When do plants lose water?
- During photosynthesis – when plants use carbon dioxide and radiant energy from the sun and electrons from hydrogens in water to create carbohydrates
- During transpiration – when water vapour is lost from plants.
A digression on transpiration…
Transpiration occurs when the tiny pores, known as stomata, on leaves are open, and water vapour escapes. Usually this happens in the day, but for some plants this happens at night. When stomata are open (how stomata open and close is in and of itself a fascinating topic!) it allows carbon dioxide outside the cell to ‘diffuse’ into the leaf cells for photosynthesis. While stomata are open it also means some water will diffuse out of the stomata. Water loss is also necessary though to help cool the plant.
Plants may close stomata when it is too hot (like midday in summer). I imagine because they can’t absorb enough water from the soil to mitigate water loss.
‘Transpirational pull’ is crucial in the mechanics of transporting water and dissolved ions, from roots up through the plant.
The Mechanics of Transpiration
From Wikipedia: Transpiration article:
“Mass flow of liquid water from the roots to the leaves is caused by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere. Water is absorbed at the roots by osmosis, and any dissolved mineral nutrients travel with it through the xylem.”
Yowzers, that’s a lot of Kool-Aid!
Fact from same Wikipedia article:
“A fully grown tree may lose several hundred gallons of water through its leaves on a hot, dry day.”
For metric-minded people, 1 gallon = 3.8 litres. So potentially 2,800 litres of water. Thousands of litres. That’s a lot of Kool-Aid!
The ways to measure the ‘rate of transpiration’ sound quite cool (‘potometer’, ‘transpirometer’, ‘heat balance sap flow gauge’, lysimeter) and fun to try and set up as a home experiment. I guess you’d need to take into account the time of day of your measurements – when it is hotter, the transpiration rate would be higher, except if the stomata are closed, like when it’s very hot, then it should be zero. The description of a potometer on Wikipedia uses a cutting from a plant. But is the transpiration rate the same for any part of the plant? And what if the cutting is a whole branch as opposed to a twig? And how could you measure taking into account the roots too, as you need an unbroken stream of water? Hmmm
It’s complicated… but… how does sap flow through xylem?
Transpirational Pull
A Digression on Surface Tension & Capillary Action…
This digression attempts to explore the scientific principles behind transpirational pull.
Step 1 – Xylem is a capillary
Xylem tissue consists of vessel elements or tracheids end to end with water flowing through them, like a thin tube or capillary.
Step 2 – Water (H2O) is cohesive
There are intermolecular forces at work! Water molecules cohere to other water molecules. The negatively charged oxygen of one water molecule bonds with the positively charged hydrogen of another water molecule.
- In the middle of water, in a glass for example, there’s molecules surrounded by other molecules in nice snug hydrogen bonds (the bonds that water molecules form with other water molecules and within themselves, I think). From Wikipedia: Surface Tension article, “In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules…”.
- But, at the surface of the water in the glass, there are no water molecules above the top layer of water molecules to even out the pull from the water molecules below. This creates an internal tension in water (surface tension) so that the water pulls inward.
- And, the water molecules are also attracted to the sides of the glass. From Wikipedia: Meniscus article: “a concave meniscus occurs when the molecules of the liquid attract those of the container’s, causing the surface of the liquid to cave downwards”. It does not end here! See Step 3!
Step 3 – Water is adhesive
The water molecules in xylem tissue adhere to the cell walls. And this is where capillary action comes into it… Actually this part is hard to explain, let alone understand, so I will for now resort to quotes:
- From Wikipedia: Meniscus article “Capillary action acts on concave menisci to pull the liquid up, increasing favorable contact area between liquid and container…” and
- From Wikipedia: Capillary Action article “Adhesion pulls the liquid column up until there is a sufficient mass of liquid for gravitational forces to overcome the intermolecular forces…”
So, it sounds like because water in the xylem of plants is attracted to the cell walls (adhesion), and is weaker at its own edges (surface tension), it moves upward (capillary action), overcoming both its own internal tension and gravity.
Continue to Root Pressure, how water gets into the plant in the first place (since plants don’t have mouths 
)
Questions to explore further…
- How does water move horizontally in the xylem – e.g. from one xylem ‘tube’ to another?
- How does water move down the xylem – e.g. if the liquid ‘chain’ is broken, or when the transpirational pull from the stomata at the top stops or reverses (and, can it reverse?) RTD mentioned something about ‘valves’ hmmmm…