Cellulose, wood structure and water movement in the plant

1. Cellulose, wood structure and water movement in the plant Objectives of the lecture: 1. Describe how secondary growth takes place and results in radial growth of stems and roots. 2. Define the principal cell components of secondary growth. 3. Describe the Transpiration-Cohesion-Adhesion Theory for water movement in plants. Text book pages: 87, 89-90, 152-153, 807-810, 813-828.

2. Stem thickening secondary xylem Bark cork cambium vascular cambium secondary phloem secondary phloem thickening Lateral meristems Two lateral meristems in older stems and roots of woody plants produce secondary growth that results in an increase in diameter (whereas apical meristems result in an increase in length). Vascular cambiumsecondary vascular tissues, xylemphloem Cork cambiumperiderm (continually replaces the epidermis), bark tissue

3. Secondary growth L4 S12

4. Figure 36-28 Lateral meristems increase the width of stems and roots. Lateral meristems (cork cambium and vascular cambium) produce bark and wood. Cork Cross section of young Linden tree Periderm Fig 36.28 Cork cambium Cork cambium adds cells primarily to the outside Phelloderm 3 year Bark Secondary phloem 2 year Vascular cambium Vascular cambium adds many cells to the inside and some cells to the outside Secondary xylem Wood Rays of parechyma cells 1 year

5. Cambium cells in yellow outer surface of stem or root differentiation division division differentiation differentiation Phloem production division division division differentiation differentiation Xylem production direction of radial growth Further cell differentiation The cambium is usually a few cells thick during periods of active growth After division then cells differentiate into specialist cells of the tissue

6. The vascular cambium produces xylem and phloem pit in cell wall one vessel member sieve plate Conifers Angiosperms Sugar transport cytoplasm absent (cells dead at maturity sieve-tube member companion cell (living) Water conduction Phloem Xylem PHLOEM SIEVE TUBE TRACHEIDS VESSEL

7. Cambium and secondary xylem of a conifer Direction of growth Late wood Early wood Cambium Ray initials Rays Tracheids with bordered pits Cambium and secondary xylem of a conifer Recall how the cambium produces xylem cells. This drawing shows what those cells differentiate to. Cambium and secondary xylem of a conifer

8. Taking sections through a stem Transverse Section Radial Longitudinal Section Tangential Longitudinal Section L4 S13

9. Transverse section of Quercus rubra wood vessel EARLY WOOD LATE WOOD toward stem surface

10. Secondary xylem is a complex tissue It conducts water, stores water, and provides support In conifers it consists of: tracheids that are long in the vertical direction, and have bordered pits that can shut when a tracheid collapses under low water potential. Tracheids can be re-saturated with water ray cells running horizontally through the xylem and are composed mainly of parenchyma and some tracheids. In angiosperms vessel elements are an additional cell type: Vessel elements are short but much wider compared to tracheids and are stacked on top of each other to form a vessel

11. Fig. 36-26a Tracheids are spindle shaped and have pits. Face view Side view Longitudinal section Pits Pits The torus at the center of the bordered pit moves and seals the pit when a tracheid aspirates

12. Tracheids and rays, pine Tangential Longitudinal Section Pits

13. Figure 36-26b Vessel elements are short and wide and have perforations as well as pits. This vessel element has a completely perforated end wall Perforations Pits Elongated vessel element provides moderate support but superior fluid conduction compared to a tracheid Longitudinal section

14. Figure 36-27 Cross section Longitudinal section Sieve-tube members Sieve-tube members Companion cells Companion cells Cross section Sieve plate Sieve plate Sieve-tube members and companion cells develop from the same progenitor cell. Sieve-tube members unite vertically to form a sieve tube. Sieve-tube members have no nucleus at maturity and depend on companion cells to regulate physiological processes.

15. Table 36-2

16. Water movement in plants Water moves along potential energy gradients—from areas of high water potential to areas of low water potential. What happens between living cells and their environment? • Water’s potential energy in plants is a combination of • 1. its tendency to move in response to differences in solute concentration • 2. the pressureexerted on it. Osmosis Turgor pressure What happens outside of the living cell? Plants do not expend energy to replace water lost in transpiration. Water moves from soil and roots to leaves along a water potential gradient. The gradient exists because water at the air-water surface in leaves is under negative pressure (tension) great enough to pull water up from the roots through xylem. The Cohesion-Tension Theory

17. What happens between living cells and their environment? Solute potential is the tendency of water to move by osmosis. Cell is placed in pure water. Its solute potential is low relative to its surroundings. Water moves into cell via osmosis. Solute potential inside cell and in surrounding solution is the same. No net movement of water. Cell Pure water Solute Fig 37.1a Water movement Hypotonic solution Isotonic solution Osmosis is the spontaneous net movement of water across a semi-permeable membrane from a region of high solute concentration to a solution with a low solute concentration, down a solute concentration gradient. Osmosis is a selective diffusion process driven by the internal energy of the solvent molecules.

