Water is the lifeblood of all living things, and is undoubtedly vital for plants. As the foundation of life on Earth, water plays an essential role in sustaining plant health, supporting growth, and enabling plants to thrive.

World Water Day is an international initiative that highlights both the importance of water and its growing scarcity. Climate change, rising temperatures, and altered rainfall patterns are making water availability increasingly challenging for plants. On this day, we want to emphasize the essential role water plays in plant life and the importance of preserving this invaluable resource.

What are the critical roles of water for plants ?

Nutrient transport

Water is the primary medium through which plants absorb nutrients and minerals from the soil, delivering them to plant cells to support growth and development.

This process begins at the roots, where water and dissolved minerals are taken up.

Water enters root cells through osmosis, a natural process in which it moves across a semi-permeable membrane.

Meanwhile, essential nutrients are absorbed through passive or active transport mechanisms, depending on their concentration gradients—the difference in concentration between the soil and plant roots.

• Passive transport occurs when the soil has a higher concentration of nutrients than the plant cells. In this case, the plant does not need to expend energy: water flows in naturally via osmosis, and some nutrients enter by diffusion. 
• In contrast, active transport is necessary when the soil has a lower concentration of nutrients than the plant cells. This process requires energy, as the plant powers specialized cellular "pumps" to move essential nutrients like potassium and nitrogen into the roots. 

By combining these transport systems, plants efficiently absorb the resources they need for healthy growth, even in nutrient-poor soils.

Once inside the roots, water is transported upwards through a specialized tissue called the xylem. This movement occurs thanks to three main forces:

• root pressure  : as more water enters the roots, internal pressure pushes it upward.
• capillary action : the natural adhesion of water molecules to the walls of the xylem helps pull water upward.
• transpiration pull : as water molecules exit through tiny pores called stomata, they pull neighboring water molecules upward, creating a continuous flow from roots to leaves.

If you are curious to learn more about irrigation and nutrition, see our latest article on fertigation : https://www.orius.co/blog/optimizing-plant-growth-the-power-of-fertigation 

Photosynthesis

Water is indispensable for photosynthesis—the vital process through which plants convert light into energy, helping them grow, thrive.

Contrary to what is sometimes thought, all the oxygen released by plants comes from the photolysis of water during photosynthesis and not from the absorbed CO₂.

Water molecules (H₂O) are split into oxygen (O₂), protons (H⁺), and electrons (e⁻) by the enzyme photosystem II (PSII).

The electrons are transferred through the electron transport chain (ETC) to generate ATP and NADPH, which power the Calvin cycle.

Ruben and Kamen’s (1941) experiment confirmed this phenomenon (1). They used water labeled with the isotope ¹⁸O (H₂¹⁸O) and CO₂ containing normal oxygen (¹⁶O).

They demonstrated that the oxygen released during photosynthesis contained the ¹⁸O isotope, proving it originated from water.

Cooling through evapotranspiration

Water also cools plants via a process called evapotranspiration.

This process helps plants to regulate temperature and water balance. 

Transpiration takes place in the leaves of the plant, at the level of the stomata, small retractable openings. 

It is influenced by multiple factors : external temperature, ambient humidity, sunlight and atmospheric radiation, wind speed, soil characteristics, available water, etc. In the example of ambient humidity : the drier the air, the greater the amount of water released by the plant. 

A huge proportion of the water absorbed by the plant will be released.  

For example, an adult oak tree can transpire up to 1,000 liters of water per day, 75 liters of water for a birch tree (2) 

Growth and plant structure

Water under pressure in plant cells, also known as turgor pressure, is one of the key elements that helps plants maintain their shape and rigidity.

This pressure is generated by water absorption in plant cells, causing them to expand and create internal pressure against the cell wall.

This pressure is essential for :

• Cell growth: turgor pressure is a key factor in cell elongation and division. It allows plant cells to stretch and grow in size, contributing to the overall growth of the plant.
• Plant rigidity and shape: water under pressure within cells maintains rigidity of the plant tissues. This is especially important for non-woody plants and young tissues, which rely on turgor pressure to maintain their form and structure.

Why do plants expel water as droplets on the leaves ? 

This phenomenon is called guttation. Guttation is mainly caused by excessive humidity, leading to an imbalance in root pressure.

If the plant absorbs more water than it can expel due to high ambient humidity and freshly watered soil, it cannot transpire enough water. This results in an imbalance in root pressure, which rises. As a consequence, the water in the leaf does not have enough time to evaporate and is expelled as droplets, similar to an overflow.

If plants did not possess this guttation mechanism, in young tissues, where transpiration rates are lower, they would not receive essential nutrients through water circulation. Calcium, in particular, which is critical for cell wall stability and signaling processes, would not be effectively supplied. Guttation allows plants to maintain calcium homeostasis - a regulatory process by which the plant maintains the various constants of the internal environment - in all tissues.

Guttation is therefore not only a sign of a fully functional root system but also a crucial process for providing non-remobilizable nutrients - elements that, once incorporated into plant tissues, are not easily reallocated to other parts of the plant, such as Calcium -  which are essential for plants.

