5 minute read
Stealth Sciences
Water Interactions
Welcome to the second part of our ‘Stealth Science’ Series! In this article, we will be discussing how important and seemingly complicated the consumption of water is within our gardens. Without it, all life on earth would cease to exist. Plants have cleverly evolved a very unique way of utilising this precious resource for a huge range of biological processes.
We will start our lesson with the importance of the transpiration stream. This essential concept describes the movement of water from the soil to the atmosphere. The controlling factor for this movement is the gradient of water potential as it amazingly moves upwards like an upside-down waterfall!
What about gravity? How do trees move such vast amounts of water towards the sky?
In a brilliant example of evolutionary prowess, plants have evolved a threefold method of mechanisms. Various theories state that plants cleverly use water’s peculiar cohesive and molecular properties to their advantage. This allows the tallest trees to lift thousands of litres of water hundreds of metres into the air.
Using a combination of root pressure, capillarity and cohesion, plants are able to move water even more efficiently than the most advanced human technology. Firstly, root pressure acts to uptake the salts and water by osmosis; through diffusion and the difference in water potential around the root hairs.
Capillarity states that water rises higher in smaller diameter pipes and that you can imagine the xylem as tiny, microscopic, thin straws (that use tracheids and vessels). As the root pressure and capillary push the water from the base, cohesion pressure from the leaf surface pulls continuous columns of water upwards. Another theory also suggests that a lack of gasses within the xylem and phloem contributes to the inward movement of water and lack of vertical pressure on the water column within the plant.
This five-part series delves into plant science to help you understand why a garden flourishes or flops. Over the next five issues, we will discuss the important topics relating to plant biology and physiology, structure and function, covering roots to shoots and everything in between!
Understanding the science behind the art of horticulture ensures we can cultivate beautiful, healthier, and more sustainable crops.
The Five Classes:
• Plant Morphology and Anatomy (see last issue)
• Water Interactions
• Plant Food and Ionic Relations
• Photosynthesis and Phloem
• Plant Hormones: The control of growth and development
A majority of the water tension comes from the cohesive pressure of the leaves. This is actually a very easy movement of water to accurately measure. A potometer (also known as a transpirometer) is a basic device you can make with simple lab equipment to measure the rate of water uptake from a leafy shoot.
During daylight hours, the water column within the plant is under the greatest tension as the water potential is much higher. A dendrometer allows us to very accurately measure and observe tree trunks shrinking during the day and actually swelling at night! This occurs as the water column is most stretched while the plant is maximising transpiration.
How does the water get inside the plant in the first place?
Let’s imagine we are looking at the transpiration stream under a potent HP microscope, travelling like a magic school bus through the different organs and processes of the plant.
Underground (or underwater in certain hydroponic setups), the water firstly passes through the apoplast (the non-living areas; cell walls and intercellular spaces) of the root cortex along the water potential gradient, and into the root.
It encounters an impervious barrier at the endodermis but enters through the symplast (the living areas bounded by different membranes e.g. cell protoplasts) and flows back into the apoplast as it is carried upward by the transpiration pull from the leaves.
Near the destination, the water leaves the xylem and travels via the apoplast in the mesophyll cell walls into the substomatal cavity. This is where an exchange of gases occurs and the water brilliantly emerges into the atmosphere as humidity. This is a very oversimplified analysis of the transpiration stream but gives you an idea of how the water moves between different cells and intercellular spaces to get to where it needs to go.
Many of us have heard of the stomata, the little bean-shaped things that control respiration. The stomatal opening is an essential organelle for photosynthesis. This is a tiny pore that is bordered by ‘guard cells’ that control and regulate gas exchanges from the leaf. As the stomatal opening size increases, so does the transpiration, but the rates will depend on water stress, light, CO 2, air toxicities, and several other factors.
Stomatal closure is regulated by Abscisic Acid ABA (we will discuss this much more in depth within Topic 5: Plant Hormones) and causes the cessation of the K+ pump. They will close during times of darkness, high CO 2 levels, or other poisons in the air. The guard cells have chloroplasts that photosynthesize to help the plant exchange gases (mostly oxygen and carbon dioxide).
It is predominantly the environmental factors of a crop that will influence the transpiration levels of a plant. As relative humidity decreases, so does transpiration. But also as the air warms, the plant will transpire significantly more to try to regulate its own temperature.
Surrounding the plant, the light intensity and air movement will also have drastic effects on the control of transpiration, as will the soil conditions, soil type, and salinity.
To truly comprehend the influence of water relations in horticulture, we need to look at the cellular aspects of H20. Universal laws state that water potential is determined by three factors: gravity, pressure, and the concentration of dissolved solutes.
Water movement inside a plant is osmotic; it moves through barriers or semi-permeable membranes as the turgor pressure maintains the structure and shape of a cell.
As it is with humans and all life on earth, water is our key to existence; we need to keep it pure and treat it as our most valuable resource
This turgor pressure describes the pressure exerted by the cell wall in response to the expansion of the vacuole (a space within the cell, usually enclosed by a membrane). It keeps the cells solid and structurally able to do their job. Only a small amount of water moves in or out of the cell, but this still has a large effect on its water potential and its turgor pressure.
When the guard cells are turgid, the stomata are open and the gas exchange is underway!
Why is water the most important limiting factor for productivity and crop growth?
As water moves throughout the plant, it is used in a variety of physiological mechanisms. The three most important needs for water are as a raw material for photosynthesis, for turgor to support and expand cells, and as a solvent for ions and organic compounds.
Plant life only actually utilizes 1% of water for metabolic activity; the remaining 99% is transpired and supports the rest of our global ecosystem! Astonishingly, a Eucalyptus regnans (Mountain Ash) can transpire more than 12,000 litres per hectare per day!
The soil type and particle size will also influence how much water a plant is able to transpire or use. Depending on the salt/silt/clay ratio, the substrate will have differing field capacities (maximum amount of water) that a plant is able to utilize.
As it is with humans and all life on earth, water is our key to existence; we need to keep it pure and treat it as our most valuable resource. Plants have evolved to adapt to changing climates and we can learn from their ingenuity and resourceful nature.
The next segment of Stealth Science will investigate how plants use this water to consume their food and the ionic relations behind mineral nutrition! 3
BIO:
Founder of Indicated Technology Pty Ltd, Tom is a certified horticulturalist and paid consultant working in the Australian medical cannabis industry. After finishing studies in production horticulture (hydroponics) and plant biology; Tom has spent the past 6 years working in the protected cropping space. Tom is passionate about sustainable yet economic cultivation methodologies and also teaches cannabis cultivation as part of University and private education programs. Tom is also the Communications Manager for Stealth Garden wholesale supplies.
