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Chapter 15
In solutions with a high concentration of nutrient elements, the needs of olive plants are covered by the absorption of a smaller quantity of H2O. Ignoring transpiration, to calculate the water requirements of olive trees, water loss from the soil surface is included. The sum of transpiration plus water loss from the soil surface by evaporation is termed evapotranspiration. Water availability is fundamental for starch hydrolysis and contributes to cell turgor, which controls the stomatal mechanism. If the leaf is not turgid then the stomatal apertures are closed, and this reduces both CO2 supply and the rate of photosynthesis. Therefore, water availability, stomatal opening, Pn and final yield are closely linked. Furthermore, water is a significant solvent for photosynthesis, and in combination with CO2 produces carbohydrates according to the equation: nCO2 + nH2O → nO2 + Cn(H2O)n Another role of water is as a solvent of nutrient salts and affecting their transport to cells and tissues. The soil solution and nutrient elements are easily absorbed from the plant providing that the osmotic pressure of the solution within the plant counteracts the force of soil nutrients. When soil water is retained by the soil with a force greater than the force with which it is absorbed, the plant withers through lack of water.
WATER QUALITY Rainfall is traditionally the main water supply in many areas of the world in arid and semi-arid regions. In these areas, irrigation from underground sources contains excess salts, affecting plant growth (Klein et al., 1994). Furthermore, the quality of surface water is sometimes compromised by its mixing with poorquality drainage water. A serious problem nowadays is the over-pumping of groundwater, which lowers the water table and leads to the entrance into aquifers of sea water. The salt content of irrigation water is usually expressed in terms of conductivity (Ecw); the units are dS/m at 25°C and measurements are conducted by using conductivity meters. Furthermore, the water quality for irrigation and the evaporation from the soil surface lead to salt accumulation in the root zone. The consequence of this is that the Ec of the soil solution is significantly higher than that of the irrigation water. The reduced yields in olives due to salinity can be predicted when the Ec of a soil solution reaches approximately 3–6 dS/m. The suitability of irrigation water depends on the total amount and the type of salts present. Sodium, chloride and boron ions at high concentrations are toxic. The suitability of irrigation water also depends on the concentrations of Na, Ca and Mg (me/l), which are used to determine the specific absorption rate with sodium adsorption ratio (SAR), according to the equation: SAR = (Na)/√(Ca + Mg)/2