TL;DR : Plants are almost entirely made of water, they also need it for photosynthesis, to carry nutrients, to exchange for carbon dioxide, for turgidity… Please don’t forget to water them !
The fact that plants need water seems so obvious to us that most gardeners never stop to ask themselves why? Even though it seems like the type of question a curious 4 year old would ask his parents (you might even be here because that is exactly what happened), it is nonetheless a very important and interesting question. Understanding the way a plant works in detail is the best way to get to know it better. If you know how your plant works you’ll be able to easily prevent most ailments and you’ll know how to treat it in case one gets by you. The more you know about plants the more likely you are to become well-versed in the art of maintaining a luscious miniature forest in your garden all year long. So let’s start understanding our green friends with this simple question: why do plants need water?
Just like us, plants need water several reasons. Living things need water (most of the time in rather larger quantities) to stay alive, and plants are, after all, living things. Plants do differ from animals in a few ways. One of them being that plants need much more water as they use much more water than most animals do. Plants also contain more water than animals. Most plants are made up of around 90% water. Humans, for example, contain a much smaller (nonetheless large) portion of water (around 60% in adults). Without proper hydration plants will just shrivel up and die. But what exactly do they use water for?
The amount of water a plant needs depends some factors such as : the type of plant, the amount of light the plant is getting, and its age. Some plants, such as water hyacinths and rice, need so much water over the course of a day that they need to have their roots standing in water in order to get enough. The most common cereals or potatoes are more likely to need watering once a day in normal conditions (of course if they are under the scorching sun it’s a different story), whereas cacti only need water every two to three days and some plants can even last throughout much longer periods of drought without dying.
To stay upright
When plants are not watered regularly enough or cannot access water easily and in large enough quantities they wilt. This is because of something called turgor (or turgidity), which is the result of the action of water pressure inside the cells that make up the plant’s skeleton. Water enters a plant through its roots and travels all the way up to its leaves. When a plant is properly hydrated, there is enough water pressure to make the leaves and the stems strong and sturdy; when a plant doesn’t get enough water, the pressure inside the stems and leaves drops and they wilt.
This is why you often see your plants all droopy if you haven’t watered them regularly. Don’t panic when this happens, plants have a remarkable capacity to recover from this “droopiness”. Just re-water lightly and watch as you plants return (sometimes in a few minutes, sometimes in a few hours) to fully upright and the leaves become normally stiff again.
It is important to note that different plants have different reactions to lack of water but this is the one you see most regularly.
Germination
Most seeds need water to activate the enzymes that orchestrate the germination process. Absorbed water usually causes the seed’s hard shell to swell, soften, and sometimes crack which makes it possible for the plant to break through.
Depending on the species different things (cold, frost, warmth, humidity, fire, animal interaction,…) are needed for a seed to germinate, but water is definitely the most important.
When plants germinate, the first thing that pushes out is the root, that digs downwards to find water to enable the aerial parts to expand upwards. This highlights the young plants’ need for water immediately after germination.
If you want to learn more about germination you should definitely read more about it.
Photosynthesis
Photosynthesis (literally “light” and “putting together” or “synthesizing”in ancient Greek) produces food for the plant by combining light energy, carbon dioxide and water to produce sugars. Each of these elements (along with nutrients from the soil) are needed in order for the plant to grow. During photosynthesis, water molecules are used up at differents stages of the process. Firstly, during the “light dependent phase”, water is broken down into oxygen in a reaction called water photolysis. Then, during the “light independent phase”, carbon dioxide is fixed from the atmosphere, and transformed into sugar (glucose), and this phase also uses up water in the process.The simplified chemical reaction can be broken down as follows: 6CO2 + 6H2O => C6H12O6 + 6O2. So plants need water for chemical reactions during photosynthesis.
Surprisingly, this represents only a minor fraction of the water used during the day because of photosynthesis. Most of the water that moves through the plant is simply lost by transpiration through the tiny pores in the leaves called stomata. These pores have to open to allow entry of carbon dioxide, the flip-side of this being increased water-loss via transpiration.
Transpiration
Often confused with evaporation, transpiration is the process of water being pulled through the roots, up the stem and out of the plant through the leaves as water-vapor. Evaporation on the other hand describes the transformation to vapor that is present in the environment (on the leaves or in the soil after rain for example) . Incidentally, this passive (i.e. it does not require any energy to function) water loss from the leaves is the motor for water-uptake from the soil and water-transport throughout the plant – as described by the cohesion-tension theory (but that’s another story).
There are three main reasons why the plant needs to “sweat”:
- Allows the intake of carbon dioxide from the air
- Cools the plant in warm summer heats especially (this process does not happen at night)
- Causes nutrients and water to flow throughout the plant, thereby feeding and hydrating it
As explained before, water is passively transpired through stomata during photosynthesis to allow carbon uptake. To make matters worse, the quantity of water lost is 200 times greater than the quantity of carbon taken in. In other words, a plant must lose 200 litres (about 50 gallons) of water just to be able to create 1kg of dry matter! If people needed that much water, an adult would have to drink approximately 75 liters of water a day. This quantity of water is actually pretty likely to kill you.
There are some benefits though. For example, this transformation from liquid to gas at the leaf surface from the leaves serves to cool leaves down when they get hot – and maximizing exposure to light tends to make them get pretty hot! When water evaporates from a plant during transpiration it cools the plant, in the same way the humans sweat to cool off in the heat. An adult plant can “sweat” its entire body weight daily.
Plants sweat to cool off, especially under the scorching summer sun.
Nutrient transfer
This one is pretty easy to guess. Water is a necessary medium for the transfer of nutrients from the soil and into the root system and then to the plant itself. Without it, the soil’s nutrients could not be absorbed by the plant and the plant could not feed itself. If your plant is not regularly watered it will not only die of thirst but also of hunger.
Water has other functions. For example it also helps absorb the resources needed directly through the leaves. One of these resources is carbon dioxide (or CO2). Once in the leaves, water evaporates, as the plant exchanges water for carbon dioxide. This process is called transpiration, and it takes place when water exits the plant’s body through tiny openings in the plant’s leaves, called stomata. The water from the leaves “evaporates” through the stomata, and carbon dioxide enters the stomata, taking the water’s place. Around 200 molecules of water are needed to allow the intake of one molecule of carbon dioxide. Plants need this carbon dioxide along with other things to make food.
So here we have it, plants keep your air moist, healthy and full of oxygen, aren’t they great? So be sure never to forget to water them because your potted plants rely on you to get them something to drink. Just don’t over-water them either because it might kill them.
Keep in mind that plants are pretty good at adapting to their environment if you give them enough time. Over time some plants have evolved to resist to many conditions even extended periods of drought. Some people are working on creating varieties of plants that are extremely resistant to harsh environments.
If you are interested in reading about this you should learn more about Pascal Poot, a french farmer. This man grows an amazing amount of more than 400 tomato varieties that are adapted to living even in the hardest conditions (in France), where most plants do not grow without any help from mankind : http://www.bioaddict.fr/article/pascal-poot-l-homme-qui-fait-pousser-400-varietes-de-tomates-sans-eau-ni-pesticides-a4895p1.html
And by the way, his tomatoes are even tastier grown this way.
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