Effect of Silicon on Root Morphological Parameter of Silicon Accumulating and Non-Accumulating Plants Under Aluminum Stress.

The main symptom caused by aluminum (Al³⁺) stress in plants is root growth inhibition, influencing negatively the water and nutrients uptake. Although silicon (Si) is not considered to be an essential nutrient for plants, it is known to effectively mitigate various abiotic stresses in plants, such as Al³⁺ stress common in acids soil. However the mechanism of this interaction between Si and Al³⁺ is not well understood, especially in Si non-accumulator plants as castor beans. The majority of researches on this role of Si had been done on Si accumulators as rice and sugarcane. Moreover there is not any information about that interaction in plants root, since the main effect of Si in plant is on the shoot. So, because of the damage caused by Al³⁺ on plants is on the root the aim of this study was to evaluate the influence of Si on the root morphological parameter of a Si accumulating plant (upland rice) and Si-non-accumulating plant (castor bean) under Al³⁺ stress. There were two experiments, the first experiment was upland rice (Si accumulating plant) and second experiment was castor bean (Si non-accumulating plant). The experiments were conducted in nutrient solutions, and were laid out in randomized blocks, (Factorial design, 2x4) with four replications. In the first experiment (upland rice), the treatments included four combinations of Si and Al³⁺; 1) 0 Si and 0 Al³⁺, 2) 1.7 mmol L^-1 Si and 0 Al³⁺, 3) 1.7 mmol L^-1 Si and 1.4 mmol L^-1 Al3⁺, 4) 0 Si and 1.4 mmol L^-1 Al³⁺ ; imposed on two upland rice cultivars Maravilha (Al³⁺ susceptible); ANA 7007 (Al³⁺ tolerant). In the second experiment (castor bean), the treatments were four combinations of Si and Al³⁺; 1) 0 Si and 0 Al³⁺, 2) 1.7 mmol L^-1 Si and 0 Al³⁺, 3) 1.7 mmol L^-1 Si and 1.1 mmol L^-1 Al³⁺, 4) 0 Si and 1.1 mmol L^-1 Al³⁺; imposed on two castor bean cultivars CRZ 6 (Al³⁺ susceptible); CRZ 18 (Al³⁺ tolerant). The root parameters were length, surface area, volume and diameter of upland rice and castor bean plants. Differences were observed in all upland rice root parameters among treatments. The treatments without Al³⁺ showed better growth over all other treatments. However, there was no significant difference between Si applied with Al³⁺ and the Al³⁺ alone treatments in the parameters. Therefore, Si application did not improve the upland rice root development under Al³⁺ stress. For castor bean root parameters, the treatments without Al³⁺ showed better root length, surface area and volume over all other treatments, and there were no difference between Si applied with Al³⁺ and the Al³⁺ alone for these treatments. However the treatment Si applied with Al³⁺ showed lower root diameter over all other treatments. The root diameter increased with Al³⁺ application, it occurred because the root become stunted according of root meristem injury or death and increase the rigidity of the cell wall and their diameter. But in this case, Si applied with Al³⁺, the Al³⁺ toxicity did not increase the root diameter of castor bean plants, maybe the Si contributed a little for that, but it needs more studies. Si did not improve upland rice and castor bean root growth under Al³⁺ stress.