Terrestrial plants pull water from the soil through their roots, move it upward through an internal plumbing network, and lose most of it as vapor through tiny pores in their leaves. That basic loop sounds simple, but it involves some of the most remarkable physics and biology in nature. A tall tree, for instance, lifts water to heights that would defeat any conventional pump, does so without a heart or any muscular organ, and manages the whole process while balancing a constant tension between needing carbon dioxide for growth and losing water every time it opens the pores that let that gas in.
Getting Water Into the Roots
Water enters a plant through its roots, specifically through a zone near the root tips where fine root hairs vastly increase the surface area in contact with soil moisture. From there, water can travel toward the center of the root through two main routes. One path runs through the spaces between cell walls, essentially flowing around the cells. The other path runs through the cells themselves, crossing cell membranes one after another. Both routes operate simultaneously, but the balance between them shifts depending on conditions.
The cell-to-cell route is controlled largely by proteins called aquaporins, which are channels embedded in cell membranes that let water pass through. Roots can adjust these channels surprisingly quickly. Research has shown that roots alter their water permeability within hours to a couple of days in response to stimuli like day-night cycles, nutrient shortages, or stress.1Annals of Botany. The Role of Aquaporins in Root Water Uptake This is not a passive system sitting there waiting for rain. Roots actively tune how much water they let in.
Salt stress illustrates how sensitive this tuning can be. In experiments with Arabidopsis (a small plant widely used in lab research), even a modest dose of salt rapidly shut down aquaporin activity in root cells, cutting water flow. Plants engineered to overexpress a specific aquaporin gene were protected from this shutdown, keeping water moving even when salt was present.2PubMed. Regulation of aquaporin-mediated water transport in Arabidopsis roots exposed to NaCl The takeaway is that the gateway between soil water and the rest of the plant is actively regulated at the molecular level, and disruptions there can cascade upward through the whole system.
How Water Climbs a Tree
Once inside the root, water enters a network of dead, hollow cells called xylem vessels that form continuous tubes running from root tip to leaf tip. The classic explanation for how water rises through these tubes is the cohesion-tension theory: as water evaporates from leaf surfaces (a process called transpiration), it creates a negative pressure, or tension, that pulls water upward from below, like sipping through a straw. Water molecules cling to each other through hydrogen bonding, so pulling from the top drags the entire column upward.
That theory has held up as the primary explanation for over a century, but researchers have found the full story is more complicated. Experimental work using non-invasive measurement techniques has produced evidence that plants may use an interplay of several mechanisms beyond simple cohesion-tension alone.3PubMed Central. Water ascent in trees and lianas: the cohesion-tension theory revisited in the wake of Otto Renner One refinement involves naturally occurring surfactants, soap-like molecules found inside xylem vessels. These surfactants coat hydrophobic surfaces and tiny nanobubbles, keeping the bubbles small enough that they do not expand and block the vessel, which would be catastrophic for water flow.4PubMed. Xylem Surfactants Introduce a New Element to the Cohesion-Tension Theory
The reason bubble formation matters so much is that the water inside xylem is under tension, meaning it is actually being pulled rather than pushed. Under those conditions, a bubble can expand explosively and fill the vessel with air, a process called embolism. An embolized vessel is useless for water transport. Plants have evolved multiple strategies to prevent this, and the surfactant system appears to be one of them.
Why Trees Cannot Grow Forever
If water transport were costless, trees could grow infinitely tall. They cannot, and water is a major reason. As a tree gets taller, the water column must fight both gravity and the friction of travelling a longer path through narrow xylem conduits. The resulting tension at the top of the tree increases, making the leaves at the crown progressively more water-stressed. Even when soil moisture is plentiful, this stress limits how much the highest leaves can expand and photosynthesize, effectively setting a ceiling on growth.5PubMed. The limits to tree height
Tall trees also face a greater risk of embolism because the tension in the water column increases with height. Taller conifers, for example, show conflicting demands in their xylem design: they need wider conduits for efficient water transport but narrower ones for safety against air-seeding, the process that triggers embolism.6Proceedings of the National Academy of Sciences. Maximum height in a conifer is associated with conflicting requirements for xylem design A global analysis of woody plants found that this safety-versus-efficiency tradeoff is real, though the correlation is weak: no species managed to be both highly efficient and highly safe at the same time.7New Phytologist. Weak tradeoff between xylem safety and xylem‐specific hydraulic efficiency across the world’s woody plant species
The Stomatal Balancing Act
Most water loss happens through stomata, the microscopic pores on leaf surfaces that open to let carbon dioxide in for photosynthesis and release oxygen and water vapor. Every molecule of CO₂ a plant captures costs it hundreds of molecules of water. Plants regulate this exchange by opening and closing stomata using guard cells that respond to light, humidity, CO₂ concentration, and a stress hormone called abscisic acid (ABA).
