【正文】 Education, Inc. publishing as Benjamin Cummings ? Changes in turgor pressure (膨壓 ) that open and close stomata – Result primarily from the reversible uptake and loss of potassium (K+) ions by the guard cells (b) Role of potassium in stomatal opening and closing. The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane and vacuolar membrane causes the turgor changes of guard cells. H2O H2O H2O H2O H2O K+ H2O H2O H2O H2O H2O Figure Copyright 169。 2022 Pearson Education, Inc. publishing as Benjamin Cummings Xerophyte Adaptations That Reduce Transpiration 旱生植物減少蒸散作用以適應(yīng)環(huán)境 ? Xerophytes (旱生植物 ) – Are plants adapted to arid climates (乾旱氣候 ) – Have various leaf modifications (葉的變形 ) that reduce the rate of transpiration Copyright 169。 2022 Pearson Education, Inc. publishing as Benjamin Cummings ? The stomata of xerophytes (旱生植物的氣孔 ) – Are concentrated on the lower leaf surface (下表皮 ) – Are often located in depressions that shelter the pores from the dry wind Figure Lower epidermal tissue Trichomes (“hairs”) 細(xì)毛 Cuticle Upper epidermal tissue Stomata 100 ?m 下表皮 上表皮 Copyright 169。 2022 Pearson Education, Inc. publishing as Benjamin Cummings ? Concept : Organic nutrients are translocated (轉(zhuǎn)移 /運(yùn)移 ) through the phloem ? Translocation (轉(zhuǎn)移 /運(yùn)移 ) – Is the transport of anic nutrients in the plant ? Phloem sap (韌皮液 ) – Is an aqueous solution that is mostly sucrose – Travels from a sugar source to a sugar sink Copyright 169。 2022 Pearson Education, Inc. publishing as Benjamin Cummings ? Phloem sap (韌皮液 ) – Is an aqueous solution that is mostly sucrose – Travels from a sugar source to a sugar sink ? A sugar source (糖的源頭 ) – Is a plant an that is a producer of sugar, such as mature leaves ? A sugar sink (糖的儲(chǔ)存區(qū) ) – Is an an that is a consumer or store of sugar, such as a tuber or bulb Movement from Sugar Sources to Sugar Sinks (自糖源頭到糖儲(chǔ)存區(qū)的移動(dòng) ) Copyright 169。 2022 Pearson Education, Inc. publishing as Benjamin Cummings Figure Mesophyll cell (sugar source) Cell walls (apoplast) Plasma membrane Plasmodesmata 伴細(xì)胞 Companion (transfer) cell Sievetube Member (篩管細(xì)胞 ) Mesophyll cell Phloem parenchyma cell Bundle sheath cell (束鞘細(xì)胞 ) (a) Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sievetube members. In some species, sucrose exits the symplast (red arrow) near sieve tubes and is actively accumulated from the apoplast by sievetube members and their panion cells. ? Sugar must be loaded into sievetube members (篩管細(xì)胞 ) before being exposed to sinks ? In many plant species, sugar moves by symplastic and apoplastic pathways (細(xì)胞質(zhì)連絡(luò)絲 ) Cytosol (symplast) Copyright 169。 2022 Pearson Education, Inc. publishing as Benjamin Cummings Figure (b) A chemiosmotic mechanism is responsible for the active transport of sucrose into panion cells and sievetube members. Proton pumps generate an H+ gradient, which drives sucrose accumulation with the help of a cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell. High H+ concentration Cotransporter Proton pump ATP Key Sucrose Apoplast Symplast H+ H+ Low H+ concentration H+ S S ? In many plants, phloem loading requires active transport (主動(dòng)運(yùn)輸 ) ? Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose Copyright 169。 2022 Pearson Education, Inc. publishing as Benjamin Cummings Vessel (xylem) H2O H2O Sieve tube (phloem) Source cell (leaf) Sucrose H2O Sink cell (storage root) 1 Sucrose (1) Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sievetube members. This causes the tube to take up water by osmosis. 2 4 3 2 (2) This uptake of water generates a positive pressure that forces the sap to flow along the tube. (3) The pressure is relieved by the unloading of sugar and the consequent loss of water from the tube at the sink. (4) In the case of leaftoroot translocation, xylem recycles water from sink to source. Transpiration stream Pressure flow Figure Pressure Flow: The Mechanism of Translocation in Angiosperms (壓力流：被子植物韌皮部轉(zhuǎn)運(yùn)韌皮液的機(jī)制 ) ? In studying angiosperms, researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure Copyright 169。 2022 Pearson Education, Inc. publishing as Benjamin Cummings Pressure flow hypothesis (壓力流假說(shuō) ) ? The pressure flow hypothesis explains why phloem sap always flows from source to sink ? Experiments have built a strong case for pressure flow as the mechanism of translocation in angiosperms Aphid feeding Stylet in sieve tube member Severed stylet exuding sap Siev e Tube member Sap droplet Stylet Sap droplet 25 ?m Sieve tube member Experiment: To test the pressure flow hypothesis,researchers used aphids that feed on phloem sap. An aphid probes with a hypodermiclike mouthpart called a stylet that perates a sievetube member. As sievetube pressure forcefeeds aphids, they can be severed from their stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different points between a source and sink. Results: The closer the stylet (口針 ) was to a sugar source, the faster the sap flowed and the higher was its sugar concentration. Conclusion: The results of such experiments support the pressure flow hypothesis. Figure 蚜蟲覓食 在篩管細(xì)胞中 的蚜蟲口針 殘存的口針 排出韌皮液 Copyright 169。 2022 Pearson Education, Inc. publishing as Benjamin Cummings 報(bào)告完畢 敬請(qǐng)指教 ??？ ！？ ！？ ！？ ?。? ??？