BIO2202 Plant Physiology 1 Water Relations Lecture 4

1
BIO2202 Plant Physiology 1 Water Relations -
Lecture 4

• Transpiration the Vascular System

2
Uptake of Water from the Soil

• Root hairs main point of water uptake due to
  large surface area contact.

TZ Fig. 4.2
3
T Z Figure 4.3
4
Uptake of Water from the Soil

• Water flows across root epidermis, cortex,
  endodermis, pericycle and stele into the xylem
• Two major routes (Fig. 4.4
• (i) Apoplast
• (ii) Symplast plasmodesmata, casparian strip

5
T Z Figure 4.7a, b Vessels 40-500um diam.
x many metres long Tracheids 30um x 5mm
6
Characteristics of Xylem Fluid

• Contains dilute salt solution -0.2 to -0.1 MPa
• Flow rates 1 to 45 m.h-1, max transpiration in
  trees
• potential gradient of 3MPa sufficient to lift
  xylem fluid 100m.

7
Characteristics of Xylem Conducting Elements

• Dead tissue at maturity - no cytoplasm
• Strong hydrophilic 2o wall thickenings
• Low flow resistance, no membranes in pathway.
• Narrowness exploits adhesive cohesive forces.
  Balances flow rate against cavitation.

8
Poiseuilles Equation

• Volume of flow rate ?r4 . ??P
• 8? ?x
• r radius
• ? viscosity

9

• Cavitation is minimised by
• pitted end walls
• pitted pores in walls
• flow around air bubbles
• redissolving of gas bubbles
• growth of new wood

T Z Figure 4.8
TZ Fig 4.7c
10
Water Movement through the Leaf

• Every leaf cell is within 2-3 cells of a vein or
  vein ending supplying water mineral nutrients
• Cell walls strongly hydrophilic
• Saturated cell walls ensure continuous connection
  from vein endings to substomatal air spaces.
• Water transpires from these cavities through the
  stomates to the surrounding air

11
Effects of Transpiration in the Leaf

• Loss of water vapour decreases humidity in leaf
  air spaces below 100
• Water from the saturated cell walls enters vapour
  phase
• Lowering of cell wall matric potential draws
  water from the veins
• Loss from the veins creates a tension in the
  xylem water columns
• This tension draws water into the roots from the
  soil

12
Ficks Law

• Js - Ds.?Cs
• ?x
• Js flux density i.e. rate of movement
• across a fixed unit of area
• Ds diffusion coefficient
• t C½ (distance)2 x K x D

13
T Z Fig. 4.11
14
Factors Governing Leaf Water Loss

• Stomatal conductance
• boundary layer resistance
• temperature
• relative humidity of exterior air

15
T Z Fig. 4.13
16
TZ Fig.4.12
17
(No Transcript)
18
(No Transcript)
19
T Z Figure 4.17