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The movement of molecules, specifically water and any solutes, is vital to understand in light of plant processes. This will be more or less a quick review of several guiding principles of water motion in reference to plants.
Roughly 90% of the water that enters a plant is lost via transpiration. Transpiration is the loss of water vapor through the leaves, just to refresh you. In addition, less than 5% of the water entering the plant is lost through the cuticle. Water is vital to plant life, not just for turgor pressure reasons, but much of the cellular activities occur in the presence of water molecules and the internal temperature of the plant is regulated by water. Recall that the xylem pathways go from the smallest part of the youngest roots all the way up the plant and out to the tip of the smallest and newest leaf. This internal plumbing system, paired with phloem and its nutrient transportation system, maintains the water needs and resources in the plant. The issue of the processes by which water is raised through columns—of considerable height at times—has been studied and debated for years in botany circles. The end result is the cohesion-tension theory.
The Cohesion-Tension Theory
Polar water molecules adhere to the walls of xylem tracheids and vessels and cohere to each other which allows an overall tension and form ‘columns’ of water in the plant. The columns of water move from root to shoot and the water content of the soil supplies the ‘columns’ with water that enters the roots via osmosis. The difference between the water potentials of the soil and the air around the stomata are capable of producing enough force to transport water through the plant—from bottom to top and thus goes the cycle.
Phloem is responsible for transporting food substance throughout the plant. As with water movement in plants, the movement of organic solutes in plants has been studied and debated for years. The currently accepted hypothesis is the pressure-flow hypothesis for the translocation of solutes.
This is essentially a source and sink hypothesis. Food substances that are in solution flow from a source, which is generally where water is taken up by osmosis (roots; food storage tissues, such as root cortex or rhizomes; and food producing tissues such as mesophyll in leaves), and the food substances are then given up at a destination or a sink where the food resources will be utilized in growth. The idea is that the organic solutes are moved along concentration gradients existing between sources and sinks.
At the source, phloem-loading occurs and sugars are moved by active transport into the sieve tubes of the smallest veins. The overall water potential in the sieve tube drops and then water enters the phloem cells via osmosis. The resulting turgor pressure from the movement of the water is enough to drive the solution through the sieve network to the sink . The sugar is unloaded at the sink via active transport and water then exits the ends of the sieve tubes. The pressure drops as the water exits, which causes a mass flow from the higher pressure at the source to the now lowered pressure at the sink. Much of the water that exits the sieve tubes will diffuse back into the xylem where it can be recirculated, transpired once it reaches the source. In a nutshell the mass flow is caused by drops in turgor pressure at the sink as the sugar molecules are removed. This generates the next push of materials toward the sink.
It is the responsibility of the stomata to regulate transpiration and gas exchange via the actions of the guard cells. The pores of the stomata are closed when turgor pressure in the guard cells is low, and they are open when turgor pressure is high. Changes occur when light intensity, carbon dioxide concentration or water concentration change. The guard cells of the stomata use energy to take up potassium ions from adjacent epidermal cells.The uptake opens the stomata because water potential in the stomata drops and water moves into the guard cells and increases turgor pressure. When the potassium ions are released, the water then leaves the cells as the water potential shifts again. There is evidence that stomata will close with water stresses, but there also seems to be some indication that hormones are involved cause a loss of potassium ions from the guard cells and thus a pore closure.
Most plants keep their stomata open during the day and close them at night. However, there are plants that do the opposite and open their stomata during the night when overall water stress is lower. These plants have a specialized form of photosynthesis called CAM photosynthesis since the standard source of carbon dioxide is shut off as the stomata are closed during daylight hours. There are desert plants that are able to store carbon dioxide in their vacuoles in the form of organic acids that are converted back into carbon dioxide during the daytime for standard photosynthetic processes. As mentioned earlier, there are also adaptations such as sunken stomata which reduce the loss of water. Submerged or partially submerged plants generally do not have stomata on the underwater portions of their leaves.
High humidity will reduce transpiration rates while low humidity accelerates the process. There is a direct correlation between temperature and water movement out of the leaf. At high temperatures the rate of transpiration increases, while the opposite occurs at lower temperatures.
Many external factors will affect growth rates and quality. The minerals available in the local soil is one such source of external input. Essential plant elements include: carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulphur, calcium, iron, magnesium sodium, chlorine, copper, manganese, cobalt, zinc, molybdenum, and boron to name the most common. Other minerals are required but they vary greatly from plant to plant. For example some algae need large amounts of iodine and silicon, while some loco weed species need selenium—which is poisonous to cattle on its own.
When any of these elements are lacking in the soil and the deficiencies are not compensated for by adding fertilizer compounds of compost the plant will demonstrate characteristic symptoms of mineral deficiencies. Most commercial fertilizers are some ratio of nitrogen, phosphorus and potassium and thus are able to compensate for a wide variety of issues. As an example of uses for the essential element in plants we will look at a few elements and how they are utilized:
As you can see by scanning through the list, all of these elements are involved to one degree or another in vital life sustaining processes!
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