Guttation is different from what is called dew. Dew is a phenomenon where atmospheric moisture condenses on cooler surfaces, typically due to nocturnal radiation for plants. Dew is pure water, whereas guttation, which is moisture secreted by the plant itself, can contain other compounds like sap and nutrients. 

What are the consequences of inappropriate water quantity and quality ?

Inappropriate water quantity and quality can have significant negative effects on plants.

Water Scarcity (poor quantity)

When plants receive insufficient water, they experience drought stress, which can hinder their growth, reduce yield, and, in extreme cases, cause the plant to die. Insufficient water also impedes the plant’s ability to carry out essential processes like nutrient absorption, photosynthesis, and transpiration. This can result in wilting, leaf drop, stunted growth, and poor development of flowers and fruits.

Waterlogging (excessive quantity)

On the other hand, excessive water can lead to waterlogging, where the soil becomes saturated and lacks the oxygen necessary for healthy root function. Waterlogging is mainly an issue because it deprives roots of oxygen, causing root rot, reduced nutrient uptake, and, ultimately, plant death. Waterlogged conditions also favor the growth of harmful pathogens (like mushrooms and bacteria), further stressing the plant.

Poor water quality

If the water is contaminated with pollutants like salts, chemicals, or heavy metals, it can lead to water toxicity. High salt levels, for example, can result in salt stress, where plants struggle to absorb water due to osmotic imbalance. This reduces their ability to grow and can cause leaf burn, wilting, and nutrient deficiencies. Additionally, contaminated water can introduce harmful microorganisms, which may infect plants and affect their quality.

How do we manage water at Orius ?

At Orius, we understand the precious value of water. We grow our plants in controlled environments, where every drop counts. We manage water and nutrients in a closed-loop system.

We capture and recycle water from evapotranspiration, reusing it to nourish our plants in an endless water cycle.

This process allows us to capture 100% of the water rejected by our plants, resulting in a 95-98% water savings compared to conventional farming methods.

As you may know, we are developing cultivation methods and systems to grow plants in future space stations, in collaboration with CNES. Water usage is a critical issue in such extreme environments.

As part of our ongoing experiments, we cultivated wheat in our systems which use 95% less water compared to conventional farming :

• on average, worldwide, 1,827 liters of water are used to produce 1 kg of wheat (4)
• at Orius, we managed to use less than 100 liters of water to produce 1 kg of wheat (5)

We use 2 main types of irrigation systems depending on the crop being cultivated and the expected outcomes. 

Ebb&Flow

Ebb&Flow is a hydroponic system where plants' roots are periodically flooded with a nutrient solution and then drained, allowing the plants to absorb nutrients and oxygen. The cycle of flooding (the "flow") and draining (the "ebb") is repeated at regular intervals.

Aeroponics

Aeroponics is a method of growing plants where their roots are suspended in the air and misted with water and nutrients.

In both systems, we manage water (and nutrients) very precisely using our Mixmaster, an intelligent liquid fertilizer dosing system designed for closed-loop fertigation management. We will measure and deliver exactly the water and nutrients the plant needs, thereby preventing resource waste and avoiding groundwater or air pollution. 

In open fields, when crops are fertilized with water and fertilizers, a significant portion is not used by the plant. Generally, more than 50% of the Nitrogen applied to crops is not assimilated by plants  (6). The rest may seep into the soil and reach groundwater or volatilize into the air.

Orius Mixmaster will also allow us to customize nutrient recipes by managing the ratio of different elements added to the tank.

We can adapt irrigation and nutrition to the plant’s specific needs at each stage of its development.

Curious to find out more about the Mixmaster? Download the product description

Conclusion

Water plays a central role in plant development. It is not so much a “fuel” for the plant, as one might think, but rather a crucial element that allows for the efficient delivery of nutrients to all parts of the plant. It also regulates its temperature, maintains its structure, and facilitates the growth of its cells. Water also participates in the process of photosynthesis, through which plants convert light into energy, allowing them to grow and thrive while releasing oxygen in the atmosphere.

Bibliography

(1) Ruben, S., Randall, M., Kamen, M. & Hyde, J. L. Heavy Oxygen (O18) as a Tracer in the Study of Photosynthesis. Journal of the American Chemical Society 63, 877–879 (1941)

(2)https://www.onf.fr/vivre-la-foret/%2B/14a1::le-pouvoir-des-arbres-levapotranspiration.html#:~:text=La%20quantit%C3%A9%20d'eau%20rejet%C3%A9e,d'eau%20pour%20un%20bouleau. 

(3) Broughton, K. J. & Conaty, W. C. Understanding and Exploiting Transpiration Response to Vapor Pressure Deficit for Water Limited Environments. Front. Plant Sci. 13, (2022).

(4) Mekonnen, M. & Hoekstra, A. A global and high-resolution assessment of the green, blue and grey water footprint of wheat. Hydrology and Earth System Sciences 14, (2010).

(5) internal datas Orius

(6) Tilman 2002, Dobermann 2004