ACS Distance Education UK
Most soils contain solutions of salt and water. The root epidermal cells of plants contain more concentrated solutions of water, sugar and salts. Thus, water will move from the soil solution and into the root epidermal cells. Water enters plant cells and is stored in the vacuole, which then expands and makes the plant cell rigid, or “turgid”. When the plant cell becomes full, water still keeps flowing into it due to osmosis. The water is stored in the cell’s central vacuole. To avoid bursting, the cell also allows water to flow out of it. The pressure inside the cell is known as turgor pressure.
Roots lose water in saline soils (i.e. salty soils). Because the solute concentration in saline soils is higher than the solute concentration in the root cells, the direction of osmotic flow is reversed. The water loss reduces the cell turgor. As a result, the plant wilts. When high amounts of water are excreted from the plant cells, plasmolysis occurs: The vacuoles shrivel and cytoplasm is drawn from the cell walls. Long-lasting plasmolysis leads to cell death.
Plants adapted to saline soils do not suffer plasmolysis, as they are capable of maintaining osmotic water uptake by storing salts at higher concentrations than the soils’.
Water moves from the soil via osmosis to the root tips, and then into the cortex cells and into the endodermis. The epidermal and endodermal cells push water across the root and up the xylem with a small pressure, which is known as “root pressure”.
Root pressure is capable of pushing water to the leaves of low-growing plants. In high-growing plants an additional force, i.e. a pulling force produced in the plant’s leaves, is needed to lift the water to the leaves.
THE TRANSPIRATION STREAM
Mesophyll cells in plant leaves contain high concentrations of sugars, and via osmosis, they pull water out of the xylem. The water is later discharged from the turgid mesophyll and turned into water vapour, which exits the leaves in a process known as transpiration. This water vapour from the mesophyll is replaced by liquid water pulled up from lower down in the plant. Water moves in a continuous stream through the xylem of the roots, stems and leaves, and is continuously pulled up through the plant and out of leaf stomatal cells into the atmosphere.
The drawing force of transpiration is known as transpirational pull. Root pressure and transpiration work together to pump water into the roots of the plant; then pull it up through the plant and out into the atmosphere.
Transpiration not only elevates water in the plant against gravity. It also transports dissolved minerals from the roots throughout the plant in the xylem’s transpiration stream. Transpiration additionally cools the plant’s leaves, as heat is removed from the leaves through evaporation.
Whilst transpiration is a very important, advantageous process in plants, it also leads to a high loss of water. Approximately 98 percent of water taken up by the roots is lost through transpiration in most plant species. Transpiration, and resulting water loss, is unavoidable as the leaves’ stomata need to be open to let in carbon dioxide for photosynthesis. However, if water loss due to transpiration is too high, plants can react by closing the stomata.
Transpiration and environmental conditions
Environmental conditions have a huge effect on the rate of transpiration. Water loss from plants is greatest during hot, dry conditions. Water loss increases in the day – a problem which many desert plants overcome by only opening their stomata (the leaf cells which release water vapour from plants) at night. When air humidity is low, transpiration increases. It also increases when there is a light breeze in the air. However when leaves are shaken by a really strong wind (or even by other means); the stomata close and transpiration decreases.
In cold winter conditions, when deciduous species have dropped their leaves and are dormant, water movement ceases. Should the water left inside the plant freeze, it would expand, rupturing the cell membranes and killing the plant. To avoid this, plants undergo a process known “cold hardening” prior to winter. During this process, plants accumulate sugars in the protoplasm of their cells – these sugars act as a kind of antifreeze. At the same time, the permeability of cells alters so that water can leak into the intercellular spaces. Thus, if ice crystals form outside the cell walls, the living cell remains intact and alive.
Water potential is a measure of the energy available in a solution to allow water molecules to move through a semi permeable membrane in the process of osmosis. Distilled water in a beaker has a water potential of zero. Adding a solute then decreases the water potential, as the resulting solution has less “energy” to flow toward an area of high concentration than pure water. Water potential is an important concept in osmosis and respiration, as plants rely on water potential to move water from the soil to the roots, and then up through the body of the plant.