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 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

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 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, 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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