Understanding and managing water stress in plants
From north to south, from east to west, July poses the same question to agriculture: how fragile has the balance between climate, water and agricultural production become?
In the Po Valley, as in the inland areas of Central Italy, the fruit and vegetable production districts of Southern Italy and the islands, July is generally one of the most critical periods of the year from an agronomic perspective. The natural water reserves stored in the soil during winter and spring are already almost entirely depleted, while solar radiation, temperatures and atmospheric evaporative demand reach high levels. At the same time, many crops are at flowering, fruit set, fruit growth or grain-filling stages: phenological phases in which even a relatively short period of stress can have significant consequences for production.
Each summer makes this vulnerability even more evident. Year after year, June and July continue to rank among the hottest months ever recorded in many parts of Europe, while drought conditions are worsening across much of the continent. Drought, extreme heat and increasing competition for water resources are affecting agricultural systems on every continent, while emerging climate risks continue to threaten crops, pastures and food security.
If crops produce less, and of lower quality, how will we be able to ensure a sufficient supply of nutritious and affordable food?
Understanding water stress in plants
Water stress is a physiological condition in which the plant’s water status is no longer compatible with the normal functioning of its physiological processes. It occurs when the availability or movement of water through the soil–plant–atmosphere system is insufficient to maintain an adequate water potential. The plant then activates a series of responses aimed at limiting water loss and protecting its tissues; if the condition becomes severe or prolonged, photosynthesis—and consequently growth and yield—will be impaired.
In agronomic terms, “water stress” generally refers to water-deficit stress, a condition in which water supply does not meet the plant’s requirements. Waterlogging, on the other hand, represents excess-water stress and, by reducing oxygen availability in the soil, can lead to root asphyxia.
Why a plant becomes water-stressed
Several factors often act at the same time, making it necessary to assess the cropping system as a whole. A plant does not come under stress because of a single cause, but because the factors that maintain the balance of the entire cropping system begin to break down. Let us look more closely at what can contribute to the onset of this physiological condition.
Before doing so, however, it is worth asking a broader question: if climate change, as a result of human activities, is making our fields increasingly difficult to manage, can we simply keep “dealing with the damage” without changing behaviours, production models and political decisions?
Insufficient water availability in the root zone
Water in the soil is not necessarily water available to the crop.
If there is water at the bottom of a well but no lifting system capable of reaching it, the water resource is present but cannot be used. Something similar happens in the soil: some water may be located too deep, some is retained by the soil matrix, and some lies outside the volume explored by the roots.
From an agronomic perspective, plant-available water lies between field capacity (the maximum amount of water a soil can retain by capillary forces against gravity) and the permanent wilting point (the minimum soil moisture level below which plants can no longer extract sufficient water). However, stress can begin before this limit is reached: as the soil dries, the remaining water is held more strongly by colloidal particles and moves more slowly towards the roots.
Soil texture also makes a difference:
- sandy soils retain relatively little water, and their water reserve can decline rapidly between one irrigation or rainfall event and the next;
- clayey soils retain larger amounts of water, but part of it remains difficult for roots to absorb;
- in medium-textured soils, the balance between water retention and plant availability is generally more favourable.
High evaporative demand and VPD
On very hot, dry and windy days, the atmosphere “demands” water faster than the roots can supply it to the canopy, and plants may experience water stress even when there is still water in the soil.
This condition can be described using VPD, or vapor pressure deficit. In practical terms, it indicates how strongly the air can promote water loss from the leaves: the hotter and drier the air, the higher the VPD and the greater the evaporative pressure on the plant.
It is a bit like laundry drying outdoors: in humid air, it dries slowly; in hot, dry and windy conditions, it loses water much more quickly. The same principle applies, figuratively, to leaves.
For this reason, during the hottest hours of the day, a crop may become water-stressed before the soil has actually exhausted its available water reserve.
The following graphic shows what changes for the plant under low versus high VPD conditions.
