Effects of silicon on growth and mineral composition of barley grown under toxic levels of aluminum

Growth chamber studies were conducted to investigate the effects of silicon (Si) on plant growth and nutrient uptake in barley (Hordeum vulgar e L.) grown hydroponically under toxic levels of aluminum (Al). Treatments were 0, 100, or 150 mu mol L-1 Al (as AlCl3), and 0 or 1.0 mmol L-1 Si (as silicic acid) in Experiment A; and 0, 50, or 75 mu mol L-1 Al (as AlCl3) and 0.5 or 1.0 mmol L-1 Si (as silicic acid) in Experiment B. The Al and Si treatments were arranged factorically in a randomized, complete block design with three replicates. Aluminum added significantly(P < 0.05) increased plasma membrane permeability, which was not alleviated by added Si. The addition of Si significantly (P < 0.05) reduced root length, dry weight, shoot nitrogen (N) and phosphorus (P) concentration, root N and potassium (K) concentration, and increased root P concentration of barley plants exposed to 75 mu mol L-1 Al and above. Nutrient uptake was significantly inhibited when plants were grown under 75 mu mol L-1 Al and above, regardless of Si. However, Si helped to alleviate the toxic effects of Al on the growth of plants grown with 50 mu mol L-1 Al, thus resulting in little difference in both dry matter weight and nutrient uptake between Al-fed treatments and Al-deprived ones. Under Al stress, increased levels of Si increased shoot Al concentration, while shout Si concentration significantly (P < 0.05) decreased with increasing Al level in nutrient solution. It can be concluded that Si alleviated the phytotoxicity of Al at a lower concentration of Al, hut exaggerated it at a higher concentration of Al. Based on the literature reports and the present study, the authors believe that the effect of Si on Al-stressed plants depends upon the plant species, Al and Si levels, and the duration of plants exposed to Al stressed environment. Si-enhanced Al toxicity in barley is partly attributed to the mineral nutrient imbalances.