\"1. Water within the whole plant forms a continuous network of liquid columns f

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"1. Water within the whole plant forms a continuous network of liquid columns from the absorbing surfaces of roots to the evaporating surfaces, mainly but not exclusively, within the leaves. Water columns within the conducting elements (vessels and tracheids) extend over 99% of the whole water pathway (vascular pathway) in a plant. The remaining 1% is constituted by the wall and cytoplasm of living cells, primarily found at the beginning and at the end of the pathway, in root and leaf parenchyma. 2. This vascular pathway has hydraulic continuity from the root-to-soil interface to the different parts and organs of the plant (see textbook Figure 4.1). The primary function of this hydraulic continuity is to transfer instantaneously (sound speed), the variations of tensions or pressure throughout the plant. Hydraulic continuity is highly dependent on the tensile strength of water (see below). 3. The driving force for water movement in the system is generated by surface tension at the evaporating surfaces. The evaporating surface is at the wall of the leaf cells where evaporation takes place. In contrast to a large flat surface of evaporating water, these surfaces are a porous medium. Consequently, evaporation generates a curvature in the water menisci within the pores of the cell wall lined by cellulose microfibrils (see textbook Figure 4.9). 4. The radius of curvature of these menisci is sufficiently small as to be able to support water columns as long as one hundred meters, (approximate size of the highest existing trees). By applying the empirical Jurin law, we find that a radius of 0.12m supports a column of 120 meters (Zimmermann, 1983). 5. Because of surface tension, and the small radius of curvature of the menisci at the evaporative surfaces, evaporation lowers the water potential of the adjacent regions including the xylem elements. This change is instantaneously transmitted throughout the whole plant. 6. In this way, evaporation establishes gradients of pressure or tension along the pathway in transpiring plants. This causes an inflow of water from the soil to the transpiring surfaces. The validity of the CTT does not depend on a specific range of xylem tensions, but it nevertheless predicts that “high tensions” could exist in plants. Typical values can be as low as –3 MPa in crop plants, –4 MPa in trees, and –10 MPa in desert species. It is noteworthy that the well established, linear relation between leaf water potential and transpiration is not predicted by the CTT. Rather, it is a consequence of the fact that in well-watered plants, the water flow through the plant closely approximates the relation: absorption = transpiration (in the absence of significant growth in volume) and the hydraulic resistances remain constant. Water flow through the plant can then be described as an electric current through a circuit of resistances in series: Flow = Difference of water potential / resistances This is the well-known Van den Honert approach to the Ohm’s law analogue of sap flow in the soil-plant- atmosphere continuum. The application of the Ohms law to sap flow encompasses many phenomena (heat transfer, water transfer in soil, Darcy law, first diffusion law, etc.) and, it is therefore independent of the underlying physical mechanisms and the nature of moving fluid. For example, the electrical approach does 8 not address whether sap is under tension or pressure. For this reason, the description of sap flow by Ohms is rather phenomenological. In contrast, the CTT explains specific physical of the water transport in plants. 7. Due to the fact that transpiration "pulls" the sap from the soil to the leaves, water in the xylem is in a metastable state of tension. In this state, the water column is susceptible to cavitation, (i.e., to the appearance of a vapor phase within the liquid phase). Whenever transpiration stops because of the absence of a humidity gradient between the leaf and atmosphere (as in the case when it rains period or in a night with high relative humidity), water will keep moving from the soil to the leaves until the difference of water potential across the water column disappears. The water potential value at this equilibrium is called predawn water potential. Under these conditions there is no more water potential difference between the soil surrounding the roots and the leaves; however if the predawn water potential is different from zero (i.e.: -0.1MPa) the sap will remain under tension. In contrast, if stem pressure (in winter) or root pressure (in spring) exists, water is pushed from the roots to the leaves. Water potential will be positive and sap under pressure. 8. As long as transpiration occurs, the xylem sap is under tension and cavitation could take place. As discussed below, the risk of cavitation has been the most controversial aspect of the CTT. However, both cohesion between water molecules, which gives water its tensile strength, and adhesion of water molecules to xylem walls (see chapter 3 in the textbook), prevent cavitation, to a certain extent. During dry conditions in summer and frost-thaw cycles in winter, air bubbles can enter the xylem and reduce the hydraulic conductivity of the conducting elements. The entry of air into the xylem under dry conditions has been explained by the air-seeding hypothesis. This hypothesis states that xylem cavitation starts when air is pulled through the pit membrane pores (see textbook figure 4.7). This occurs when the air pressure Pa, (usually near zero), minus the xylem pressure Px (negative under these conditions), across the air-water meniscus at the pore creates a pressure difference sufficient to displace the meniscus. The first phase of this event, the cavitation proper, is an extremely rapid invasion of the conducting element mainly by water vapor. A slower entrance of air, which corresponds to embolism follows cavitation. Special features of the conducting system design (interconnected conduits much shorter than the tree and pit membrane) and stomatal regulation limit the spread of air. "

The primary driving force of water being pulled up trees is:

Explanation / Answer

The primary driving force of water being pulled up trees is-d) Surface tension of tiny menisci of gas-liquid interface within leaves.

Water movement in the system occurs due to surface tension at the evaporating surfaces. Leaf is the site of evaporation.

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