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[Screaming Eagle]

Water Transport in Plants

Watering is one of the first tasks that budding bonsai artists are required to learn. Very simply, if it is not learned properly early in one's bonsai experience, chances are that the trees will suffer, may die and the world may have lost one more potential bonsai artist. Tradition says that young bonsai artists in Japan may be allowed to water trees for as long as a year before actually being allowed to prune or wire the sensei's trees. All of us know how important water is to plants but chances are, not many of us know exactly how water is moved through plants. That is the subject of this article. We'll attempt to answer 3 basic questions. How do plants get water from the soil throughout the plant?.
Exactly what is transpiration and what does that have to do with anything? And finally, what is water stress and how does it happen?
You will recall that this series of articles regarding basic Botany is based on information that I have obtained from several sources, but primarily 'The Nature of Life' by John Postlethwaite and Janet Hopson. It was published by McGraw-Hill in 1995 and is a basic college biology text. While it is written without regard to bonsai practices, we will attempt to relate the presented information to our wonderful art at every opportunity.
Here along the Gulf of Mexico, we are blessed with numerous sources of water and a shallow freshwater table provided by an aquifer. There are also hundreds of pockets of water in the marsh that creates one of the most productive saltwater and brackish estuaries in the world. Because of the amounts of water that we have available, we sometimes take for granted what an extremely valuable resource this is and we certainly don't protect it enough from pollution and saltwater intrusion. It is also to our detriment that we don't understand the physical properties of water and why plants depend on these special qualities.
The avenue for movement of these molecules of water and nutrition throughout all plants is the vascular system that we described in an earlier monologue. This system of transport tubes connects all parts of the plant in much the same way that blood vessels transport nutrients in the bodies of mammals. Plants also require a mechanism for maintaining water balance within the plant (homeostasis). Later we will explore how critical soil quality is to plants, but our primary task at this juncture is to understand how water flows within plants.
Plants use tremendous amounts of water in their daily functions. It's reported that tomato plants use 32 gallons of water in their yearly growth cycle and that 98 % or more of this water will evaporate. The main reason is that water follows a one way path through plants rather than being recycled as in animals. A steady supply of water is essential for plants to carry out photosynthesis, carry out cooling in hot weather and to transport substances (nutrients) internally.
You will recall from our discussion on leaves and leaf anatomy that they are covered with a waxy cuticle that is waterproof. This coating protects plants from losing water and is so efficient that plants have to have special openings to allow CO2 to enter (for photosynthesis). To remedy this situation, plants have stomata which are tiny openings usually on the underside of leaves that open and close depending on their hydration status. There are guard cells that surround the stoma (singular) that change size according to the amount of water within a plant and either open or close the stoma to allow carbon dioxide to enter and to prevent excess water loss through evaporation.
Root hairs by the hundreds of thousands extend from roots and, in cooperation with mycorrhizae (in some plants) fill the soil spaces to form a huge combined surface area to explore water reservoirs in the soil. Normally, water enters roots by diffusion. Plants don't have to expend any energy to obtain the water which moves from areas of higher concentration to areas of lower concentration. The water moves into the root hairs and through the root cortex. Some of it enters the cytoplasm of root cells, but most of it passes within cell walls. Remember that there are cells that separate the root cortex from the vascular xylem and phloem and water cannot follow the route of the cell walls between the cells. Water has to pass through the cytoplasm of the endodermal cells before it enters the hollow xylem to be transported throughout the plant.
Okay, so water uses simple diffusion to enter the root hairs, but how do minerals and water reach the rest of the plant? The plot thickens. It seems that minerals enter roots without the plant having to use any energy. Plants do have to use a system of protein pumps to move nutrients into the xylem vessels. Osmosis takes care of water entering the xylem after that, because water simply follows the minerals into the xylem. The minerals are dissolved in water in the xylem and transported throughout the plant.
Okay, water enters roots by simple diffusion, and the minerals are pumped into the xylem (tubes) , but how does this mixture get to the top of a tree? How do those leaves on top of a 125' Bald Cypress monarch get water?
Scientists have considered 4 possible ways. 1) Water could be pushed up from the roots. 2) Pumps in the xylem could move water up like a bucket brigade. 3) Capillary action of water moving up a thin tube could account for the action. 4) Water could be pulled up by evaporation of water from leaves. All of these are possibilities, but which one gets the job done?
