Root-System Development and Water-Extraction Model Considering Hydrotropism

certain conditions. However, because root-system development is not considered in these water-extraction A two-dimensional model that combines root-system development models, the root length per unit soil volume should be and water extraction by roots is proposed to simulate the dynamic interaction between root growth and soil-water flow. Both of hydrotrogiven a priori as a function of soil depth. That is, the pism and gravitropism were considered as the controlling factors of water-extraction model alone cannot simulate the beroot growth in the proposed root-system development model. The havior of soil-water extraction by roots with active finite-element method was employed to compute the soil-water flow growth, which changes the distribution of the root syscaused by water extraction, evaporation, and irrigation. We succeeded tem over time. in simulating the plagiogravitropic elongations of lateral roots under As a pioneer study of modeling plant-root system dea plane condition, and the asymmetric architecture of root system under velopment, Lungley proposed a two-dimensional model a slope condition by the proposed model in which the root hydrotro(Lungley, 1973). Diggle and Pages developed threepism is considered. On the other hand, we cannot simulate such mordimensional root-system architecture models, and they phological characteristics of a root system by the use of the conventional model in which a random elongation factor is employed, and successfully simulated crop plant-root system developroot hydrotropism is not considered. The results support the imporment and morphological architecture three-dimensiontance of hydrotropism in root-system development and the availability ally (Diggle, 1988; Pages et al., 1989). It seems that the of the proposed model in which the hydrotropism is considered. fundamental part of root-system development modeling has been completed by these models. Recently, rootsystem development models focus on the interactions T interaction between plant-root systems and between plant-root systems and soil-water flow (Claussoil, especially soil with moisture, is very important nitzer and Hopmans, 1994; Doussan et al., 1998), or in many respects. For example, root-water uptake from nutrient supply (Somma et al., 1998), which combines the soil plays an important role in the hydrological prothe water uptake models or nutrient uptake models. cess of water flow through soil, plants, and air. In the The models shown above are not the only ones that field of crop science, many experimental studies indicate exist. Many other models have been proposed and some that variations in soil-water conditions affect the rootof them have succeeded in simulating root-system develsystem architecture of various kinds of crops, and that opment in various plant species, under various condiarchitecture affects absorption efficiency. However, the tions (e.g., Lynch et al., 1997; Jourdan and Rey, 1997). interaction between the roots and soil, which is hidden However, all applications of these models were made under the ground, is difficult to investigate and studies to the root systems developing under plane conditions, of them are lagging when compared with the studies of and no application under slope conditions can be found. shoots or leaves. The modeling of plant-root system It has been generally known that a plant that grows on development and of soil-water extraction by plant roots a slope has an asymmetric root system, and this asymis therefore very useful for understanding the interacmetric architecture of the root system has been contion between plant-root systems and soil. firmed by some experimental studies (Yamadera, 1990; The water extraction by plant-root system can be calScippa et al., 2001). Recently, the contribution of plantculated by adding a term of water-extraction intensity, root system architecture to slope stability has become S (s 1), to the Richards’ equation, which is the fundaone of the main interests in the fields of erosion control mental equation of water flow in unsaturated soil. Variand revegetation technology. Some studies have shown ous models that give the extraction intensity S have that plant-root systems growing under hillside slope conbeen proposed (Gardner, 1964; Herkelrath et al., 1977a; tributes to the slope stability, increasing the soil strength Herkelrath et al., 1977b; Feddes et al., 1978). Herkelrath by the their architecture, and decreasing the soil-water et al. (1977a, 1977b) proposed that the extraction intencontent by water uptake (Greenway, 1987). This contrisity S is proportional to the potential difference between bution has been investigated, considering root strength, roots and soil, volume saturation of the soil space, and growth, and rate of decay (Watson et al., 1999). root length per unit soil volume. Feddes et al. (1978) inTo simulate the root-system development and soiltroduced water-extraction efficiency as a function of soilwater flow under a slope condition, it is necessary to take water potential and succeeded in showing the behavior into account the effect of the slope condition on root of soil-water extraction by a plant-root system under growth. Some experiment results have shown that root growth is influenced by hydrotropism, which is the root elongation toward water (Takahashi, 1994; Takano D. Tsutsumi, K. Kosugi, and T. Mizuyama, Division of Forest Science, Graduate School of Agriculture, Kyoto University, Oiwakecyo Kitaet al., 1995). Because soil moisture exhibits asymmetric shirakawa Sakyo-ku Kyoto-city Kyoto 6068502, Japan. Received 26 distribution under slope conditions, root hydrotropism Nov. 2001. *Corresponding author ([email protected]). can be one of the main factors causing the asymmetric root-system development. Published in Soil Sci. Soc. Am. J. 67:387–401 (2003).