WHAT NEEDS TO BE TRANSPORTED?
Water and dissolved elements
Photosynthate
Xylem
Movement
is unidirectional
Roots -> Stem -> Leaves
Phloem
Movement can be bidirectional
Source -> Sink
XYLEM TRANSPIRATION
I. Anatomical variation in xylem
A. Vessel radius increases from protoxylem to metaxylem within a vascular bundle
B. Number of vascular bundles entering a petiole
3. Numerous vascular bundles
C. Primary leaf traces form interconnections in stem
1. Depends on number of vascular bundles associated with each leaf
2. Leaf arrangement
3. Amount of parenchyma formed in association with leaf gaps
4. Vessel size and number change with vertical distance along leaf trace
D. Pattern of protoxylem and metaxylem arrangement changes in the transition zone
1. Endarch in stems
2. Exarch in roots
E. Primary vasculature at tips of stems and roots functionally continuous with secondary xylem in stem (A,B,C) and roots (A,B)
F. Variation within annual growth rings
1. Dicots
a. Constant size = diffuse porous
b. Steady decrease in size = semi-ring porous
c. Abrupt decrease in size = Ring porous
2. Conifers
a. Constant size
b. Cell wall thickness increases gradually
c. Cell wall thickness increases abruptly
II. Transpiration in the Tomato Stem studied by Dimond (1965) Link to UC, Davis Virtual Tomato Site
A. Three vascular bundles (x,y,z) enter each leaf
B. At any transverse level in the stem 6 vascular bundles are apparent
1. Three small bundles (a,c,e) that are the central (y) bundles associated with leaves at different successive nodes
2. Three large bundles (b,d,f) that are functionally continuous with bundles x,y,z and each other at different vertical levels
3. The basic vascular unit bundle pattern (b,c,d + x,y,z)
a. Spans three nodes (Nodes 2 -> 5)
b. Interconnects laterally with two other basic vascular units
1. d,e,f at node 0
2. f,a,b at node 1
c. Interconnects vertically with another basic vascular unit at node 2
d. Each basic vascular unit is offset by one node vertically
e. Each basic vascular unit is offset by 137 degrees radially
III. Theory of vascular flow
A. Flow of water is laminar rather than turbulent
B. Laminar flow in capillary follows Poiseuille's Law
p = (8 * l * u * v) / ( p * r4) or v = (p * r4 * p) / (8 * l * u)
p = driving pressure required to move a fluid
with viscosity = u, at a
rate = v (ml per sec), through a
capillary of radius = r and
length = l.
p = (8 * l * u * v) / p * S( n * r4)
S ( n * r4) describes attributes of a vascular bundle consisting of n vessels
IV. Properties of xylem transport system inferred from the model system
A. Conductance (rate of flow through a vascular bundle under unit difference in pressure) is the reciprocal of its resistance to flow
1. Conductance of an unbranched bundle varies from one internode to the next, which reflects variation in size and number of vessels along a bundle
2. Conductance of the three large vascular bundles in the stem varies with vertical level in the stem, which reflects the sum total of the size and number of vessels in these bundles
3. Conductance of larger vascular bundles was 50-100 times greater than smaller vascular bundles in the lower part of the stem
4. Conductance of larger vascular bundles was similar to that of smaller vascular bundles in the upper part of the stem, which reflects the decrease in size of the larger bundles from base to apex
B. Relative contributions of different size vessel elements within a vascular bundle to the conductance of that vascular bundle
1. In large vascular bundles
a. Large vessel element radius > 10X radius of small vessel elements
b. Implies that flow rate in large vessels > 10,000X flow rate of small vessels
2. In a "typical " large vascular bundle
a. The six largest vessels should transport 60% of total fluid
b. The ten smallest vessels should transport 0.05% of total fluid
C. Relative probability of disruption of transpiration by air embolisms (cavitation)
1. Larger vessel elements with low resistance to fluid flow
a. Higher probability of cavitation
b. Slower rate of recovery
2. Smaller vessel elements with high resistance to fluid flow
a. Lower probability of cavitation
b. Faster rate of recovery
D. Conductance pathway dependent on environmental conditions
1. In unstressed plants the bulk of fluid flow is through large vessel elements in large vascular bundles
2. Under stress conditions the bulk of fluid flow is through small vessel elements in both large and small vascular bundles
