The mere fact that plants need water appears much obvious that most gardeners never pause to ask themselves why they do.
Even though this may seem like a question any 3-year old would ask his parents, it is nonetheless an interesting and very important question.
Just like animals, plants need water for several reasons. Staying alive is the primary reason why humans need water, and plants are not an exception either.
Factually, plants do differ from animals in several ways. One-way being that they require and use more water than animals do as they are made up of up to 90 percent water.
Before outlining the main reasons, it is important to note that the amount of water plants need to depend on a number of factors.
Among them include the type of plant, amount of light exposed to the plant and the age of the plant. That said, some of the principle reasons why plants need water include the following.
1. To stay upright
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As mentioned before, plants wilt if they don’t access water in the required amounts.
This is due to turgor, an effect resulting from the action of water pressure inside the cells of the plant, something that makes up its skeleton.
As you may know, water enters the plant through the roots and travel all the way to its leaves.
A properly hydrated plant has enough water pressure to make the stems and leaves sturdy and strong.
Lack of sufficient water reduces the turgor pressure inside the stems and leaves, resulting in wilting. This explains the observation made on the behavior of plants that haven’t been watered regularly.
The enzymes that orchestrate the germination process in plants require water to activate.
Absorbed water causes the hard outer shell of seedlings to soften, swell and sometimes crack, making it possible for the growing plant to break through and germinate.
Plants require a variety of conditions such as cold, warmth, fire, humidity and frost to germinate. However, above all, water is definitely the most important of all.
Further adding to this, the first part of the plant that breaks through is the root. It digs downwards to find water that enables aerial growth of the plant. This clearly depicts the young plant’s need for water/moisture immediately for the germination process.
By definition, photosynthesis is the process by which plants use energy absorbed from sunlight to produce sugars.
Plants use a combination of light energy, water, and carbon dioxide to produce foods, something key for its survival. Each of the elements mentioned prior, including nutrients absorbed from the soil, are essential for the plant to grow.
During this process, water molecules are used at different stages. There are two main stages of photosynthesis, and all require water. The first stage, also known as light-dependent phase, absorbed water is broken down to oxygen, a process called water photolysis.
In the second stage, the light-independent phase, carbon dioxide absorbed from the atmosphere is combined with water to form sugars/glucose.
Startlingly, water used in photosynthesis represents a minor fraction of water used by a plant during the day. This is because a huge percentage of water moving through the plant during the day is lost by transpiration.
Transpiration represents water lost through the tiny pores found in the leaves, known as stomata. As a normal process, the pores should open to allow for free entry of carbon dioxide. The flip side of this being increased water loss through transpiration.
As mentioned above, transpiration is the process through which water absorbed through the roots, up through the stem is lost as water vapor.
On the other hand, evaporation, a term often confused with transpiration, refers to the transformation of water found in the environment to vapor. Plants need to “sweat” because of the following three reasons.
- The counter-current process allows intake of carbon dioxide from the air.
- It cools the plan, especially during warm summer heats.
- It causes water and nutrients to flow throughout the plant thus feeding and hydrating it.
5. For nutrient transfer
Through transpiration, water becomes a necessary medium for the transfer of nutrients absorbed from the soil through the root system to the plant’s leaves.
Without water, soil nutrients absorbed cannot reach the leaves. This explains the reason why plants not regularly watered not only die of thirst but also hunger.
How Plants Absorb Water
Though it may seem pretty obvious, it entirely true that plants absorb water only through roots. Plants absorb water through its entire surface, including the roots, stems, and leaves.
Nonetheless, majority of water in a plant is absorbed through the root hairs. These are thin-walled single-celled outgrowth of the root epidermis.
Due to their growth, root hairs are in close proximity to the water film surrounding soil particles.
The cell wall of root hair is permeable to water and minerals. However, the cell membrane and vacuole membrane form a semi-permeable membrane. The soil solution is lowly concentrated compared to cell sap in the root hair, allowing water to be absorbed through osmosis.
With the absorption of water, the root hair cells become turgid, and its osmotic pressure falls. The cells of the cortex, in turn, have higher osmotic pressure, causing the diffusion of water from the root hair cells to cortical hair cells.
It is through this mechanism that water in plants moves into deeper cortical cells to the endodermis of the root under an osmotic concentration gradient. Upon reaching the endodermis of the root, water is forced into the xylem tubes through plant’s passage cells.
This describes the pressure that forces water into the xylem tubes. Due to this pressure, the water column sent is maintained up to a certain height. This whole system and mechanism is known as active absorption of water.
This plays a minor role in tall trees. The main and most efficient mechanism through which rooted plants absorb water is by passive absorption.
This occurs without utilization of metabolic energy. In this mechanism, roots act as an organ of absorption and passage. This is perhaps the reason why this is often referred to as water absorption “through roots,” rather than “by roots”.
It occurs rapidly in transpiring plants during the daytime, probably because of the opening of stomata and atmospheric conditions.
The force of water absorption is created at the leaf end, often referred to as transpiration pull. Loss of water through the leaf, transpiration, creates a dragging force from the leaf end.
The force is transmitted down the root through the water column in the xylem tubes. The force of continuity is maintained by cohesion forces between the water molecules, acting as a rope.
In passive absorption of water, roots simply act as a passive organ of absorption. As transpiration proceeds, water absorption occurs simultaneously to compensate for water loss from the leaf end.
Water is an essential limiting non-living/abiotic factor that affects plants growth. It is also a key principal determinant of vegetation distribution worldwide.
The importance of water to plant’s stems from its central role in photosynthesis, growth and the distribution of both organic and inorganic molecules.
The reasons as to why plants need water lies solely on the role of water in photosynthesis. To make glucose, plants must absorb carbon dioxide from the atmosphere.
The balance between transpiration and photosynthesis is the essential compromise as to the existence of plants.