In this article we will discuss about the relationship between plants and water.
Water plays a central role in the metabolism of plants, as a source of hydrogen for the reduction of carbon dioxide in photosynthesis and as a product of respiration. Water is the solvent and hence the conveyor of transportable ions and compounds into, within and out of all living plants.
It is a major structural component, often constituting more than 90 per cent of the vegetative biomass. Only a small fraction of water absorbed by plants is used in photosynthesis, while most (as much as 99 per cent) escapes as vapour in the process of transpiration from plant canopies.
Mesophytes, including most crops, control their water economy by developing extensive root system to extract water from deep soil layers and regulating stomatal apertures to limit water loss from the leaves. Many physiological processes related to photosynthesis, growth and development of plants are affected by water stress sometime before the plant actually wilts.
Loss of water from plant canopies by transpiration sets up a chain of reactions to replace the water lost. Potential energy of plant-water decreases due to reduction in plant-water content. Lowering of water potential at the root surface below that of the soil-water is the driving force for passive water uptake from the soil by the plant.
When the potential of water in the atmosphere is less than that of water in the soil, water tend to move from the latter to the former. The proportion of water that passes through the plant depend upon the resistance offered by the path through the plant. Xylem provides the path of least resistance in the stem. Greater the diameter and fewer the cross walls, greater the rate of flow. Most of the flow outside the xylem appears to be along the walls.
Water moves form root surface to xylem in a radial manner through the cell walls or through intercellular spaces. Suberisation of radial walls of the endodermal cells tend to make them impervious to water. Hence, water and solutes may pass through protoplasm rather than around them.
The difference in water potential between the surrounding medium and the roots is due to either the osmatic effects of solutes accumulated in the roots xylem of slowly transpiring plants (active absorption) or by the tension developed in the hydrodynamic system of more rapidly transpiring plants (passive absorption).
Water status of plant tissue is primarily described by two basic parameters:
(i) The amount of water contained relative to what it can hold when fully turgid (relative water content) and
(ii) Energy status of the contained water, usually expressed as the total water potential.
Relative water content (RWC) which is the ratio of actual water content to water content at saturation (fully turgid), is generally, expressed as percentage. Actual water content is obtained by subtracting dry weight (DW) of the sample from the fresh weight (FW). Water content at saturation is the difference between saturation weight or turgid weight (TW) and dry weight.
Actual water content (per cent): [(FW-DW)/FW] x 100
Water content at saturation (per cent): [(TW-FW)/(TW-DW)] x 100
Relative water content (per cent): [(FW-DW)/(TW-DW)] x 100
For saturation, tissues are to be floated on water for 3 to 24 hours. While floated on water, the samples are illuminated by light of compensatory intensity to prevent changes in tissue dry weight. Samples are to be dried in oven at 65° to 80°C or at 35°C under vacuum.
There are several indirect methods of estimating plant water status by measuring leaf temperature, canopy temperature, canopy air temperature differentials, diffusive resistance, transpiration rate etc.
Leaf temperature of stressed plants is high due to reduced transpiration compared with plants adequately irrigated. Thermocouples or infrared thermometers are used for measuring leaf temperature. Porometers are used to measure leaf temperature in addition to transpiration and stomatal resistance measurements.