Transcript Long-Distance Transport in Plants

Long-Distance Transport
in Plants
Biology 1001
November 23, 2005
4. The Control of Transpiration
 The need for transpiration is part of the cost of doing
photosynthesis
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The large surface area of a leaf maximizes photosynthesis
but also increases water loss due to transpiration
Under drought conditions regular plants wilt due to loss of
turgour pressure
But transpiration also contributes to evaporative cooling
The rates of transpiration are highest on sunny, warm, windy
and dry days
Transpiration is controlled by opening and closing of
stomata
The Mechanism of Stomatal Opening and Closing
 About 90% of plant water loss
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occurs through the stomata
Guard cells control the diameter of
the stomata by changing shape
When they take in water they
become turgid and bowed due to
radially oriented microfibrils and
unevenly thickened walls, opening
the pore
When guard cells lose water they
become flaccid and the pore closes
The changes in turgour pressure
that open and close stomata result
from the reversible uptake and loss
of potassium ions (K+)
Figure 36.15!
5. Translocation in Phloem
 The transport of organic nutrients in a plant is called
translocation
 The direction of translocation is from a sugar source to a
sugar sink
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Mature leaves are the primary sugar sources
Growing roots, buds, stems and fruits are sugar sinks
Storage organs such as bulbs are sinks during the summer and
sources during the spring
 Phloem sap is an aqueous solution of 30% sucrose,
minerals, amino acids, and hormones
 Sugars must be loaded into sieve-tube members of the
phloem for translocation
Loading Sucrose into the Phloem
 Sucrose manufactured in mesophyll cells can travel via the
symplast to sieve-tube members
 In some species sucrose exits the symplast near sieve tubes
and is actively accumulated from the apoplast by sieve-tube
members and their companion cells
 Loading sucrose at the source often involves active transport
 Unloading sucrose at the sink occurs by diffusion
Figure 36.17!
The Mechanism of Translocation is Pressure Flow
 Bulk flow driven by positive pressure,
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called pressure flow, moves phloem
sap at a rate as high as 1m/h
At the sugar source sugars are loaded
into sieve tubes, and water follows by
osmosis
At the sink, sugar leaves the sieve
tube and water follows by osmosis
This creates a pressure differential that
pushes water through the sieve tube
from source to sink
In the case of leaf-to-root
translocation, xylem recycles the
water from sink to source
Figure 36.18!
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