Differential effects of monomeric and polymeric aluminium on the root growth and on the biomass production of root and shoot of corn in solution culture

Isolated studies carried out in solution culture for wheat (Triticum aestivum L.) and soybean (Glycine max L.) have shown that Al in its polymeric form ('Al(13)') is more phytotoxic than Al in its monomeric form, especially with regard to root growth. The objective of this study is to find out whether polymeric Al is more toxic than monomeric Al on root growth and biomass production of corn (Zea mays L.). This study was carried out with two corn cultivars differing in terms of their sensitivity to monomeric Al toxicity, in solution culture: HS7777, a sensitive single hybrid and C525M, a tolerant single hybrid. The results showed that the increase of Al concentration in solution, in general, causes a significant reduction on seedling root elongation, with a different intensity of reduction for each Al form. Monomeric Al significantly reduced root elongation of cultivar HS7777 at concentrations equal to or above 16 micromolar Al, after the second day, as it did for cultivar C525M at concentrations equal to or above 22 micromolar Al. On the other hand, polymeric Al(13) caused root growth to cease completely at concentrations above 5 micromolar Al after the second day. Cultivar C525M was more affected than HS7777. On the seventh day, the typical symptoms of Al toxicity could be observed: decrease in root elongation, reduction of secondary root counts and root diameter increase, the effects being all the more evident for polymeric Al, which also induced curling of the main root apical area. The effects of monomeric Al were less evident for root and shoot biomass than they were for root growth. However, cultivar C525M was not affected, in general terms, by the increase of Al concentration in solution, whereas HS7777 was prone to biomass reduction. As for Al(13) treatments, root and shoot dry matter was far more affected at concentrations equal to or above 5 micromolar, regardless of cultivar. Tolerance differences between cultivars HS7777 and C525M for different Al forms in solution suggest that there are physiological differences in the toxicity mechanism of monomeric and polymeric forms of Al, as previously detected by other authors working with wheat.