Title: BIO2202 Plant Physiology 1 Water Relations Lecture 5
1BIO2202 Plant Physiology 1 Water Relations -
Lecture 5
- Stomata and the Leaf Epidermis
2Occurence
- 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
3Anatomy 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
4Hydrophobic 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.
5T Z Figure 13.1
6Hydrophobic 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.
7TZ Fig 13.2
8Cell 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.
9Cell 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.
10Cell 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.
11Cell 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.
12T Z Figure 4.16 (A) Dicot stoma (B) Monocot
stoma
13The 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
14T Z Figure 4.16 (A) Dicot stoma (B) Monocot
stoma
15TZ Fig. 4.15
16Opening 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.
17From Willmer, C.W. (1983) Stomata. Longman,
London.
18From Willmer, C.W. (1983) Stomata. Longman,
London.