Different species handle this tradeoff in fundamentally different ways. Some plants, described as isohydric, keep their leaf water status relatively constant by aggressively closing stomata as soon as drought begins. They play it safe but sacrifice photosynthesis. Others, described as anisohydric, keep their stomata open longer, maintaining higher photosynthetic rates even as their leaf water potential drops.8PubMed Central. Risk-taking plants: anisohydric behavior as a stress-resistance trait This is a genuine risk: keeping stomata open during drought means leaves can desiccate and xylem embolism becomes more likely. But in environments where droughts are short or unpredictable, this gamble often pays off through higher carbon gain over the growing season.9Plant, Cell & Environment. Differences in osmotic adjustment, foliar abscisic acid dynamics, and stomatal regulation between an isohydric and anisohydric woody angiosperm during drought
Recent genetic engineering work has tried to split the difference. Researchers overexpressed specific ABA receptor genes only in guard cells and found that the modified plants reduced water loss without penalizing leaf growth. Under drought, these engineered lines showed improved water-use efficiency without losing biomass.10New Phytologist. Guard‐cell expression of abscisic acid receptors for engineering water‐use‐efficient plants without trade‐offs in growth That result is promising for crop breeding, because historically, making a plant more conservative with water has usually meant accepting lower yields.
Internal Water Reserves
Plants do not rely entirely on real-time water uptake from soil. Many species store water internally, particularly in their trunks. In temperate broad-leaved trees around 25 to 28 meters tall, stored stem water can supply roughly 10 to 22 percent of daily transpiration in four diffuse-porous species, with an estimated 5 to 12 kilograms withdrawn per day. Ring-porous species like ash trees, by contrast, draw far less from storage.11Tree Physiology. Stem water storage in five coexisting temperate broad-leaved tree species: significance, temporal dynamics and dependence on tree functional traits
Interestingly, the contribution of stored water can actually increase as drought intensifies, even as the total amount stored drops. In Mediterranean oaks, stored water accounted for about 2 percent of transpiration early in the growing season but rose to about 5 percent by late summer, as the trees drew down their reserves to keep functioning during dry conditions.12Plant, Cell & Environment. Stem hydraulic capacitance decreases with drought stress: implications for modelling tree hydraulics in the Mediterranean oak Quercus ilex Stored water is not enough to survive a prolonged drought, but it acts as a buffer, buying the tree time during peak demand.
Some leaves have a physical trick of their own. Rather than cavitate under extreme water stress, xylem conduits in certain leaves collapse inward, deforming instead of filling with air. Researchers imaging xylem conduits in leaves found that as water potential dropped below a certain threshold, conduits became progressively more squashed but very few actually embolized. Out of thousands of conduits examined, embolism was found in fewer than 4 percent even at the driest conditions measured.13Plant Physiology. Reversible Leaf Xylem Collapse: A Potential “Circuit Breaker” against Cavitation Because the collapse is reversible when water returns, this acts like a circuit breaker, protecting the leaf’s plumbing from permanent damage.
