BIO2202 Plant Physiology 1 Water Relations Lecture 5

1
BIO2202 Plant Physiology 1 Water Relations -
Lecture 5

• Stomata and the Leaf Epidermis

2
Occurence

• Appeared during the Devonian period 390 mill.
  years ago.
• Co-evolved with cuticle and vascular system.
• Occur in all higher plants above Bryophytes.
• Mainly on leaves green stems. Also flowers,
  fruits, pods.
• In dicots scattered randomly across the
  epidermis.
• In monocots and Gymnosperms typically in rows
  along long axis of leaf

3
Anatomy of the Epidermis

• The shoot system of vascular plants is covered by
  a waxy cuticle. What is its function?
• Reduces water loss
• Repels entry by hydrophillic substances
• Important deterent to pathogenic attack
• Thickness is responsive to environmental
  conditions

4
Hydrophobic Structural Components of Plants

• Cutin
• Makes up the bulk of the cuticular material.
• Long chain (16-18C) saturated hydroxy fatty
  acids.
• Esterified to each other to form a 3-D network.
• Waxes
• Long single chain alkanes and saturated fatty
  acid esters.
• Not cross-linked.

5
T Z Figure 13.1
6
Hydrophobic Structural Components of Plants

• Suberin
• polymer, contains dicarboxylic acids,
• components have longer chain lengths than in
  cutin and some phenolic groups (esp.ferulic
  acid).
• Generally not found in the cuticle, but a major
  component of the surface layer of underground
  plant parts, the periderm of bark, the Casparian
  strip of the root endodermis.
• Suberin formation is a frequent response to
  stress or physiological factors which require
  erection of a diffusion barrier, e.g. leaf
  abscission sites and wound sealing.

7
TZ Fig 13.2
8
Cell Types of the Leaf Surface

• Epidermal cells
• Typically large and highly vacuolated, /-
  chloroplasts.
• Often contain crystals of calcium oxalate.
• Cuticularised outer walls prevent water loss and
  act as a barrier to resist attack by insects and
  pathogens.

9
Cell Types of the Leaf Surface

• Trichomes
• Single or multicelled adaptations of epidermal
  tissue.
• Form hairs, scales, secretory glands.
• May ? H2O loss by ? boundary layer resistance.
• May excrete unwanted products e.g. salt in
  halophytes.
• Frequently exhibit high metabolic activity.

10
Cell Types of the Leaf Surface

• Subsidiary (accessory) cells
• Smaller, specialised epidermal cells lying
  adjacent to stomatal guard cells.
• Usually contain a dense cytoplasm with many
  organelles.
• Usually without chloroplasts or crystalline
  inclusions.
• Are involved in the stomatal mechanism.

11
Cell Types of the Leaf Surface

• Guard cells
• Complex cells which occur in pairs either side of
  the stomatal pore.
• Have a high metabolic activity, contain high
  numbers of mitochondria, but few chloroplasts.
• Two anatomic forms of stomata those with
  kidney-shaped guard cells and elliptical
  pores(dicots) and those with dumbbell-shaped
  guard cells (grasses).
• Appear to have few or no plasmodesmata, thus are
  isolated from the symplast of surrounding cells.

12
T Z Figure 4.16 (A) Dicot stoma (B) Monocot
stoma
13
The Importance of Guard Cell Microfibrils

• The pattern and direction of cellulose
  microfibril deposition determines the shape of
  the guard cells and the directions in which they
  expand and contract

14
T Z Figure 4.16 (A) Dicot stoma (B) Monocot
stoma
15
TZ Fig. 4.15
16
Opening and Closing of Stomates

• Stomata open due to an influx of water into the
  guard cells which causes them to swell
• Water enters the guard cells because there is a
  reduction in the osmotic potential of the guard
  cells
• This drop in osmotic potential ( and hence total
  water potential) is a result of the accumulation
  of solutes in the guard cells, in particular K.

17
From Willmer, C.W. (1983) Stomata. Longman,
London.
18
From Willmer, C.W. (1983) Stomata. Longman,
London.