The transpiration–cohesion–tension mechanism accounts for xylem transport

The current model of xylem transport involves three processes (Focus: Key Figure 34.7):

  1. Transpiration of water molecules from the leaves by evaporation

  2. Tension (negative pressure) in the xylem sap resulting from transpiration from the leaves

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  3. Cohesion of water molecules in the xylem sap, from the leaves to the roots

focus: key figure

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Figure 34.7 The Transpiration–Cohesion–Tension Mechanism Transpiration causes evaporation from mesophyll cell walls, generating tension on the xylem. Cohesion among water molecules in the xylem transmits the tension from the leaf to the root, causing water to flow in the xylem from the roots to the atmosphere.

Question

Q: What would be the effect of high relative humidity in the air around the leaves on root uptake of water? Explain.

With high humidity, there would be reduced transpiration from the leaf. This would in turn reduce water pull from the leaf veins and in turn reduce water from the xylem and root uptake.

Media Clip 34.1 Inside the Xylem

www.life11e.com/mc34.1

Animation 34.1 Xylem Transport

www.life11e.com/a34.1

The concentration of water vapor in the atmosphere is lower than that in the air spaces inside the leaf. Because of this difference, water vapor diffuses from the intercellular spaces of the leaf to the outside air, in a process called transpiration. Within the leaf, water evaporates from the moist walls of the mesophyll cells and enters the intercellular spaces. As water evaporates from the aqueous film coating each cell, the film shrinks back into tiny spaces in the cell walls, increasing the curvature of the water surface and thus increasing its surface tension. This increased tension (negative pressure potential) in the surface film draws more water into the walls from the cells, replacing that which was lost. The resulting tension in the mesophyll draws water from the xylem of the nearest vein into the apoplast surrounding the mesophyll cells. The removal of water from the veins, in turn, establishes tension on the entire column of water contained in the xylem. Cohesion between water molecules in the column prevents the column from breaking. So these three forces—transpiration, tension, and cohesion—operate together to draw water up the xylem, all the way from the roots to the leaves.

Each part of this theory is supported by evidence:

The transpiration–cohesion–tension mechanism accounts for the movement of water through the xylem. Dissolved mineral ions are carried along with the water to all of the plant’s living tissues, where the ions are used for various cellular processes (see Chapter 35 for more on plant nutrition). In addition to promoting the transport of minerals, transpiration has an added benefit of cooling a plant’s leaves. The evaporation of water from mesophyll cells consumes heat, thereby decreasing the leaf temperature. A farmer can hold a leaf between thumb and forefinger to estimate its temperature; if the leaf doesn’t feel cool, that means transpiration is not occurring and it must be time to water.

As you saw in the opening investigation of this chapter, farmers strive to improve the water-use efficiency of their crops. The roles of the root system and transpiration are important. A gene from the model plant Arabidopsis has been shown to improve water-use efficiency by encoding a transcription factor that regulates genes involved in these processes (Investigating Life: Improving Water-Use Efficiency in Rice).

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investigating life

Improving Water-Use Efficiency in Rice

experiment

Original Paper: Karaba, A. et al. 2007. Improvement of water use efficiency in rice by expression of HARDY, an Arabidopsis drought and salt resistance gene. Proceedings of the National Academy of Sciences USA 104: 15270–15275.

The objective of improving rice production involves cultivating a large amount of plant material (biomass)—meaning a large amount of grain—while using less water. The relationship between biomass produced and water consumed is called water-use efficiency. A team at Virginia Polytechnic University led by Andrew Pereira approached this problem by isolating a gene that confers drought resistance from the model plant Arabidopisis. This gene, called HARDY, encodes a transcription factor in the AP/ERF family that activates transcription of genes involved in the plant’s response to stresses. The team showed that Arabidopsis strains that expressed the HARDY transcription factor were much more resistant to drought and to an environment high in dissolved salts. They then inserted the HARDY gene into a high-expressing vector and made transgenic rice plants. Initial studies showed that the plants were drought-resistant.

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work with the data

The investigators measured a number of parameters in both wild-type and transgenic HARDY rice:

  • Water-use efficiency: g carbon fixed/kg water used

  • Transpiration rate: water lost in g/cm2 plant area/day

  • Carbon fixation rate: g/cm2 plant area/day

  • Biomass accumulation: g in root + shoot

Results are shown in the table and expressed as mean ± SD.

Parameter Wild-type HARDY
Water-use efficiency 1.5 ± 0.06 3.0 ± 0.5
Transpiration rate 5.2 ± 0.2 4.0 ± 0.4
Carbon fixation rate 0.7 ± 0.06 1.3 ± 0.06
Biomass accumulation 7.5 ± 0.5 13.0 ± 0.6

QUESTIONS

Question 1

Explain each result.

There was increased water-use efficiency (biomass accumulation divided by transpiration) in plants overexpressing HARDY because there was decreased transpiration (line 2) and increased photosynthesis–carbon fixation (line 3).

Question 2

What statistical test would you use to evaluate the significance of any differences found?

Use a t-test for paired samples.

A similar work with the data exercise may be assigned in LaunchPad.