Water management in irrigated rice - Rice Knowledge Bank ...

The plant-soil-water system22.1 Water movement in the soil-plantatmospherecontinuumRice plants take up water from the soil and transportit upward through the roots and stems and releaseit through the leaves and stems as vapor in the atmosphere(called transpiration). The movement ofwater through the plant is driven by differences inwater potential: water flows from a high potentialto a low potential (imagine free water flow over asloping surface: water flows from the top, with ahigh potential, to the bottom, with a low potential).Different units express water potential (Table 2.1)and, unfortunately, different authors report differentunits. In this report, we usually use the termPascal (P).In the soil-plant-atmosphere continuum, thewater flows from the soil, with a relatively highpotential, through the plant to the atmosphere justoutside the leaves, which has a relatively low potential.Potentials in the soil-plant-atmosphere areusually negative and we also use the term “tension,”which has the opposite value. For example, a tensionof +10 kPa is the equivalent of a potential of–10 kPa. The term tension is intuitively easier tounderstand: a high tension suggests a high “pullingforce.” Thus, water flows from a low tension inthe soil (low pulling force) to a high tension in theatmosphere (high pulling force). The water tensionin the atmosphere outside the leaves is determinedby climatic factors: relative humidity, wind speed,temperature, and solar radiation. This atmospherictension translates into the “evaporative demand”of the atmosphere, which determines potentialtranspiration rates. The water tension in the soilis determined by the amount of water in the soiland by soil physical properties such as texture andbulk density. The speed with which water movesthrough the plant is determined by the difference inwater tension between the soil and the atmosphere(the higher the differences, the faster the water willflow) and by the resistance to water flow in the plant(Ehlers and Goss 2003). First, soil water needs toovercome a physical resistance (the epidermis)to enter the roots. Then it flows through cells orthrough spaces between cells into so-called xylemor vascular bundles that will transport it upward.The water flows easier and faster in wide bundlesTable 2.1. Units to express water potential and some corresponding values. pF is calculated as 10 log(–H), where H is waterheight in cm.Unit nameCorresponding valueWater height (cm) 0 −10 −100 −1,000 −15,850pF (–) –∞ 1 2 3 4.2Bar (bar) 0 0.01 0.1 1 15.85Pascal (Pa) 0 1,000 10,000 100,000 1,585,000Kilo Pascal (kPa) 0 1 10 100 1,585Mega Pascal (MPa) 0 0.001 0.01 0.1 1.58511