Root pressure can indeed be considered as a real possibility. Through a process called guttation, tiny droplets of water are sometimes forced out of some plants. But is it enough to push water to the top of that old flat-top? Guttation works when soil is nearly saturated with water and leaves are not losing much water through evaporation. When the xylem is full of water, the water does not usually move back down the cylinder so it has to go somewhere. It moves by osmosis to the area of least pressure which is the openings in the plant leaves where it is forced out by simple root pressure. It's not very likely to be the answer here simply because the root pressure force would have to be very high to get the water up to the top of the tree. Some trees (Pines for example) don't develop root pressure, so that can't be the answer for all trees. A simple test would prove that it doesn't work very well. If you puncture a plant stem, water does not spew out the opening, air is sucked in. If there were xylem pumps and the puncture happened near a pump, water would be forced out and that doesn't happen either. Forget the root pressure and the bucket brigade for right now.
Water is in fact moved upward through the xylem in narrow tubes via capillary action. Only problem with that is that the water is only moved a few inches up the plant which is not enough to explain what happens with tall plants (>18"). Somehow, water has to be pulled up.
Transpiration is the major explanation for water transport in most plants. Through some pretty fancy engineering feats, a cooperative system that relies both on the characteristics of plants and the qualities of water is what works. Root pressure does exist and capillary action does work to some limited extent, but it's the pull from transpiration that accounts for the process working.
Transpiration is essentially evaporation of water through the stomata on stems and leaves. Water molecules move from higher concentration to lower concentration (outside the leaf) through these openings. It's the physical properties of water that make the process work so efficiently.
Water molecules tend to cohere very strongly to other water molecules (which is why spiders can walk on water). This essentially forms a chain of water molecules within the xylem pipeline. Water evaporates from the open stoma and the next water molecule in line moves up to take it's place. Each molecule lost moves the chain further up and maintains a constant flow of water in the xylem column. As the chain moves up, pressure in the roots is decreased so water diffuses into the roots (via the root hairs) to replace the lost molecules and the process continues and maintains a constant tension. This process will continue as long as evaporation continues and as long as the water column remains unbroken.
Air enters the xylem of cut flowers the same way. Holding the base of cut flowers under water and cutting an additional 2 inches or so off while the plants are underwater restores the flow of the water column, removes the air bubbles and transpiration can continue and the flowers remain fresh longer (hint, hint). I had heard people say that it was important to take cuttings while they were under water and never understood why that was important to maintaining water balance until I understood this process.
This also gives us a better idea of what water stress really is. When water molecules evaporate from a moist leaf cell and diffuse through an open stoma, the solute concentrations in that cell increase, water enters via osmosis and pulls more water molecules into adjacent cells with it. As each water molecule is pulled up through the xylem, water ordinarily enters the roots because they have been depleted to keep the molecules moving up through the column. Ordinarily. In well watered plants.
On hot, dry days, plants in full sun evaporate water faster than water can be absorbed by the roots. Tension on the water chain becomes tighter and tighter and the forces of water molecules that pull inward against the xylem walls (adhesion) can grow so strong that a tree trunk literally shrinks. When the tension is too great, the column snaps, air bubbles enter the xylem tube and the constant flow of the water column is broken. According to Postlethwaite and Hodson, plant physiologists with sensitive microphones can actually hear snapping and popping inside plants on a hot day. If the temperature drops and evaporation slows, or the soil is watered again, root pressure is reestablished, capillary action resumes within the xylem tubes and the water column is reestablished. If the stress is not relieved, wilting can occur and destruction of all or part of the plant results.
Some plants simply drop all of their leaves in situations of extreme water stress. I must admit, I've had that happen with plants that I ignored for one reason or another. Ordinarily though, the kidney shaped guard cells become flaccid, slump together and close off the stoma which blocks further transpiration. As the plant acquires enough water, potassium ions (the K in N-P-K) enter the guard cells, water follows via osmosis, guard cells swell, stoma opens, carbon dioxide enters the leaf, photosynthesis resumes and life is good again. Balance is restored (homeostasis). My ignored cuttings popped new leaves within a week or so and flowered later that year. As long as the mineral nutrients are available, this process will continue.
There are other factors that can affect the way guard cells and stomata work. Availability of carbon dioxide, light, temperature and daily plant rhythms all make a difference, but generally, stomata are open for gas exchange when the threat of water loss is not imminent and are closed when dehydration is a threat. Some plants simply open their stomata only at night during very hot weather. They store carbon dioxide and carry on photosynthesis when sunlight is present but with stomata closed. As long as mineral nutrients are available along with water, this process will continue.
Next, we will examine exactly what sap is and what causes sap to rise. Have fun.