The amelioration of aluminium toxicity by silicon in higher plants: Solution chemistry or an in planta mechanism?

Abstract

Aluminium (Al) toxicity is a very important factor limiting the growth of plants on acidic soils. Recently, a number of workers have shown that, under certain conditions, silicon (Si) can ameliorate the toxic effects of A1 in hydroponic culture. The mechanism of the amelioration is unclear, but three suggestions have been put forward: Si‐induced increase in solution pH during the preparation of hydroponic solutions; reduced availability of Al due to the formation of hydroxyaluminosilicate (HAS) species in those solutions during plant growth; or in planta detoxification. It is now known that it is possible to make up Al and Si solutions in an order in which pH is lowered prior to Al addition; in these cases amelioration has still been observed. Amelioration has also been noted in experiments where HAS formation is minimal. These observations would suggest that, at least under some circumstances, there is an in planta component to the amelioration phenomenon. Several microanalytical investigations have noted codeposition of Al and Si in root cell walls. We propose a model in which root cell walls are the main internal sites of aluminosilicate (AS) and/or HAS formation and of Al detoxification. Factors promoting AS/HAS formation in this compartment include: high apoplastic pH; the presence of organic substances (e.g. malate); and the presence of suitable local concentrations of reactive forms of Al and Si, on or within the surfaces of the wall matrix. All these are likely to be important in the amelioration of Al toxicity.

Apoplastic Binding of Aluminum Is Involved in Silicon-Induced Amelioration of Aluminum Toxicity in Maize

“…However, little is known of Al reactions in the root apoplast, and Al and Si interactions in this compartment are likely to be even more complex (Cocker et al, 1998a). The formation of an Al-Si complex is dependent on pH, and Al and Si concentration.…”

“…It may be assumed that under our experimental conditions, the apoplastic pH in the root tip was also higher than that of the root surface because we observed an increase in the bulk solution pH, if the pH was not kept constant using a pH-stat during the experiment (data not shown). Cocker et al (1998a) suggested that the concentrations of Al and Si and pH within the apoplast are likely to decide on HAS formation. The fact that no ameliorative effect of Si on Al toxicity was observed after 1 h of Al treatment, according to Al-induced callose formation, but a clear effect could be detected after 12 h Al treatment may reflect such dose requirement in cortical cell walls.…”

“…Little or no effect of Si on Al resistance has been found in wheat (Triticum aestivum), pea (Pisum sativum; Hodson and Evans, 1995), and cotton (Gossypium hirsutum; Li et al, 1989), but this may have been due to methodological shortcomings (Ryder et al, 2003). The beneficial role of Si has been suggested to be based on two aspects: solution chemistry and in planta mechanisms (Cocker et al, 1998a). Ma et al (1997) suggested that the ameliorative effect of Si on Al toxicity resulted from decreasing the toxic Al 31 concentration in solution by forming Al-Si complexes.…”

Atributos químicos de solos influenciados pela substituição do carbonato por silicato de cálcio

“…O Al inibe o crescimento radicular e, conseqüentemente, a absorção de nutrientes. Efeitos de amenização da toxicidade de Al pela adição de Si têm sido reportados em muitas espécies (Cocker et al, 1998), supostamente devido à redução na concentração de Al tóxico. Segundo esse mesmo autor, o uso de Si para reduzir a toxidez de Al tem-se mostrado bastante efetivo.…”

Effects of silicon (Si) fertilization on gas exchange and production in Brachiaria

“…On a short-and mid-term basis, more than 30 million hectares are going to be recovered, to be degraded or sharp production decline process (Sano et al, 2008). A reason for the resistance of Brachiaria grass in Cerrado soil may be its ability to absorb and accumulate Si in the epidermis of leaves, thereby reducing the toxic effects of aluminum (Al), manganese (Mn), and iron (Fe) and increasing phosphorus (P) availability (Cocker et al, 1998;. Tropical soils are highly weathered, acidic, and low in Si availability for plants.…”