Biomechanics

All vascular plants have a plumbing system of conduits that carry water from the roots to the leaves. In nature, water evaporates out the pores (stomata) in the leaves, pulling water out of the ground, through the roots, through the stem and leaves, and into the air. This process is much like drinking through a straw. However, when it is a hot day or the ground is dry, there is a lot of tension on the water in the stem.

Eventually, the tension becomes so strong that air is pulled into the vessels through tiny pores in the side of the vessels. The air bubble grows until it blocks the vessel and prevents the flow of water up the plant. This is called an air embolism (blockage). The effects of an embolism in the plant vascular system are similar to the effects of an embolism in a human blood vessel. If many vessels are blocked by air, the plant will die from lack of water.

Flowering plants have supportive fibers surrounding the conduits. These supportive fibers have two functions: 1) mechanical strength (stiffness), 2) resistance to air embolism formation in the vessels. A previous study (Jacobsen et al, 2005) revealed a correlation in flowering plants between mechanical strength and air embolism resistance due to the presence of these dual-function fibers. However, since ferns do not have supportive fibers surrounding the conduits, we anticipate that there will be no correlation between mechanical strength and air embolism resistance in ferns.

Mechanical Strength

SURB2013HelenBreahnaLab1SDavis 976
Instron Mechanical Testing Device breaking a fern stem

To measure stem stiffness, we used an Instron Mechanical Testing Device (pictured above). As the Instron breaks the stem, the sensor generates a stress/strain graph. From this graph we use the initial slope (Modulus of Elasticity, MOE) and the breaking point (Modulus of Rupture, MOR).

StressStrain

Air Embolism Resistance

To measure air embolism resistance, we are using the classical centrifuge method described by Alder et al (1997).

2014Fernincentrifuge.med

Spinning the stem in the centrifuge generates a tension on the water column like the tension observed in nature. In the lab, we control how fast we spin the stem (the amount of tension on the water) and then measure the effects on hydraulic conductivity in the stem.


Literature Cited:

Alder N.N., et al. (1997) “Use of centrifugal force in the study of xylem cavitation.” Journal of Experimental Botany 48.308: 665-674.

Jacobsen, Anna L., et al. (2005)”Do xylem fibers affect vessel cavitation resistance?.”Plant Physiology 139.1: 546-556.

Advertisements