Chapter 4. Water Balance of Plants

Transcription

1 Chapter 4 Water Balance of Plants

2 Main driving forces for water flow from the soil through the plant to the atmosphere

3 Water flow in soil The rate of water flow in soil depends on The size of pressure gradient through the soil Soil hydraulic conductivity How easily the water moves through the soil Sandy soil > silt soil > clay soil Water content

4 Field capacity Low Medium High

5 Soil hydraulic conductivity (mh -1 MPa -1 ) For a clay soil (Soil space filled with water) Field capacity Soil water potential (MPa) More soil space filled with air Permanent wilting point for many species The decrease in soil hydraulic conductivity as the soil dries is due primarily to the movement of air into the soil to replace the water. As air moves in, the pathways for water flow between soil particles become smaller and more tortuous, and flow becomes more difficult.

6 Permanent wilting point At this point, plant can not regain turgor pressure even the transpiration stops. w < s

7 Field capacity The capacity of a soil to hold water The water content of a soil after it has been saturated with water and excess water has been allowed to drain away.

8 Concave menisci

9 Soil water potential w = s + p + g s is generally negligible because solute concentration is low ( ~ MPa). p is very close to 0 MPa in wet soil, but decrease as soil dried out.

10 Why p is negative in most soil? Soil dried out or water absorbed by plants Air space expand Because of adhesive force, water tends to cling to the surface of soil particles. Concave menisci developed => surface tension p = -2T/r T : surface tension of water = 7.28 X 10-8 MPa r : the radius of curvature of the menisci The radius of curvature of air-water surface become very small in drying soils.

11 Water flow in the soil Plants absorb water from the soil, and they deplete water near the surface of the roots. Ψ p decrease The depletion reduces p in the water near the root surface and establishes a pressure gradient with respect to neighboring region of soil that have higher p.

12 Water absorption by the root : Three pathways that water moves through the roots apoplast symplast transmembrane pathway

13 Suberin exists mainly in roots and contains very long chain (C20- C26) fatty acids and fatty alcohols and high phenol contents.

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15 Root hydraulic conductance Jv = Lroot X Δ w Jv : the rate of water flow Lroot : root hydraulic conductance Δ w : the radial water potential difference across the root Lroot is low when roots are subjected to low temp or anaerobic conditions, or treated with respiratory inhibitors; that is water uptake of roots decreases in these conditions.

16 Regulation of aquaporins

17 Regulation of aquaporins Permeability of aquaporins can be regulated in response to intracellular ph. Decreased rate of respiration leads to increases in intracellular ph. This increase in cytoplasmic ph alters the conductance of aquaporins involved in the movement of water across roots, resulting in roots that are less permeable to water. Closure in response to drought results from the dephosphorylation of two highly conserved serine residues, whereas closure during flooding results from the protonation of a conserved histidine.

18 Root pressure A positive hydrostatic pressure ( p ) builds in the xylem of roots. For example : The cut stump will exude sap from the cut xylem. Guttation, liquid droplets on the edges of their leaves. Most of the time, the transpiration rates are high, and water is taken up so rapidly into the leaves and lost to the atmosphere that a positive pressure never develops in the xylem. Root pressure usually occurs when soil water potentials are high and transpiration rates are low.

19 Dewdrops Transpiration is suppressed, and RH is high. Guttation in leaves from strawberry in the early morning. Leaves secrete water droplets through the hydathodes.

20 Root pressure A positive hydrostatic pressure ( p ) in the xylem of roots Mechanism : Root absorb ions from the dilute soil solution Transport ions into the xylem Build up the solute in the xylem sap Decrease the xylem osmotic potential ( s ) s = -RTC s Decrease the xylem water potential ( w ) C s increase => s decrease The lowering of xylem w provides a driving force for water absorption, which in turn leads to a positive hydrostatic pressure in the xylem.

21 Note Well hydrated plants, high humidity and less transpiration => root pressure increase (but usually less than 0.1 MPa) Dried condition, transpiration rate increase => negative hydrostatic pressure in xylem

22 Tracheary elements Tracheids angiosperms and gymnosperms Vessel elements angiosperms and a small group of gymnosperms (usually found in higher plants)

23 Preventing gas bubbles from spreading into neighboring tracheids Two primary walls & a middle lamella

24 Cavitation (or embolism) As the tension in water increase there is a tendency for air to be pulled through microscopic pores in the xylem cell walls and finally formed an air bubble => cavitation or embolism.

