Evapotranspiration Integrated Model for Analysis of Soil Salinization Affected by Root Selective Absorption

Soil salinization occurs in crop fields of arid and semiarid regions under desertification (Dregne, 2002). Soil salinity reduces the crop’s ion absorbing power, which quickly reduces growth rate (Munns, 2002), and presents a serious problem for sustainable agriculture (Food and Agriculture Organization, 2002). Generally, salts are introduced through poor irrigation water and accumulated in the root zone soil (Oster, 1994; Rengasamy, 2006). This salt accumulation results from the following processes: 1) the transport of water and ions from groundwater to the root zone soil is mainly driven by crop transpiration (i.e., root water absorption), 2) these ions are selectively absorbed by crop roots, and 3) ions mainly responsible for soil salinization (such as Na and Cl ) accumulate in the root zone soil (Kitano et al., 2006; Yasutake et al., 2006; 2007; 2009a; Araki et al., 2011). Therefore, it is important to understand active and selective ion absorption by crop roots in the soil salinization process. Active and selective ion absorption by crop roots is regulated through ion-specific transport proteins on root cell membranes (Taiz and Zeiger, 2006). Focusing on this function of membrane transport proteins, Epstain and Hagen (1952) expressed the characteristics of ion absorption with the Michaelis-Menten equation, which was proposed based on the dependence of ion absorption on ion concentration in the root zone. Sago et al. (2011a; 2011b) investigated the effect of environmental factors on root ion absorption and observed that ion absorption depended on not only ion concentration in the root zone but also on ion mass flow to the root surface, which was defined as ion concentration in the root zone multiplied by water flow driven by crop transpiration (Barber, 1962). Therefore, Sago et al. (2011c) modified the Michaelis-Menten equation and newly proposed the transpiration-integrated model, which represents ion absorption affected by ion mass flow. Nomiyama et al. (2012b) applied the transpirationintegrated model to the data of Yasutake et al. (2009b), to analyze ion absorption by maize and sunflower plants in soil-less culture under salinized conditions. The results indicated that the dynamics of salt accumulation in the simplified condition of root zone in soil-less culture can be explained reliably by the transpiration-integrated model. On the other hand, in soil-based culture, both soil evaporation and transpiration induce a complicated process of water transport accompanied by ion transport in the root zone soil. To investigate this complicated process, Kitano et al. (2009) developed a large-sized soil column system for analyzing the dynamics of water and ion transport in soilplant systems. Ebihara et al. (2010) examined salt accumu-