Lecture 9 Transport in Plants: Xylem Dr. Alan McElligott Chapters 34 & 35

XYLEM Aims: To introduce the structure of the xylem To study the transport of water and minerals in the xylem To introduce transpiration and stomata

XYLEM Aims: To introduce the structure of the xylem To study the transport of water and minerals in the xylem To introduce transpiration and stomata These lecture aims form part of the knowledge required for learning outcome 3: Describe mechanisms for the life processes (LOC3)

Essential reading pages pages XYLEM 34.2 How Are Plant Cells Unique? (pages )

Figure 34.6 Three Tissue Systems Extend Throughout the Plant Body

34.2 How Are Plant Cells Unique? Xylem contains cells called tracheary elements— die before assuming their function. Gymnosperms have tracheids with pits in the secondary walls that allow materials to move freely.

34.2 How Are Plant Cells Unique? Angiosperms have vessel elements with pits. Larger diameter than tracheids; lignin in secondary cell walls; end walls break down after death, forming hollow tubes. Xylem of many angiosperms also contains tracheids.

Figure 34.9 Plant Cell Types

35 Transport in Plants 35.2 How Are Water and Minerals Transported in the Xylem? 35.3 How Do Stomata Control the Loss of Water and the Uptake of CO 2 ?

Figure 35.1 The Pathways of Water and Solutes in the Plant

35.2 How Are Water and Minerals Transported in the Xylem? Xylem transport: a maple tree in summer loses 220 liters of water per hour. Xylem must transport 220 litres per hour to prevent wilting. The tallest trees exceed 110 meters. Xylem must transport water to great heights. Several models for xylem transport have been proposed.

35.2 How Are Water and Minerals Transported in the Xylem? First proposal was pumping action by living cells. Ruled out in 1893 by an experiment in which cut trees were placed in a poison solution. Solution rose through trunk to leaves (which died), then stopped rising.

35.2 How Are Water and Minerals Transported in the Xylem? Experiment established three points: Living, pumping cells were not involved. Leaves were crucial: solution continued to rise until leaves were dead. Movement was not caused by the roots.

35.2 How Are Water and Minerals Transported in the Xylem? Some hypothesized that xylem transport is based on root pressure. Higher solute concentration and more negative water potential in roots than in soil solution; water enters stele and from there has no where to go but up.

35.2 How Are Water and Minerals Transported in the Xylem? Guttation is evidence of root pressure; water is forced out through openings in leaves. Root pressure also causes sap to ooze from cut stumps. But it cannot account for ascent of sap in trees.

35.2 How Are Water and Minerals Transported in the Xylem? If root pressure was pushing sap up the xylem, there would be a positive pressure potential in the xylem at all times. But xylem sap in most trees is under tension; negative pressure potential.

35.2 How Are Water and Minerals Transported in the Xylem? An alternative to pushing is pulling. Leaves pull the xylem sap upwards. Evaporative water loss from the leaves creates a pulling force or tension on the water in the apoplast of leaves. Hydrogen bonding between water molecules makes sap cohesive enough to withstand the tension and rise by bulk flow.

35.2 How Are Water and Minerals Transported in the Xylem? Concentration of water vapour in the atmosphere is lower than in the leaf. Water vapour diffuses from leaf through the stomata: transpiration. Within the leaf, water evaporates from walls of mesophyll cells, film of water on cells shrinks, creating more surface tension (negative pressure potential). Draws more water into cell walls to replace what was lost.

35.2 How Are Water and Minerals Transported in the Xylem? Resulting tension in mesophyll cells draws water from nearest vein. Removal of water from veins results in tension on the entire column of water in the xylem, so that water is drawn up.

35.2 How Are Water and Minerals Transported in the Xylem? Ability of water to be drawn up through tiny tubes is due to cohesion: water molecules stick together because of hydrogen-bonding. The narrower the tube, the greater the tension the water column can withstand. Water also adheres to the xylem walls.

35.2 How Are Water and Minerals Transported in the Xylem? This transpiration–cohesion–tension mechanism requires no energy from the plant. Water moves passively toward a region of more negative water potential. Dry air has the most negative water potential, and soil solution has the least. Mineral ions in xylem sap rise passively with the water.

