Dynamic change in surface pH and organic acid content in rice seedling root-tip exposed to aluminum stress

Surface pH and cytoplasm organic acid contents of root-tips of rice seedlings under aluminum stress were monitored via a modified agarose gelling plate technique, with bromocresol purple as pH indicator. While no obvious changes are noted in surface color of rice seedling root-tip under Al stress for 1 h, 3 h and 6 h, significant changes occur under no-Al stress, showing little orange in 1 h and deep orange in 3~6 h. Surface pH of Al-free rice seedling root-tip is time-dependent and strongly alkaline under an initial pH of 4.4 condition. On the contrary, increase in surface pH of rice seedling root-tip is inhibited by Al stress. Under 200 μmol·L-1 Al stress, three out of a total 10 detected organic acids in cell of rice seedling root-tip, show varying, time-dependent dynamics. There is insignificant change in oxalic acid in rice seedling root-tip during the test and a minimal change in citric acid in the initial 3 h of Al stress. However, citric acid rapidly and significantly increases then, reaching the maximum in 12 h (P<0.05) before decreasing by 16.27% in 24 h. Though change in citric acid is small (P>0.05) after 24 h of Al stress, it is still higher than that in 6 h. Malic acid in rice seedling root-tip decreases, but at an insignificant rate in 3 h of Al stress. After 3 h, malic acid decreases significantly, but the decrease ceases to be significant after 6 h of exposure to Al stress.