cap

11. Primary roots, cont. • Epidermis protects the root • Root hairs are short-lived extensions of epidermal cells • Increase surface area of root in contact with soil • Aid in absorption of water and dissolved nutrient minerals

12. Root hairs on a radish seedling(length approx. 5 cm)

13. Cortex consists of parenchyma cells that often store starch • Endodermis • Is innermost layer of cortex • Regulates movement of nutrient minerals into root xylem

14. Endodermal cells • Have a Casparian strip around their radial and transverse walls • Strip is impermeable to water and dissolved nutrient minerals. • Have carrier proteins in their plasma membranes • Proteins actively transport nutrient minerals

15. Endodermis and nutrient mineral uptake

16. Root’s stele, or vascular cylinder, consists of • Pericycle • Xylem • Phloem

17. Pericycle gives rise to • Lateral roots • Lateral meristems • Xylem conducts • Water • Dissolved nutrient minerals • Phloem conducts • Dissolved sugar

18. Monocot roots • Often consist of pith surrounded by a ring of alternating bundles of • Xylem • Phloem • Lack a vascular cambium and therefore do not have secondary growth

19. LM of a cross section of a monocot root (Smilax)

20. Herbaceous dicot roots • Xylem forms a solid core in the centre of the root

21. Pathway of water and nutrient mineral ions • Root hair/epidermis • Cortex • Endodermis • Pericycle • Root xylem

22. Water and dissolved nutrient minerals move through epidermis and cortex along • Either the apoplast (along interconnected porous cell walls) • Or the symplast (from one cell’s cytoplasm to the next through plasmodesmata)

23. Pathways of water and dissolved nutrient minerals in the root

24. Roots of gymnosperms and woody dicots develop secondary tissues • Production of these is result of • Activity of two lateral meristems • Vascular cambium • Cork cambium

25. Vascular cambium produces • Secondary xylem (wood) • Secondary phloem (inner bark) • Cork cambium produces • Periderm (outer bark)

26. The tissues in a primary root At onset of secondary growth, vascular cambium extends out to pericycle, forming continuous, non-circular loop

27. Vascular cambium produces secondary xylem to its inside and secondary phloem to its outside Ring of vascular cambium gradually becomes circular

28. Prop roots • Develop either from branches or from a vertical stem • Grow downwards into the soil to help support certain plants in an upright position

29. Buttress roots • Have swollen bases or braces • Support certain tropical rainforest trees that have shallow root systems

30. Pneumatophores • Aerial “breathing” roots • May assist in getting oxygen to submerged roots

31. Some epiphytes • Have roots modified to photosynthesize • Absorb moisture • Or, if parasitic, penetrate host tissues

32. Corms and bulbs • Often have contractile roots • These grow into the soil and then contract • Corm or bulb is thereby pulled deeper into the soil

33. Mutualistic relationships between roots and other organisms • Mycorrhizae • Root nodules • Root grafts

34. Mycorrhizae • Mutually beneficial associations between • Roots • Soil fungi

35. LM of ecto-mycorrhizae

36. LM of endo-mycorrhizae

37. Root nodules • Swellings that develop on roots of leguminous plants • Swellings house millions of rhizobia (nitrogen-fixing bacteria)

38. Root graft • Natural connection between roots of trees belonging to the same or different species

39. Factors influencing soil formation include • Parent material • Climate • Organisms • Passage of time • Topography

40. Most soils are formed from parent material that is broken into smaller and smaller particles by weathering processes • Climate and organisms work together in weathering rock

41. Important in forming soil and in cycling nutrient materials are organisms such as • Plants • Algae • Fungi • Worms • Insects • Spiders • Bacteria

42. Topography affects soil formation • Steep slopes have little or no soil on them • Moderate slopes often have deep soils

43. Soil is composed of • Inorganic minerals • Organic matter • Air • Water

44. Inorganic minerals provide plants with • Anchorage • Essential nutrient minerals • Organic matter • Increases soil’s water-holding capacity • Releases essential nutrient minerals into the soil as it decomposes

45. Soil air provides oxygen for soil organisms to use during aerobic respiration • Soil water provides water and dissolved nutrient minerals to plants and other organisms

46. Cation exchange • Cations are attracted and reversibly bound to clay particles • Clay particles have predominantly negative charges on their outer surfaces • Roots secrete protons (H+)

47. Cation exchange, cont. • Protons are exchanged for other positively charged mineral ions • Mineral ions are freed into the soil water to be absorbed by roots

48. Cation exchange

49. How acid alters soil chemistry

50. Carbon Oxygen Potassium Magnesium Sulfur Hydrogen Nitrogen Calcium Phosphorus Silicon Ten of the nineteen essential elements plants require are macronutrients (required in quantity)