D1 Water Flow Measurement

Topic

Water transport within the soil-plant-atmosphere continuum begins in the soil, where soil water potential determines the amount of water available for root absorption: the higher the soil water potential, the easier it is for the root to absorb water, whereas the lower the potential, the more difficult it is for the root to sustain that transport. A suite of measurement tools allows for the quantification of each stage of this water transport across the entire soil-plant-atmosphere continuum.

Once the root absorbs water from the soil, the water moves through the xylem to the leaf; xylem hydraulic conductivity quantifies the ease with which the xylem facilitates this flow: higher hydraulic conductivity allows more water to pass through the xylem per unit of time, while lower conductivity restricts water transport to the leaf.

Upon reaching the leaf, water transport is quantified by leaf water potential; this, combined with soil water potential, determines the potential difference driving water transport across the entire continuum. Finally, water exits the leaf into the atmosphere, and the transpiration rate quantifies the amount of water released into the air per unit of time: a higher transpiration rate indicates faster completion of water transport through the entire soil-plant-atmosphere continuum.

When water transport through the xylem exceeds the system's capacity, cavitation occurs—air bubbles disrupt the xylem conduits. This phenomenon is quantified by the degree of cavitation, a measure that directly reduces the xylem hydraulic conductivity available for water transport. When the degree of cavitation causes the loss of xylem hydraulic conductivity to exceed a threshold of approximately 88%, the xylem sustains irreversible damage; however, leaf water potential, xylem hydraulic conductivity, transpiration rate, and the degree of cavitation collectively allow for the diagnosis of this water stress before such irreversible xylem damage actually occurs.

When combined with soil measurements, these tools—measuring soil water potential, leaf water potential, xylem hydraulic conductivity, transpiration rate, and the degree of cavitation—make it possible to identify where the bottleneck limiting water transport is located within the soil-plant-atmosphere continuum: the bottleneck may lie in soil conductivity, where the soil itself restricts the amount of water delivered to the root; it may lie in the root, where the root fails to absorb or transport water that is otherwise available in the soil; or it may lie in the aerial xylem, where the section of the xylem above ground restricts water transport even when the soil and root are functioning normally.

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