Insufficient or compromised root system
A crop’s ability to use soil water depends on the extent, depth and functionality of its root system. Young roots, fine roots and root hairs play a particularly important role in water uptake: when their development is limited, the volume of soil effectively explored by the roots is reduced, and the amount of water available to the plant decreases.
Soil compaction, plough pans and low porosity can restrict root growth and confine roots to the upper soil layers, which are more exposed to drying. Mechanical damage, root diseases and prolonged waterlogging can also reduce root activity. Under oxygen-deficient conditions, limited oxygen availability restricts cellular respiration and the energy required for absorption processes.
A plant with a compromised root system may therefore experience water stress even in moist soil. In such cases, simply increasing irrigation does not necessarily solve the problem and may even make it worse when the underlying cause is poor soil aeration.
The BioAksxter® depolluting fertilizer promotes soil oxygenation and rebalancing, creating favourable conditions for root system development and helping reduce root vulnerability to asphyxia, rots and soil-borne pathogens.
Salinity and osmotic stress
In saline soil, water may be present but more difficult for the plant to absorb. A high concentration of salts lowers the osmotic potential of the soil solution: to take up water, the roots must therefore develop an even lower water potential. This places greater physiological demand on the plant, which may show symptoms similar to those of water deficiency even when the soil appears moist.
For more information on the causes, effects and management of salinity in agricultural soils, see the dedicated article published in BioAksxter Magazine.
Irrigation management errors
A functioning irrigation system does not necessarily ensure that water reaches active roots at the right time and in the required amounts. Irrigation intervals that are too long can allow the readily available water reserve to become depleted, while frequent but insufficient applications may wet only the upper soil layers, encouraging shallow root development and making roots more vulnerable to temperature fluctuations and drying.
Excessive irrigation volumes can also create problems. In poorly drained soils, water fills pores that would normally contain air, reducing oxygen availability and impairing root function. The plant may then show symptoms similar to water stress, not because water is lacking, but because poor oxygenation compromises root function and therefore water uptake.
Poorly positioned drippers, uneven pressure, emitter clogging and differences in soil texture can also create over-irrigated areas alongside zones that receive too little water. Scheduling irrigation solely according to the calendar, without considering climate, rooting depth, soil characteristics and crop phenological stage, therefore increases the risk of alternating between water deficit and excess.
What happens inside the plant during water stress
The first responses to water stress generally begin before the crop shows any visible symptoms: the plant detects changes in its water status and progressively alters its growth, gas exchange and metabolism.
From loss of turgor to hydraulic and chemical signals
As the soil dries or atmospheric demand increases, the plant’s water potential becomes progressively more negative. One of the first processes to slow down is cell expansion, because tissue growth depends on the turgor pressure generated by water inside the cells.
However, the plant does not wait until wilting occurs before responding. Changes in water availability are perceived both by the roots and by the above-ground organs and are transmitted through hydraulic and chemical signals. These signals coordinate tissue responses, slowing growth and activating the mechanisms that regulate gas exchange.
For this reason, a crop may already have undergone substantial physiological changes while still appearing visually normal.
Abscisic acid and stomatal regulation
One of the main signals involved in the response to water deficit is abscisic acid, or ABA, a plant hormone whose concentration increases as the plant’s water status deteriorates. ABA acts on the guard cells surrounding the stomata, altering their ionic balance: the cells lose water and turgor, leading to a progressive reduction in stomatal opening.
Stomatal closure is a defence response. By reducing the exchange between the internal air spaces of the leaf and the atmosphere, the plant limits transpiration and slows water loss. This regulation can begin before wilting becomes visible and helps temporarily protect the integrity of the vascular system. However, stomatal closure also carries a physiological cost, because it restricts the entry of carbon dioxide required for photosynthesis.
