Eureka! Institutional Repository

The purpose of this postgraduate dissertation was to study the effect of silicon on the growth of mint plants grown under toxic aluminum levels. Experiments conducted showed that plant exposure at 2 or 4 mM Si improved plant growth causing an increase in the weight of fresh shoots compared to controls. In addition, this exposure caused a decrease in proline concentrations compared to controls. These results indicate that there might be some form of stress limited by the addition of Si, since weights of fresh shoots increased, whereas proline concentrations gradually decreased with the increase in Si concentration. To similar findings have also led the significant increases of the parameter 'Area', which is associated with the electron donor stock in electron transport chain. The measurements of the concentrations of malondialdehyde (MDA), chlorophyll counts, transpiration rate (E) and intercellular CO2 concentration (Ci) did not reveal any significant differences, while the maximum photochemical performance (Fv/Fm), Fv/Fo and the performance index (PI) as well have produced contradictory results, which could not lead to final conclusions. These contradictory results are probably due to the rather limited number of experimental units in all treatments, which allowed the estimation of some trends, but could not extraction of reliable statistical results. Exposure of plants to 350 μM Al and in all combinations of 200 or 350 μM Al with 2 or 4 mM Si, did not statistically affect the weight of the fresh shoots, except for 200 μM Al, which caused an unexpected weight increase. Exposure of the plants to 200 μM Al, also caused 20.7% reduction in proline concentrations relative to controls. However, the subsequent addition of 2 or 4 mM Si triggered a considerable increase in proline concentration in leaves and shoots compared to the previous value. These indicate that the addition of only 200 μM Al was not sufficient to provide the capability of immediate activation of the proline biosynthesis mechanisms. In contrast, plant exposure to 350 μM Al in absence of Si, as well as the subsequent addition of 2 or 4 mM Si, triggered an increase in proline concentration in leaves and shoots compared to controls. This gradual vi increase in proline by increasing the concentration of Si probably indicates that even the concentration of 4 mM Si was not high enough to limit the induced by 350 μM Al toxicity. In contrast to the above stress indicators, MDA measurements suggested that the exposure of plants to 200 or 350 μM Al caused significant stresses that were expressed as significant increases in MDA concentrations relative to controls. Furthermore, the addition of 2 or 4 mM Si caused a gradual decrease in MDA concentrations in the 200 or 350 μM Al-treated plants. Stress from the addition of 200 or 350 μM Al to mint plants were also identified by significant reductions in the Fv/Fm ratios relative to controls. These reductions were, in fact, proportional to the Al addition levels. However, when the addition of 200 or 350 μM Al was accompanied by the addition of 4 mMSi instead of 2, the Fv/Fm ratios showed significant increases with values ranging from 0.800 to 0.850, indicating plants with excellent photosynthetic activity. Stress from the addition of 200 or 350 μM Al to mint plants were still discovered by the significant Fv/Fο reductions compared to controls, which were proportional to the levels of Al addition. However, when the addition of 200 or 350 μM Al was accompanied by the addition of 4 mM Si, the Fv/Fo ratios showed significant increases ranging between 4.3 and 4.5. Nevertheless, if the addition of 200 or 350 μM Al was accompanied by an addition of 2 instead of 4 mM Si, the Fv/Fo ratios, although showing significant increases, were still below 4.0. In contrast to the above parameters, "Area" and the total chlorophyll content by SPAD, did not show significant differences in the different treatments relative to controls. Nonetheless, the performance index revealed that adding 200 or 350 μM Al to mint plants caused significant index decreases relative to controls, which were proportional to the levels of Al addition. However, the subsequent addition of 2 or 4 mM Si respectively, showed significant increases in the index over the previous values, which were, however, still lower than those of the controls. Regarding the CO2 assimilation ratio (A) and the stomatal conduction (gs), the addition of 200 or 350 mM Al to mint plants caused significant decreases in A values compared to controls, which were proportional to the levels of addition of Al. These reduced values could not be increased by further addition of 2 or 4 mM Si. It might be vii particularly worth pointing out that the addition of 2 mM Si to plants to which 350 mM Al had been already added, caused a significant reduction in the values instead of an increase, relative to the controls. Finally, concerning the transpiration rate (E) and the intercellular concentration of CO2 (Ci), the addition of 200 or 350 μM Al to mint plants did not cause significant changes in E and Ci values relative to controls. However, when the addition of 350 μM Al was accompanied by the addition of 2 mM Si or when the addition of 200 μM Al was accompanied by the addition of 4 mM Si, E or Ci values presented significant reductions or increases, respectively,compared to controls.