Effects of the Supply of Silicon in Different Forms on Injury and Distribution of Aluminum in Rice Root Tissues

Abstract

In order to evaluate the effects of Si supply on the amelioration of Al injury of rice plants (Koshihikari), plants were water-cultured for 3 d with Al (0.8 mmol L^<-1>) and low molecular Si (3.0 mmol L^<-1>) or high molecular Si (3.0 mmol L^<-1> low molecular Si+21.7 mmol L^<-1> high molecular Si) in 0.5 mmol L^<-1> Ca(NO_3)_2 solution. The distribution of aluminum in roots was measured, and root-tip tissues were observed microscopically after staining with hematoxylin, fluorescein diacetate-propidium iodide and neutral red. The results were as follows. 1) The root elongation relative to the control plants (without Al and Si) decreased with aluminum treatment. Aluminum injury appeared 24 h after Al treatment in elongation parts of the roots and Al had entered the epidermis and outer layers of the cortex, resulting in a decrease in the physiological activity of these cells. The supply of Si ameliorated Al injury, and no damage was observed even at 72 h after Al treatment in the root tissues, especially in the Al plus high molecular Si treatment. 2) The monomer Al concentration in culture solution decreased slightly with Si addition. Total Al content in shoots and roots was higher in the Al plus low molecular Si treatment and lower in the Al plus high molecular Si treatment than in the only Al treatment. Thirty-Fifty percent of the total Al amount in roots was present in the apoplastic fraction and 1-2% in the symplastic fraction. The Al amount in each fraction was markedly decreased in the Al plus high molecular Si treatment. These results indicate that the ameliorative effect of Si on Al injury is attributed mainly to changes in the chemical forms of aluminum ions in the culture solution and plants due to the formation of aluminum silicate complexes which is less toxic than monomer aluminum.