25 Water movement through the xylem vs. through living cells For trees with wide vessels (r = 100 ~ 200 m), velocity = m/h (=4 13 mm/s) Trees with smaller vessels (r = 25 ~ 75 m), velocity = 1 6 m/h (= mm/s) The driving force of water movement through the xylem is about 0.02 MPa/m. However, the driving force of water movement through a living cell (through membrane) is about 2 X 10 8 MPa/m.

26 High pressure difference is needed to lift water 100 meters to a treetop. A pressure difference of roughly 3 MPa from the base to the top branches, is needed to carry water to the tallest trees of 100 m. How are the water lifted to a treetop?

27 Cohesion-tension theory of sap ascent Because of the transpiration,the water at the top of a tree develops a large tension (a negative hydrostatic pressure), and this tension pulls water up the long water column in the xylem. However, as the tension in water increase there is an tendency for air to be pulled through microscopic pores in the xylem cell walls and finally formed a air bubble => cavitation or embolism.

28 Pushing on the plunger compresses the fluid, and a positive pressure builds up. => positive hydrostatic pressure ( 流體靜壓 ) Pulling on the plunger causes the fluid to develop a tension, or a negative pressure. => negative hydrostatic pressure. The hydrostatic pressure is positive in plant cells and is referred as turgor pressure ( p ).

29 Water evaporation in the leaf generates a negative pressure in the xylem which in turn causes water to move up through the xylem. Venation

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32 As water evaporates from the surface film that covers the cell walls of the mesophyll, water withdraws farther into the interstices of the cell wall, and surface tension causes a negative pressure in the liquid phase. Development of negative hydrostatic pressure at the surface of the cell walls in the leaf causes water to move up through the xylem.

33 Water movement from the leaf to the atmosphere

34 Transpiration diffusion of water vapor Diffusion is primary means of any further movement of the water out of the leaf. That is water movement is controlled by the concentration gradient of water vapor. Two factors determine transpiration : Difference in water vapor concentration (Cwv) Resistance of the pathway for vapor diffusion (Diffusional resistance) Leaf stomatal resistance (rs) Boundary layer resistance (rb)

35 E = Cwv (leaf) Cwv (air) rs + rb E : transpiration rate (mol/m 2 /s) rs : resistance at the stomatal pore (s/m) (leaf stomatal resistance) rb : resistance due to the boundary layer boundary layer : the layer of unstirred air at the leaf surface (boundary layer resistance) However, it is difficult to measure the Cwv (leaf). Sometimes vapor pressure are used instead of concentrations, and the difference is called water vapor pressure deficit. Water vapor pressure deficit = Pwv (leaf) - Pwv (air)

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37 Air boundary layer A thin film of still air on the surface of leaf and its resistance to water vapor diffusion is proportional to its thickness. The thickness of the boundary layer is determined primarily by wind speed.

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39 When air surrounding the leaf is still, increases in stomatal aperture have little effect on transpiration rate. The thickness of the boundary layer is the primary deterrent to water vapor loss from the leaf. When wind velocity is high, the stomatal resistance has the largest amount of control over water loss.

40 Relative humidity (RH) RH = Cwv / Cwv (sat.) The Cwv (sat.) is strongly dependent on temp.

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43 Water loss Water loss is regulated by : Difference in water vapor concentration The pathway resistance

44 The concentration gradient for CO 2 uptake is smaller than the concentration gradient driving water loss. How can plant prevent water loss without simultaneously excluding CO 2 uptake? Temporal regulation When water is abundant : At night, no photosynthesis Stomata close, preventing unnecessary loss of water. Sunny morning, photosynthesis is demanding, supply of water is abundant. Stomata are wide open, decreasing the stomatal resistance to CO 2 diffusion. When soil water is less abundant The stomata will open less or even remain closed on a sunny morning.

45 Transpiration ratio A measure of the relationship between water loss and carbon gain Transpiration ratio = moles of H 2 O transpired moles of CO 2 fixed The factors that cause large ratio of H 2 O efflux to CO 2 influx : The concentration gradient driving water loss is about 50 times larger than driving CO 2 influx. CO 2 diffuse slower than water does. MW of CO 2 > MW of H 2 O CO 2 has longer diffusion path membrane -> cytoplasm -> chloroplast envelop -> chloroplast

46 Stomata In dicot and non grass monocot Usually no subsidiary cell Kidney-shaped guard cells Grass stomatal complex pore

47 Oat 35 34

48 Ventral wall (thicker ~ 5mm) dorsal wall (thinner)

49 Ion and organic molecules Guard cell s Decrease ( s = -RTCs) w decrease Water moves into the guard cell p increase Stoma open

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51 END

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