Figure 35.6 The Transpiration–Cohesion–Tension Mechanism

35.2 How Are Water and Minerals Transported in the Xylem? Transpiration also helps cool plants. As water evaporates from mesophyll cells, heat is taken up from the cells, and leaf temperature drops. Important for plants in hot environments.

35.2 How Are Water and Minerals Transported in the Xylem? A demonstration of the negative pressure potential, or tension, in xylem sap was done by measuring tension with a pressure chamber. Also determined that tension disappeared at night in some plants, when transpiration stopped.

Figure 35.7 A Pressure Chamber

35.2 How Are Water and Minerals Transported in the Xylem? Xylem sap does not rise at night, when there is no transpiration. During the day, rate of ascension depends on temperature, light intensity, and wind velocity, which all affect transpiration. Rate of flow may also depend on concentration of K + —seems to change size of pits.

Figure 35.8 Potassium Ions Speed Transport in the Xylem (Part 1)

Figure 35.8 Potassium Ions Speed Transport in the Xylem (Part 2)

Figure 35.8 Potassium Ions Speed Transport in the Xylem

35.3 How Do Stomata Control the Loss of Water and the Uptake of CO 2 ? Leaf and stem epidermis has a waxy cuticle to minimize water loss, but it also prevents gas exchange. Stomata, or pores in the leaf epidermis, allow CO 2 to enter by diffusion. Guard cells control opening and closing.

35.3 How Do Stomata Control the Loss of Water and the Uptake of CO 2 ? Most plants open stomata when light intensity is enough for moderate rate of photosynthesis. At night, stomata remained closed; CO 2 not needed, and no water is lost. During the day stomata close if water is being lost too rapidly.

35.3 How Do Stomata Control the Loss of Water and the Uptake of CO 2 ? Cues for stomatal opening include light, and concentration of CO 2 in intercellular spaces in the leaf: low levels favor opening of stomata. If plant is under water stress or water potential of mesophyll cells is too negative, they release the hormone abscisic acid—acts on guard cells and causes them to close.

35.3 How Do Stomata Control the Loss of Water and the Uptake of CO 2 ? Opening and closing of stomata is controlled by K + in the guard cells. Blue light is absorbed by pigments in guard cells and activates a proton pump. Resulting gradient drives K + into guard cell, making its water potential more negative. Water enters cell by osmosis. Increased pressure potential causes guard cells to change shape, and a gap appears between them.

Figure 35.9 Stomata

35.3 How Do Stomata Control the Loss of Water and the Uptake of CO 2 ? The process is reversed when active transport of protons stops. K + diffuse out of guard cells passively, and water follows by osmosis. Pressure potential decreases, and cells sag, closing the gap between them. Demonstration of how much K + moves in and out of guard cells was done by using an electron probe microanalyzer.

Figure Measuring Potassium Ion Concentration in Guard Cells (Part 1)

Figure Measuring Potassium Ion Concentration in Guard Cells (Part 2)

35.3 How Do Stomata Control the Loss of Water and the Uptake of CO 2 ? Farmers and gardeners would like to reduce the amount of transpiration from crops, to reduce need for irrigation. An antitranspirant: a compound to reduce transpiration without limiting CO 2 uptake. Abscisic acid is too expensive for large- scale trials.

35.3 How Do Stomata Control the Loss of Water and the Uptake of CO 2 ? Transgenic plants with a mutant allele for the era gene are very sensitive to abscisic acid and thus resistant to wilting in droughts. Some compounds form a polymer film around leaves, and seal the stomata. They are mostly used for transplants of nursery stock.

Table 35.1 Mechanisms of Sap Flow in Plant Vascular Tissues

Check out 35.2 Recap, page Recap, page Chapter summary, page Chapter summary, page 778 Self Quiz Page : Chapter 35, questions 6-8 For Discussion Page 779: Chapter 35, Question 4 XYLEM

Key terms: anti-transpirant, abscisic acid (ABA), cohesion, guard cell, guttation, hydrogen bonding, proton gradient, stomata, tension, tracheary element, tracheid, transpiration, turgor pressure, xylem, vessel element XYLEM