K. Kosugi

Effect of Hydrotropism on Root System Development in Soybean (Glycine max): Growth Experiments and a Model Simulation

To observe root system development, soybean plants (Glycine max) were grown in root boxes that were set horizontally to reduce the effect of gravity. Along with the root system development, the two-dimensional distribution of soil water content in the root boxes was measured continuously by the time domain reflectometry (TDR) method. Root system development and its morphological architecture were strongly affected by the positions of the water supply. It is suggested that root hydrotropism plays the dominant role in root system development. In addition to root hydrotropism, the importance of root compensatory growth is suggested. A combined model of root system development and soil water flow considering root hydrotropism and compensatory growth was used to simulate root system development and soil water flow. The morphological architecture of root systems and the distribution of soil water content obtained in the experiment were successfully explained by the model simulation. These results confirmed that root hydrotropism and compensatory growth are dominant factors in root system development under a reduced effect of gravity. The validity of the model was confirmed, and its applications for various purposes were suggested.

Three-Dimensional Modeling of Hydrotropism Effects on Plant Root Architecture along a Hillslope

A three-dimensional model of root system development and soil water flow is described and applied to actual conditions along a hillslope. In the model, gravitropism, hydrotropism, and circumnutation were employed as the main factors controlling root elongation. Root systems of 2-yr-old pine trees ( Pinus massoniana Lamb.) on natural slopes in southern China were excavated and examined, and their development was simulated through the use of continuously monitored temperature and rainfall data. In the simulated root systems, angles between first-order lateral root segments and the vertical direction on the upslope portion of a tree were larger than those on the downslope portion of the tree; hence, root systems exhibited asymmetric architectures. This asymmetry was more obvious for root systems developed on the downslope side of the hillslope. Because root systems simulated without the effect of hydrotropism did not develop asymmetric architectures, the direction of soil water flux and the effect of hydrotropism appear to be the main factors contributing to the observed architectural asymmetry, typical of root systems along hillslopes. Calculations with the proposed root system model were helpful in elucidating and understanding the predominant processes affecting root system development on hillslopes.