Organic Acid Secretion from Roots in the Al-Tolerant and Al-Sensitive Maize Inbred Lines

In this study, the character of root organic acid secretion in the 14.3 μmol/L Al 3+(in 227.5 μmol/L CaCl 2 solution) and in the 0.1 mmol/L Al 3+ (in nutrient solution) in two maize (Zea mays L.) genotypes , inbred line Z1 (Al-tolerant) and Z2 (Al-sensitive) was examined. The effects of 0.1 mmol/L Al 3+ stress on the root organic acid secretion and content in different period (after 1, 3, 7, 15, 20 days treatment) in Z1 and Z2 were comparatively studied, and relationships between malate dehydrogenase activity and root organic acid content or secretion were also examined. The results showed that the sensitivity of two inbred lines to Al was different. Z1 was found to be more tolerant to Al than that of Z2. After 3 days treatment, Z1 significantly exceeded Z2 in the relative biomass of roots and plant (Fig.1). Organic acid content and secretion from the entire root system were investigated via HPLC. The analysis showed that kinds of secreted organic acids, such as oxalic,malic,acetic,maleic,citric, succinic ect, were various in different periods, in which malic acid always remained(Table 1). The secreted malic acid of Z1 could be increased remarkably by Al stress, however Z2 kept low in different period in CaCl 2 solution (Fig.2A) and in nutrient solution (Fig.3). The root organic acids included malic,succinic,acetic, maleic ect, in which succinic acid was the highest in content. Aluminum stress increased root malic acid concentrations in Z1 and total root organic acid concentrations in both inbred lines (Fig.4). In the roots, NADP-malate dehydrogenases activity was increased significantly by Al stress in Z1, but decreased in Z2. NAD-malate dehydrogenase activity had a tendency of reducing in the both inbred lines. In the leaves, both enzyme activities were reduced by Al stress (Fig.5). The results suggested that the secreted malic acid was probably synthesized by the second way (PEP→OAA→Malate) in the roots.