[Paper Review] The Water Circuit of the Plants - Do Plants have Hearts ?

Wolfgang Kundt, Éva Gruber|ArXiv.org|Mar 17, 2006

Plant Water Relations and Carbon Dynamics31 references20 citations

TL;DR

The paper proposes that plants use subcellular mechanical pumps in the root endodermis and exodermis to generate root pressure via reverse osmosis, enabling active water transport against gravity. This mechanism, powered by ATP, drives the continuous circulation of sap—functionally analogous to an animal heart—enabling water ascent in tall plants up to 140 m, with experimental evidence from guttation and exudation in multiple species.

ABSTRACT

There is a correspondence between the circulation of blood in all higher animals and the circulation of sap in all higher plants - up to heights h of 140 m - through the xylem and phloem vessels. Plants suck in water from the soil, osmotically through the roothair zone, and subsequently lift it osmotically again, and by capillary suction (via their buds, leaves, and fruits) into their crowns. In between happens a reverse osmosis - the endodermis jump - realized by two layers of subcellular mechanical pumps in the endodermis walls which are powered by ATP, or in addition by two analogous layers of such pumps in the exodermis. The thus established root pressure helps forcing the absorbed ground water upward, through the whole plant, and often out again, in the form of guttation, or exudation.

Motivation & Objective

• To resolve the long-standing biological puzzle of how water is actively transported to the crowns of tall plants against gravity.
• To challenge the prevailing cohesion-tension theory by proposing an active pumping mechanism instead of passive transpiration-driven ascent.
• To establish a functional analogy between plant water transport and animal blood circulation, suggesting plants possess a 'heart' in the form of root-based mechanical pumps.
• To quantify the role of root pressure and reverse osmosis in sustaining long-term water circulation in vascular plants.
• To provide a unified physical-chemical explanation for guttation, exudation, and water ascent in plants using water potential and pressure gradients.

Proposed method

• Proposes that root pressure is generated by ATP-powered mechanical pumps in the endodermis and exodermis, enabling reverse osmosis to concentrate water and create pressure.
• Models water transport using the generalized water potential ψ = ρgz + p − π − τ, incorporating gravity, hydrostatic pressure, osmotic pressure, and matric potential.
• Applies the principle that fluid flows from higher to lower water potential (ψ), with flow driven by gradients in ψ, not just gravity or transpiration.
• Analyzes guttation and exudation as direct evidence of root pressure, occurring primarily at night when transpiration is low.
• Uses the analogy of hydraulic systems in cities (water towers, pumps) to illustrate the need for active pumping in plants, contrasting passive osmotic and capillary forces.
• Integrates experimental observations—such as bleeding in cut branches of maple, oak, and birch—with theoretical modeling to validate the pump mechanism.

Experimental results

Research questions

• RQ1 How do plants actively lift water to their crowns against gravity, especially in the absence of transpiration at night?
• RQ2 What mechanism explains the observed root pressure (up to 10 bar) and guttation in higher plants?
• RQ3 Why is the cohesion-tension theory insufficient to explain water ascent in tall trees, particularly given mechanical constraints on leaf structures?
• RQ4 How does reverse osmosis in root endodermis and exodermis contribute to sustained water circulation in vascular plants?
• RQ5 To what extent can the water potential framework (ψ = ρgz + p − π − τ) explain the direction and magnitude of water flow in plant xylem and phloem?

Key findings

• Plants generate root pressure up to 10 bar via ATP-powered mechanical pumps in the endodermis and exodermis, enabling active water transport against gravity.
• Guttation and exudation in species like maple, birch, and oak provide direct experimental evidence of root pressure, especially prominent at night.
• The water potential ψ, defined as ρgz + p − π − τ, governs water flow direction, with fluid moving from higher to lower ψ, not just from high pressure to low.
• Osmotic and capillary forces alone cannot sustain circulation; they are limited to one-time work and require restoration, unlike active pumps.
• The root pump mechanism enables water transport in plants up to 140 m in height, consistent with observed water flow rates of up to one ton per day in large trees.
• The paper corrects the misconception that transpiration generates suction forces of up to 1 kbar, showing such values are physically implausible and based on flawed textbook interpretations.