1. Long-Distance Transport in Plants
Biology 1001
November 23, 2005

2. 4. The Control of Transpiration
The need for transpiration is part of the cost of doing photosynthesis
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

3. The Mechanism of Stomatal Opening and Closing
About 90% of plant water loss 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!

4. 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
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

5. 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!

6. The Mechanism of Translocation is Pressure Flow
Bulk flow driven by positive pressure, 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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