Roots That Move Water Between Soil Layers
Root systems do more than absorb water — they redistribute it. In a process called hydraulic redistribution, plant roots transfer water from wetter soil layers to drier ones. This occurs because water follows gradients of water potential, and root systems that span different soil depths become conduits for passive water movement, even when the plant itself is not actively transpiring.14PubMed Central. Roots to the rescue: how plants harness hydraulic redistribution to survive drought across contrasting soil textures
The phenomenon has been documented worldwide across a wide range of ecosystems and species.15New Phytologist. The magnitude of hydraulic redistribution by plant roots: a review and synthesis of empirical and modeling studies In deep-rooted trees, this can be dramatic. Measurements of roots extending to 20 meters deep showed that during drought, deep roots lifted water to shallow layers, while after rain, shallow roots redistributed water laterally among themselves.16Plant, Cell & Environment. Water uptake and hydraulic redistribution across large woody root systems to 20 m depth This benefits the plant by keeping fine absorbing roots in upper soil layers hydrated, but it also affects neighboring plants, soil microbes, and nutrient cycling. A deep-rooted tree acting as a water elevator can effectively subsidize the shallow-rooted plants growing nearby.
Fungal Partners and Water Delivery
About 85 percent of land plants form symbiotic relationships with arbuscular mycorrhizal fungi, which extend thread-like filaments (hyphae) into the soil far beyond the reach of roots alone.17PubMed Central. Battle of Arbuscular Mycorrhizal Fungi Against Drought Stress: A Gateway to Sustainable Agriculture These fungal networks are often discussed in terms of nutrient delivery, particularly phosphorus, but their role in water supply deserves just as much attention.
Experiments with isotope-labeled water have provided direct evidence that mycorrhizal fungi transport water to host plants. In one study, plants grown with fungal partners that had access to a separate water source transpired almost twice as much as plants whose fungi were excluded from that source. Using isotope tracking, the researchers estimated that water delivered by the fungal network accounted for about a third of the water the plant transpired.18New Phytologist. Routes to roots: direct evidence of water transport by arbuscular mycorrhizal fungi to host plants Modeling work supports the broader picture: mycorrhizal symbiosis extends the effective root length and helps maintain water flow at the root surface, delaying the point at which the plant crosses into water stress as soil dries out.19Journal of Experimental Botany. The role of arbuscular mycorrhizal symbiosis in improving plant water status under drought
Drinking Through Leaves
Roots and fungi are not the only way water gets in. Plants can also absorb water directly through their leaf surfaces when dew, rain, or fog wets the crown. Research in California’s coast redwood forests found that 80 percent of the dominant species there, from canopy trees to understory ferns, could take up water through their leaves. Foliar uptake increased leaf water content by 2 to 11 percent depending on species.20PubMed Central. Foliar water uptake: a common water acquisition strategy for plants of the redwood forest In those ecosystems, summertime fog arrives during an otherwise dry season, and leaves that can drink directly from the fog get a water subsidy that roots alone could not provide.
This capacity is not limited to fog forests. Evidence has grown over recent decades that foliar water uptake contributes to ecosystem-level water balances in a variety of native species worldwide.21The Plant Journal. Foliar water and solute absorption: an update Water can enter leaves through both the cuticle (the waxy outer coating) and through open stomata, though the relative importance of each route is still being studied.22Plant, Cell & Environment. Unravelling foliar water uptake pathways: The contribution of stomata and the cuticle
When the Water Runs Out
Drought kills trees, but the exact mechanism has been debated. Two main hypotheses have competed for decades. One says trees die from hydraulic failure: embolism blocks enough xylem that the tree can no longer supply its leaves with water. The other says trees die from carbon starvation: prolonged stomatal closure cuts off photosynthesis, and the tree eventually runs out of stored sugars needed for metabolism and defense.
