The importance of water relates to its essential functions in perpetuating both plant and animal life. It is an absolute requirement for all living organisms. Anaerobic organisms can live without oxygen, but they cannot without water.
In crop agriculture, water is an important climatic factor. It affects or determines plant growth and development. Its availability, or scarcity, can mean a successful harvest, or diminution in yield, or total failure. According to FAO (2011), irrigation typically doubles farm yields and the number of crops grown in one year is increased from 1 to 2.
But plant responses differ and the importance of water likewise differ depending on plant species. Most plants are mesophytes, that is, they are adapted to conditions with moderate supply of water. But some, called hydrophytes, require watery or water-logged habitats while others, called xerophytes, are more tolerant to dry conditions. The resurrection plants are in fact capable of surviving near complete dessication. They are capable of losing 90% or more of cellular water in their vegetative tissues and still remain alive. They can remain dried and appear somewhat dead for several years but, when rehydrated, suddenly spring back to life (Le and McQueen-Mason 2006).
Water is a chemical compound consisting of two atoms of hydrogen joined to one atom of oxygen (H2O). But there are more about it. There is simply no single generic answer to the question that will apply in all situations and to everybody.
Water is a substance with the unique property of being able to exist in three states: liquid, solid, and gas. As a liquid it is clear, colorless and odorless. In such state of matter it has been described as a fluid, a substance which flows freely without fixed shape. It melts and freezes at 0°C (32°F) and boils at 100°C (212°F) under normal atmospheric pressure.
Through precipitation mainly rainfall, liquid water is made available to plants as surface water, soil moisture, or groundwater. It comprises about 70-90% of the body or even more on fresh weight basis, although only a small fraction of the water absorbed is utilized. Most of the absorbed water in plants is lost through transpiration and only about 1 percent or less is used in the various biochemical processes (Devlin, 1975; Mader, 1993).
Water participates directly or indirectly in all metabolic processes in living organisms. It is considered the universal solvent because it dissolves many substances. As a solvent, it also serves as a transport medium for mineral nutrients from the soil, as well as in the translocation of organic substances within the plant. It is a chemical reactant in photosynthesis hence vital to life. It is a product of respiration. It is also believed responsible, or at least contributing, to the cooling of plants through the process of transpiration. In addition, it serves as a growth medium in hydroponics, the culture of plants in soilless nutrient solutions.
As with other climatic factors, water can possibly cause unfavorable effects on plant growth and development. Excess water in the soil can injure flood prone plants, like corn (maize), due to lack of oxygen. In this case water stress due to flooding means oxygen stress by deficiency (hypoxia) or total absence (anoxia).
Excess water within the plant can also cause injury. Edmond, et al. (1978) explained that under conditions that favor high absorption and low transpiration rates, there is build-up of high turgor pressure in the region of cell elongation which causes maximum swelling of the cells. This results to the development of leggy seedlings. Likewise, under similar conditions, growth cracks occur as exemplified by bursting heads of cabbage and cracked fruits of tomato and roots of carrot and sweet potato.
On the other hand, the injury caused by acid rain indicates that purity and quality relate to the importance of water. With dissolved sulfuric acid and nitric acid that are formed in the air from sulfur dioxide and nitric oxide generated by power plants, smelters, other industrial plants, factories and cars, acid rain can seriously injure plants (Miller, 2001). To be clear, however, it is not water per se that causes damage in this case. Rather, it is the acid(s) which is dissolved in water.
[FAO] Food and Agriculture Organization of the United Nations. 2011. Fast facts: The state of world’s land and water resources. Retrieved Mar. 24, 2013 from http://www.fao.org/fileadmin/user_upload/newsroom/docs/en-solaw-facts_1.pdf.
DEVLIN R. 1975. Plant Physiology. New York, NY: D. Van Nostrand Company. 600 p.
EDMOND JB, SENN TL, ANDREWS FS, HALFACRE RG. 1978. Fundamentals of Horticulture. 4th ed. McGraw-Hill, Inc. p. 87-130.
LE TN, MCQUEEN-MASON SJ. 2006. Dessication-tolerant plants in dry environments. In: Amils R, Ellis-Evans C, Hinghofer-Szalkay H, editors. 2007. Life in Extreme Environments. Dordrecht, Netherlands: Springer. p. 269-279.
MADER SS. 1993. Biology Part 1: The Cell. 4th ed. Duburque, IA, USA: Wm. C. Brown Communications, Inc. 152 p.
MILLER GTJr. 2001. Environmental Science: Working With the Earth. 8th ed. Pacific Grove, CA: Brooks/Cole. 549 p.
(Ben G. Bareja March 2013)