18. Pressure potential is the tendency of water to move in response to pressure. Inside of cell Turgor pressure is an important source of pressure on water in cells Expanding volume of cell pushes membrane out. Turgor pressure Plasma membrane Fig 37.1b Cell wall Wall pressure Stiff cell wall pushes back with equal and opposite force. Outside of cell

19. What happens outside of the living cell? The text book refers to “The Cohesion-Tension Theory” A more descriptive term is: TRANSPIRATION-COHESION-ADHESION THEORY: WATER POTENTIAL: Xylem sap rises against gravity, driven by a gradient of water potential. Water flows from an area of high potential to an area of low potential. Water Potential is expressed in units of pressure: 1 bar is the pressure needed to push up a column of water 10 meters. 1 megapascal = 10 bars. Pure water has a potential of 0. Addition of pressure increases water potential. Placing water under tension decreases water potential. Addition of solutes decreases water potential. The crucial question is how is a gradient water potential maintained? It is maintained by creation and maintenance of a gradient of tension.

20. Hydrogen Bonds and Cohesion Water molecules have weak negative charges at the oxygen atom and positive charges at the hydrogen atoms. H H O + H O H H H O O H H H O H Positive and negative regions are attracted. The force of attraction, dotted line, is called a hydrogen bond. Each water molecule is hydrogen bonded to four other water molecules – the force of Cohesion. The hydrogen bond has ~ 5% of the strength of a covalent bond. However, when many hydrogen bonds form, the resulting Cohesion is sufficiently strong as to be quite stable. Adhesion is the tendency of molecules of differentkinds to stick together – by a similar process. Water sticks to cellulose molecules in the walls of the xylem, counteracting the force of gravity. It is the fine structure of cellulose in tracheids and vessels that enables tension to be maintained. http://www.ultranet.com/~jkimball/BiologyPages/H/HydrogenBonds.html

21. Table 5-1

22. A water potential gradient exists between the soil, plants, and the atmosphere. Plants tend to gain water from the soil and lose it to the atmosphere. Low water potential Atmosphere : –95.2 MPa (Changes with humidity; usually very low) Fig 37.4 a). water exits leaf through stomata. b). this water is replaced by evaporation from mesophyll cells, lowering their water potential, causing them to take water from neighboring cells. Leaf : –0.8 MPa (Depends on transpiration rate; low when stomata are open) c). the process connects back to the tracheids and/or vessels causing water to be taken from the xylem. d).Water travels from the tracheids to the air following a water potential gradient. e). The cohesive and adhesive properties of water and the small diameter of xylem aid in its vertical movement. Root : –0.6 MPa (Medium-high) Soil : –0.3 MPa (High if moist; low if extremely dry) f). This pull decreases water potential in the xylem causing the roots to take water from the soil. High water potential

23. Measuring water potential The pressure bomb! See Fig 37.14 Compressed air

24. Field measurements of shoot Forest laboratory in south west Scotland Measurement every hour for 7 days

25. Diurnal pattern of shoot water potential Midnight Midday 500 400 300 200 100 Shoot water 0 potential -1 MPa -2 30 Jul 31 Jul 1 Aug 2 Aug 3 Aug 4 Aug 5 Aug 6 Aug Transpiration measured by eddy correlation mg/sec/tree  During daylight water loss from foliage exceeds water gain from soil so shoot water potential decreases. On sunny days  reaches –2 Mpa

26. Cessation of physiological processes: Cell growth and wall synthesis are very sensitive and may stop at -0.5 MPa Photosynthesis, respiration and sugar accumulation are less sensitive. They may be affected between -1 and -2 MPa

27. Stomatal control clip L11 S10

28. Things you need to know ... 1. How secondary cambia produce wood and bark. 2. The cell types found in conifer and angiosperm wood and how differences between them reflect the ecology of these two groups of plants. 3. The structure of phloem. 4. How water moves in plants from roots to leaves including typical water potential values: the transpiration-cohesion-adhesion theory.