Careful experiments have shown that both mechanisms can operate in the same species, even in the same drought event. Trees that died quickly during drought showed the classic signs of hydraulic failure: rapid drops in leaf water potential, complete loss of xylem conductivity, and relatively untouched sugar reserves. Trees that died slowly showed both depleted sugar reserves and progressive hydraulic collapse, suggesting the two processes are coupled rather than competing.23PubMed Central. How do trees die? A test of the hydraulic failure and carbon starvation hypotheses Field observations of adult conifers have further confirmed that hydraulic collapse can happen in a sudden, nonlinear fashion, commencing at surprisingly low levels of embolism and escalating rapidly to fatal failure.24Proceedings of the National Academy of Sciences. Rapid hydraulic collapse as cause of drought-induced mortality in conifers Gymnosperms (conifers and their relatives) tend to maintain wider hydraulic safety margins than flowering trees, which may reflect their greater vulnerability to accumulating embolism once it begins.25Ecology Letters. Optimal balancing of xylem efficiency and safety explains plant vulnerability to drought
Pathogens That Attack the Plumbing
Drought is not the only threat to a plant’s water supply. Vascular wilt diseases, caused by soil-borne fungi, bacteria, and oomycetes, invade through the roots and colonize the xylem, where they proliferate and physically block water flow. The result is wilting and, in severe cases, plant death.26PubMed Central. The xylem as battleground for plant hosts and vascular wilt pathogens
Plants fight back by forming physical barriers inside the vessels. Tyloses, balloon-like outgrowths from neighboring cells, and gel deposits plug infected vessels to prevent the pathogen from spreading further. But these defenses are a double-edged sword: the same structures that wall off the pathogen also block water flow in the sealed vessels.27PubMed Central. Blocking intruders: inducible physico-chemical barriers against plant vascular wilt pathogens In avocado trees infected with laurel wilt, tyloses were found in 55 to 60 percent of xylem vessels, compared with 1 percent or less in healthy controls.28Tree Physiology. Sap flow, xylem anatomy and photosynthetic variables of three Persea species in response to laurel wilt The tree’s own immune response, while necessary, compounds the water-transport problem the pathogen started.
Farming With Less Water
Understanding plant water relations has practical consequences in agriculture, where irrigation accounts for a huge share of freshwater use globally. One strategy that has gained traction is deficit irrigation: intentionally giving crops less water than they would use under unrestricted conditions. The idea is that plants can tolerate moderate water stress without proportional yield losses, and the water saved can be redirected.
Field trials with maize illustrate the tradeoff. In one study, providing about 70 percent of the crop’s full water requirement produced a roughly 13.5 percent yield penalty but improved irrigation water-use efficiency by about 23 percent and saved 30 percent of the water.29Frontiers in Agronomy. Optimizing water use efficiency in maize (Zea mays L.) production through deficit irrigation in Gazhen-Fuafuat Kebele, Northwest Ethiopia Another study found that applying 20 percent less water from a key growth stage onward actually produced the highest yield of any treatment, including full irrigation.30Agricultural Water Management. Deficit irrigation improves maize yield and water use efficiency in a semi-arid environment These results are not contradictory — timing and severity of the deficit matter enormously. Mild stress at the right growth stage can trigger the plant to invest more in grain rather than vegetative growth, a response breeders and agronomists can exploit.
Coping With Flood and Salt
Too much water presents the opposite problem. When soil is waterlogged, oxygen cannot reach root cells, and aerobic metabolism shuts down. Some plants cope by forming aerenchyma, internal air channels that allow oxygen to diffuse from above-water parts of the plant down to submerged roots. In soybean, flooding triggered rapid formation of adventitious roots and aerenchyma through the stem and taproot within days, with root porosity increasing substantially between the fourth and seventh days of flooding.31PubMed Central. Aerenchyma Formation and Recovery from Hypoxia of the Flooded Root System of Nodulated Soybean
Saline environments pose yet another challenge. Even when water is physically present in salty soil, it is harder for roots to extract because dissolved salts lower the soil water’s potential energy, effectively making the plant “thirstier.” Halophytes, plants adapted to high-salt habitats like salt marshes, have evolved multiple strategies to deal with this: salt glands that excrete excess ions, cellular mechanisms to keep salt concentrations in check, and enhanced detoxification of the reactive oxygen species that salt stress produces.32PubMed Central. Mechanisms of Salt Tolerance in Halophytes: Current Understanding and Recent Advances These are not minor tweaks. Halophytes have fundamentally restructured their water-handling systems to function in conditions that would kill most crop species. Understanding how they do it is one of the active frontiers in developing salt-tolerant crops for a world where arable land is increasingly affected by salinization.