BY TOM FORREST, STEALTH GARDEN SUPPLIES
Stealth Science
W
elcome to the second par t of our ‘Stealth Science’ Series! In this ar ticle, we will be discussing
how impor tant and seemingly complicated the
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!
consumption of water is within our gardens. Without it, all life on ear th 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.
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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.