Effects on transpiration and photosynthesis
Reduced stomatal opening limits transpiration and allows the plant to conserve water. At the same time, however, evaporative cooling of the canopy decreases, so during the hottest hours leaf temperature may rise compared with that of a well-watered crop.
Reduced stomatal opening also restricts the entry of carbon dioxide into leaf tissues, progressively lowering photosynthesis and the production of organic compounds required for growth. As a result, the plant has fewer assimilates available for vegetative development, fruit growth and storage organs.
If the stress is moderate and temporary, these changes may be largely reversible. When stress becomes severe or prolonged, stomatal limitation may be accompanied by damage to cellular structures, resulting in a persistent reduction in productive capacity.
For further information on this topic, see the BioAksxter Magazine article “How to improve photosynthetic efficiency in plants?”
Changes in nutrient uptake and transport
Water is the medium through which nutrients dissolved in the soil solution reach the roots and are transported to the above-ground organs. As the soil dries, nutrient mobility decreases and their movement toward the root surface becomes slower. Water stress also reduces the roots’ uptake capacity and the flow of sap.
As a result, the crop may show slower growth or symptoms resembling nutrient deficiency, even when the soil itself is not necessarily deficient in nutrients. Under these conditions, increasing fertilization without correcting the water imbalance is ineffective and increases salt concentration in the rhizosphere.
When an adaptive response becomes damage: oxidative stress and cellular alterations
During water stress, the production of reactive oxygen species (ROS) increases. In controlled amounts, these molecules are involved in defence signalling; however, when their production exceeds the capacity of the plant’s antioxidant systems, oxidative stress develops.
Excess ROS can damage cell membranes, alter proteins and enzymes, and impair the function of chloroplasts and the photosynthetic apparatus. The plant responds by activating antioxidant enzymes and other protective compounds, but the effectiveness of these defences depends on the intensity and duration of the stress.
If the water deficit persists, these alterations become progressively less reversible: tissue senescence increases, recovery capacity declines and, in the most severe cases, cell death occurs. This is the point at which an initially protective response turns into physiological and productive damage.
Soil pollution and water stress: when stresses combine
Scientific literature shows that soil pollution interferes with the processes that regulate plant water relations. A review published in Acta Physiologiae Plantarum reports that toxic concentrations of metals can reduce primary root elongation, impair root hair development and decrease the absorbing surface area. The same contaminants can also slow water transfer to the vascular system and reduce the hydraulic conductivity of roots, stems and leaf veins, thereby limiting water supply to the above-ground parts of the plant.
When water stress occurs alongside pollution, the effects can become even more complex. A 2026 review published in Plants, specifically addressing the interaction between heavy metals and other abiotic stresses, highlights that combined conditions can produce cumulative or synergistic effects on plant physiology, biochemistry and development.
Experimental findings support this interaction: in wheat simultaneously exposed to cadmium and water deficit, reductions in biomass and photosynthetic activity have been observed together with an increase in oxidative stress. Other studies on wheat have also shown that limited water availability can increase cadmium accumulation in the grain.
How to recognize water stress in agricultural conditions
A stressed plant rarely goes from a “normal” condition to sudden wilting. It is a bit like a car displaying its first warning lights before coming to a stop: recognizing the early signs makes it possible to intervene before stress develops into yield loss.
From early signs to visible symptoms
Among the earliest signs are reduced leaf expansion, changes in leaf orientation, increased canopy temperature and slower growth. At this stage, the crop may still appear normal even though transpiration and photosynthesis have already declined.
As the plant’s water status deteriorates, the following symptoms become visible:
- loss of turgor during the hottest hours
- leaf rolling or folding
- slower growth of shoots, fruits or grain
Under more severe stress, persistent wilting, premature senescence, fruit drop, marginal necrosis and tissue desiccation may occur.
Reversible and irreversible water stress
Moderate, short-term stress can be largely reversible: when atmospheric demand decreases or water availability is restored, the plant can recover turgor and physiological activity.
