Abstract:

Aluminum (Al) toxicity is a serious agricultural problem in acid-soil area that is 40% of the arable land in the world. Aluminum toxicity can inhibit root growth and the uptake of water and nutrients, and ultimately result in production loss. However, some crop cultivars have been selected and bred to avoid or tolerate Al toxicity with specific biochemical mechanisms, of which organic acids play an important role. The inhibitory effect of Al on root elongation in many plants is alleviated by hydroponic culture with organic acids. However, there is little information about the physiological mechanism of the alleviation.
In present study, buckwheat (Fagopyrum esculentum Moench) was used to investigate the detoxification role of exogenous organic acids for aluminum. Buckwheat seedlings were cultured in nutrient solution with 0.4 mmol·L^-1 Al and different concentrations (0.2, 0.4, 0.8 and 1.2 mmol·L^-1) of exogenous organic acids (oxalic acid, citric acid and malic acid). The seedlings were used to determine the morphological characteristics, root activity, malondialdehyde (MDA) content, peroxidase activity in leaves, electrolyte leakage of leaf cells and the distribution of aluminum in buckwheat plants.
The results indicated that the 0.4 mmol·L^-1 Al treatment had a significant toxic effect on buckwheat seedlings, the total root elongation was restrained by 57.2%, the root activity decreased by 74.7%, and the electrolyte leakage of leaf cells increased by 54.4% as compared with the control (0 mmol·L-1 Al). The application of different concentrations of exogenous organic acids could alleviate the above inhibitory effect of Al. Compared with the treatment of 0.4 mmol·L^-1 Al without organic acids, in the treatments of 0.4 mmol·L^-1 Al and 0.2 mmol·L^-1 oxalic acid, citric acid and malic acid, the total root length was increased by 16.2%, 19.3% and 2.4% (Table 2), root activity promoted by 35.4%, 176% and 123% (Fig.1), electrolyte leakage of leaf cells reduced by 26.1%, 32.3% and 31.5% respectively (Table 3). With the increase of concentrations of exogenous organic acids, above indices were increased and MDA content and electrolyte leakage decreased. Al content in buckwheat plants especially in leaves was decreased by exogenous organic acids also. Compared with 0.4 mmol·L^-1 Al treatment, in other treatments, the decrease of Al content was significantly (P<0.05) in leaves, while a little in stems affected by exogenous organic acid added. Al content in roots was significantly (P<0.05) decreased in the treatments with 1.2 mmol·L^-1 oxalic acid and 1.2 mmol·L^-1 citric acid, and less changes in other treatments (Fig.2). It was suggested that the organic acids might combine Al located in rhizosphere soil and in plants, therefore reduce the content and the toxicity of Al.

Key words: Exogenous organic acid, Buckwheat (Fagopyrum esculentum Moench), Aluminum toxicity, Ameliorating effect