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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