Root secretion of oxalic and malic acids mitigates the rubber tree aluminum toxicity

Aluminum (Al) toxicity is a major constraint for crop growth and production in tropical acidic soils, and organic acids (OAs) play a critical role in mitigating Al toxicity. However, the types of OAs secreted and Al tolerance varies across plants. To better understand the mechanisms that rubber trees use to mitigate Al toxicity, Reyan7-33-97 rubber tree tissue culture saplings were treated with different concentrations of AlCl3 to investigate the secretion of OAs under Al stress, along with the mitigation effects of exogenous OA application. The results showed that the pH value of root exudates was significantly decreased in response to the increase of Al concentration and duration, which resulted in 1.19–2.49 fold increases to total OA levels, particularly oxalic acid as its content was considerately higher than other OAs. When OAs, such as oxalic and malic acids were exogenously applied to Al treatment solutions, the rubber tree chlorophyll content, leaf relative water content and root activity were increased significantly, whereas the Al content in roots and leaves, antioxidant enzyme activities (POD, SOD), MDA and free proline contents decreased significantly in comparison with the Al treatment alone. Oxalic acid was found to be superior to malic acid in alleviating Al toxicity. Taken together, our results demonstrate that root secretion of OAs, particularly oxalic acid, under Al stress is a mechanism for rubber tree Al detoxification.