Long distance transport water
Long distance transport of substances within a plant cannot be by diffusion alone. Diffusion is a slow process. It can account for only short distance movement of molecules. For example, the movement of a molecule across a typical plant cell (about 50 μm) takes approximately 2.5 seconds.
It may takes years for the movement of molecules over a distance of 1 meter within a plant by diffusion alone.
In large and complex organisms the substances have to be moved to long distances frequently. Sometimes the sites of production or absorption and sites of storage are too far from each other therefore diffusion or active transport would not suffice.
The long distance transport systems become necessary so as to move substances across long distances with much faster rate. Water, minerals and food are generally moved by a mass or bulk flow system.
Bulk flow can be achieved either through a positive hydrostatic pressure gradient or a negative hydrostatic pressure gradient. The bulk movement of substances through the conducting or vascular tissues of plants is called translocation.
Mass flow is the movement of substances in bulk from one point to another as a result of pressure differences between the two points. It is the characteristic of mass flow that substances including in solution or suspension are swept along at the same pace similar to flowing river.
The higher plants have highly specialised vascular tissues (xylem and phloem).
The Xylem is associated with translocation of water mainly and other mineral salts (some organic nitrogen and hormones) from roots to the aerial parts of the plants.
The phloem translocates a variety of organic and inorganic solutes mainly from the leaves to other parts of the plants.
How do Plants absorb Water?
As we know that roots absorb most of the water that goes into plants that is why we apply water to the soil and not on the leaves.
The responsibility of absorption of water and minerals is more specifically the function of the root hairs that are present in millions at the tips of the roots.
The root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption. The water is absorbed along with mineral solutes through root hairs purely by diffusion.
After absorbing water by the root hairs, it can move deeper into root layers by two distinct pathways:
- Apoplast pathway
- Symplast pathway
The apoplast is the system of adjacent cell walls that is continuous throughout the plant, except at the casparian strips of the endodermis in the roots .
The apoplastic movement of water occurs exclusively through the intercellular spaces and the walls of the cells.
Movement through the apoplast does not involve crossing the cell membrane. This movement is dependent on the gradient.
The apoplast does not provide any barrier to water movement and water movement is through mass flow.
As water evaporates into the intercellular spaces or the atmosphere, tension develop in the continuous stream of water in the apoplast therefore mass flow of water occurs due to the adhesive and cohesive properties of water.
The symplastic system is the system of interconnected protoplasts. Neighbouring cells are connected through cytoplasmic strands that extend through plasmodesmata.
During symplastic movement the water travels through the cells (their cytoplasm), intercellular movement is through the lasmodesmata.
The water has to enter in the cells through the cell membrane that is why, the movement is relatively slower. Movement is again down a potential gradient. Symplastic movement may be aided by cytoplasmic streaming.
We have observed cytoplasmic streaming in cells of the Hydrilla leaf, the movement of chloroplast due to streaming is easily visible.
Maximum water flow in the roots occurs via the apoplast since the cortical cells are loosely packed and hence offer no resistance to water movement.
The inner boundary of the cortex, the endodermis is impervious to water because of a band of suberised matrix called the casparian strip.The water molecules are unable to penetrate the layer, so they are directed to wall regions that are not suberised into the cells proper through the membranes.
The water then moves through the symplast and again crosses a membrane to reach the cells of the xylem. The water movement through the root layers is ultimately symplastic in the endodermis. This is the only way water and other solutes can enter the vascular cylinder.
In xylem, water is again free to move between cells as well as through them. In young roots water enters directly into the xylem vessels and/or tracheids. These are non-living conduits and so are parts of the apoplast.
Some plants have additional structures associated with them that help in water (and mineral) absorption. A mycorrhiza is a symbiotic association of a fungus with a root system.
The fungal filaments form a network around the young root or they penetrate the root cells. The hyphae have a very large surface area that absorb mineral ions and water from the soil from a much larger volume of soil that perhaps a root cannot do.
The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae.
Some plants have an obligate association with the mycorrhizae. (Pinus seeds cannot germinate and establish without the presence of mycorrhizae)
Water Movement up a Plant
We understood how plants absorb water from the soil and move it into the vascular tissues. Now we have to try and understand how this water is transported to various parts of the plant. Is the water movement active or is it still passive? Since the water has to be moved up a stem against gravity, what provides the energy for this? etc.
Root Pressure
As various ions from the soil are actively transported into the vascular tissues of the roots, water follows (its potential gradient) and increases the pressure inside the xylem.
This positive pressure is called root pressure and can be responsible for pushing up water to small heights in the stem.
Experiment : How can we see that root pressure exists?
Choose a small soft-stemmed plant and on a day when there is plenty of atmospheric moisture, cut the stem horizontally near the base with a sharp blade, early in the morning.
We will see drops of solution ooze out of the cut stem, this comes out due to the positive root pressure.
If we fix a rubber tube to the cut stem as a sleeve, we can actually collect and measure the rate of exudation and also determine the composition of the exudates.
Effects of root pressure is also observable at night and early morning when evaporation is low and excess water collects in the form of droplets around special openings of veins near the tip of grass blades and leaves of many herbaceous parts. Such water loss in its liquid phase is known as guttation.
At best, The root pressure can only provide a modest push in the overall process of water transport. They obviously do not play a major role in water movement up tall trees.
The greatest contribution of root pressure may be to re-establish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration.
The root pressure does not account for the majority of water transport, The most of the plants meet their need by transpiratory pull.
Transpiration pull
Despite the absence of a heart or a circulatory system in plants, the upward flow of water through the xylem in plants can achieve fairly high rates, up to 15 metres/hour.
How is this movement accomplished?
A long standing question is, whether water is ‘pushed’ or ‘pulled’ through the plant.
The most of researchers agree that water is mainly ‘pulled’ through the plant and that the driving force for this process is transpiration from the leaves. This is referred to as the cohesion-tension-transpiration pull model of water transport.
What generates this transpirational pull?
Water is transient in plants. Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost through the stomata in the leaves. This water loss is known as transpiration.
We have studied transpiration in an earlier class by enclosing a healthy plant in polythene bag and observing the droplets of water formed inside the bag.
We could also study water loss from a leaf using cobalt chloride paper, which turns colour on absorbing water.