Like all life on earth, plants need water to survive and grow. Indeed, like humans, water is the primary element that makes up the structure of plants. Human bodies are comprised of around 70 percent water, but in plants this proportion can be as much as 95 percent. Water is also essential to the way a plant receives nutrients and provides energy for itself. Thus, water is arguably the most essential substance required by plants. That is why many elements of permaculture design – from using swales and contouring to slow runoff from the land and allowing it to sink into the soil to mulching to prevent evaporation of moisture from the soil surface – emphasis the preservation of water so that it can be provided to plants.
There are four primary ways in which plants use water to survive and grow.
Unlike animals, plants do not have and internal or external skeleton to give them strength of structure. While vertebrates have an internal jointed skeleton, which gives the framework to protect the internal organs and to provide structure for limbs and corporeal elements, and invertebrates have an external skeleton, or carapace, to protect their internal physiology, plants do not have an overarching structure like this. But they do need to have some form of structure to enable them to grow and maintain rigidity. Some trees and woody shrubs get this rigidity from the presence of lignin – a polymer that binds fibrous material together, but the majority of plants get it solely from the pressure of the water in their cells.
Within each cell are a number of elements, such as mitochondria that converts sugars into energy the plant can use, and chloroplasts that contain the chlorophyll the plant uses in photosynthesis. But by far the largest portion of each cell is the vacuole, a space filled with water that ensures the cell maintains its shape. If the plant receives enough water, each vacuole in each cell keeps the cell walls at the right tension, and in combination all the cells give the plant its strength. This water pressure within the cells is called ‘turgor’ and because the strength is derived from a liquid source, the strength retains a flexibility that animal skeletons lack. This allows the plants to adapt to surroundings, bend in the wind and move towards the sun as it traverses the sky during the day to get the most energy for photosynthesis.
Photosynthesis is the process by which plants produce the energy they need to survive and grow. And water is central to the process. Photosynthesis uses the energy from the sun to create energy in the form of sugars. For the molecules of sugar to form they need carbon dioxide (which they absorb from the air) and hydrogen, which is sourced from the water in the plant, which comes up through the plant from the roots to the leaves. While plants release oxygen as a by-product of the photosynthesis process, they also need a small amount of it in solution to help facilitate the function; water provides this as well.
The movement of water through the parts of a plant is called translocation. It is via translocation that nutrients are moved around the plants to where they are needed. A plant absorbs nutrients in solution; so having sufficient water in the soil is essential for good plant growth (which is why a lot of permaculture practices emphasize the preservation of water in the ground, via mulching, ground cover, and other methods). Soil nutrients are taken up by the roots in a water solution and moved via a process called capillary action that uses the tension of the water itself to maneuver it around. This allows the plant to get nutrients to the leaves where it is required for photosynthesis.
Translocation not only allows the plant to shift nutrient-rich water from the roots to the leaves, it also allows the sugars that result from photosynthesis in the leaves to be transported back down to the roots and out to blossoms and crops to enable them grow.
During photosynthesis, water evaporates from the surface of the leaves in a process called transpiration. This occurs when stomata, a kind of pore, open on the leaf to allow for the exchange of oxygen and carbon dioxide with the atmosphere during photosynthesis. Indeed, some of the oxygen that the plant releases is contained in the water vapor that is transpired. Not only does the transpiration effectively provide the space into which the essential carbon dioxide can flow, it also prompts the plant to take up more water from the soil (bringing with it nutrients), so helping to keeping the internal system of the plant in balance. It also keeps the plant cool – think of transpiration as being a bit like sweating in humans!
Besides the individual rates that different species have, rates of transpiration will vary depending on a number of factors. These include temperature (warmer temperatures causes more transpiration), light (plants transpire less in the dark) and humidity (it is easier for a plant to release moisture into drier rather than saturated air. Wind will also cause more transpiration to occur, partly as still air tends to become more humid, and the wind moves this air away from the leaves. And transpiration, like photosynthesis and translocation, is affected by the amount of moisture in the soil that is available for uptake by the plant. If there is insufficient water in the soil, transpiration slows, as do the other process, and the plant will begin to exhibit signs of distress, such as curling and browning of leaves.
All four of these processes that plants use water for are interlinked. They may occur at different rates in different species, but essentially all plants perform them. And as long as they have sufficient water in the soil, they will do them naturally, without interference, helping grow strong specimens with good crops. Get the water right and your plants will reward you.