Pineapple (Ananas comosus (L.) Merrill) is commonly grown in strongly acid soil, in which high aluminum (Al) concentration is often toxic to the roots of plants. The root apices of plants are most sensitive to Al toxicity. This paper is to evaluate the effect of Al on the growth of roots of four pineapple cultivars: Cayenne, Tainung No.6, Tainung No.13 and Tainung No.17 in Taiwan. The differences in the amount of callose and malondialdehyde (MDA) in root apices (1 cm in length) between Al-resistant and Al-sensitive pineapple treated with 0 and 300 µM AlCl3 were determined. The role that the cell wall of root apices plays in Al-resistant characteristics is also discussed. After treating with 300 µM AlCl3 in hydroponic solution (pH 4.5) for 72 h, the root elongation of Cayenne, Tainung No. 6, Tainung No. 13 and Tainung No. 17 was 115, 85, 93 and 73% of the values obtained compared to that without Al treatment, respectively. AlCl3 treatment did not increase callose and MDA contents for Cayenne, but caused significant increase for Tainung No.17. Upon exposure to Al, Al adsorption on cell walls of root apices increased with time and AlCl3 concentrations for Cayenne and Tainung No.17, but relatively greater in Tainung No.17. It reflects the fact that, Cayenne is Al-resistant and Tainung No.17 is Al-sensitive. When root apices were pretreated with 1 and 10 mM of malic acid, the Al adsorption of the cell walls of Cayenne’s root apex was lower by 18 and 31%, respectively, relative to the values obtained without the malic acid treatment. It indicates that root apex of Cayenne may secrete more malic acid that is capable of Al complexation, as well as reducing the Al binding to cell walls in order to better resist Al toxicity.
Key words: Aluminum, pineapple, root apices, cell wall, adsorption.
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