Magnified cross section of the internal cellular structure of a young tree.
The internal structure of a young tree—including its xylem, through which it pumps sap—is shown in this cross section at 100× magnification. Credit: Berkshire Community College Bioscience Image Library, Public Domain
Source: Journal of Geophysical Research: Biogeosciences

As part of photosynthesis, the world’s plants transpire about 4 quadrillion liters of water every day. A giant redwood, for instance, can move up to 2,000 liters per day up to its canopy. Moving all that water from roots to leaves takes a lot of power.

For the first time, scientists have figured out just how much power it takes for the world’s vascular plants to pump sap—fluids, including water, that flow through plants. Quetin et al. examine the “natural pump” that drives sap upward. The pump, which involves surface tension, capillary movement, and hydrogen bonds in the xylem, moves sap from parts of a plant with more water to parts with less. It’s a passive process, driven by water continually evaporating from leaves, but plants must overcome gravity and resistance from their own internal structure.

To determine how much power it takes for plants around the world to suck up sap, the researchers first developed a global map of plant conductivities and resistances, organized by plant functional type (e.g., deciduous and needleleaf trees, grasses, and forbs). They then calculated average values for biomes on the basis of the composition of different plant types in each biome. And they factored in the height of plants in an ecosystem, the ecosystem’s sap flow volumes (measured by transpiration), and the resistivity inside plants.

The researchers found that globally, forest-dominated ecosystems consume 0.06 watt per square meter, or 9.4 petawatt-hours per year of energy to pump sap. That’s about as much energy as is generated by the world’s hydropower systems combined and about one third of U.S. energy use in 2021.

This substantial amount of power and energy has had overlooked benefits for the world’s plants: Because the sap-lifting process is driven by solar-powered transpiration, plants can harvest energy from sap ascent. This is most evident in forests, with their tall canopies, where sap ascent provides energy equivalent to about 14% of the amount gathered from photosynthesis.

“The fact that there is a global energy stream benefiting the biosphere to this magnitude is surprising,” said Gregory Quetin, a climate scientist at the University of California, Santa Barbara, and lead author of the study. “While the theory of sap ascent is well explored, how much power it takes has slipped through the cracks between multiple disciplines.”

Now that the researchers have made this first estimate, they say they’ll next look to improve measurements of plant hydraulic traits and transpiration to refine their power estimates and detail how sap ascent power varies among ecosystems. (Journal of Geophysical Research: Biogeosciences,, 2022)

—Rebecca Dzombak, Science Writer

Citation: Dzombak, R. (2022), Plants need a lot of power to pump sap, Eos, 103, Published on 16 September 2022.
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