Visible recovery, however, does not always correspond to a complete recovery in productivity. If the stress occurs during flowering, fruit set or the growth of reproductive organs, part of the damage may persist even after rehydration.
When the water deficit is severe or prolonged, alterations to the photosynthetic apparatus and vascular tissues progressively reduce the plant’s ability to recover. Fruit drop, flower abortion, necrosis and cell death are instead irreversible consequences.
The rate at which the deficit develops is also important. A gradual decline in water availability may allow the plant to activate adaptive responses, whereas a rapid and severe deficit leaves less time to react. Repeated episodes may also have cumulative effects, reducing recovery capacity even when each individual event appears relatively mild.
Effects of water stress on major agricultural crops
| Crop | Particularly sensitive growth stages | Main agronomic consequences |
| Grapevine | Flowering, fruit set and the early stages of berry growth | Reduced fruit set and berry size, looser clusters and lower bud fertility for the following season, together with possible interruption of berry ripening. Grapes may also show poorer quality parameters, including an abnormal increase in must pH and a loss of acidity and typical aromas. |
| Olive | Pre-flowering period, fruit set, drupe growth and oil accumulation | Reduced fruit set, fruit drop, smaller olive size and, when stress occurs during the oil accumulation phase, lower oil accumulation. The chemical and sensory characteristics of the final product may also be impaired. |
| Citrus | Flowering, fruit set and fruit growth | Increased fruit drop, fewer fruits reaching maturity and reduced marketable fruit size. Crop quality may also be affected, with an abnormal increase in peel thickness and a lower juice percentage. |
| Tomato | Flowering, fruit set and berry growth | Flower abortion, irregular fruit set, slower fruit growth, and reduced yield and production uniformity. Quality parameters may also be negatively affected, with poor colour development and fruit cracking. |
| Maize | Tassel emergence, silk emergence, fertilization and the beginning of grain filling | Poor synchronization between male and female flowering, incomplete fertilization, fewer kernels and lower grain weight. Quality parameters may also be negatively affected, with a sharp increase in mycotoxin contamination, reduced starch content, increased protein concentration and lower grain digestibility. |
| Wheat | Stem elongation, booting, anthesis and grain filling | Reduced spike fertility, fewer kernels and lower individual grain weight. Quality parameters may decline sharply, leading to grain downgrading and poorer gluten quality, with consequences for bread-making and pasta-making performance. |
| Potato | Beginning of tuber initiation and tuber growth | Reduced tuber number and size, uneven maturation and loss of marketable yield. Quality may also deteriorate, with tuber cracking and skin splitting, lower dry matter and starch content, brown or rust-coloured flesh spots, internal fissures and black bruising following minor mechanical impacts. |
How to manage water stress in the plant–soil system
Managing a crop under water stress is a bit like dealing with a tractor that is losing power: filling the tank is not enough if the fuel cannot reach the engine or if the system is unable to sustain its operation. In the same way, increasing irrigation is not sufficient when the cropping system is already affected by soil degradation, biological imbalances and increasingly intense climatic pressures that reduce the crop’s ability to use available water efficiently.
Prevention must begin with a balanced agronomic environment, in which the physical, chemical and biological components of the soil–plant system can interact properly.
Increasing plant resilience to water stress
The BioAksxter® decontaminating fertilizer helps rebalance the plant–soil system by supporting the continuity of the processes that regulate soil functionality, root development and the crop’s physiological response. The focus is therefore not only on water availability, but also on the conditions that enable the plant to absorb and use water effectively (plant reprogramming). BioAksxter® does not replace proper irrigation management: it creates the conditions in which the soil can regulate water resources more effectively.
Managing water stress is not only a concern for individual farms, but for the global agricultural system as a whole. If agriculture is being forced to adapt every year to increasingly extreme conditions, when will governments, businesses and citizens truly begin to address the causes that are making those conditions more severe?
“It is time to wake up… and not because it is morning.”
