Effects Of Aluminum Toxicity On Root Physiology Of Soybean And Aluminum Subcellular Distribution In Root Tip Cell

In red soil region, acid - aluminum(Al) is a key factor of limiting plant growth. In order to enrich the resistance - aluminum mechanism, using two soybean (Glycine max) with different Al - toxicity tolerance as experimental materials, the effects of different acid - aluminum treatments on their root physiological characteristic were studied through solution culture. The Al treatments were set as following: T₁, mmol·L^-1 + pH 3; T₂, 0 mmol·L^-1 + pH 4; T₃, 0 mmol·L^-1 + pH 5; T₄, 0.5 mmol·L^-1 + pH 5; T₅, 1 mmol·L^-1 + pH 5; T₆, 1 mmol·L^-1 + pH 3 and treatment time was set as 2, 4, 6, 12 h and 24 h. The results showed that root growth and root activity were inhbited by the decreasing of pH value, as the protein cotent, anti - oxidative enzyme were increased firstly, and then decreased. At pH 5, along with the increasing of external Al, the root growth, root activity, protein content, and SOD activity in two culitvars were increased firstrly, and decreased afterwards, and the maximums were observed at T4 treatment. Meanwhile, the acitivities of POD and CAT were both gradually increased by the Al³⁺ stresss. The most serioust inhibition to roots appeared at T₆ treatment, the root activity, protein cotent and the acitivities of anti-oxidativeenzymes all attained their minimal value, simultaneously, the permeability of plasma membrane achieved its apex. Hematoxylin staining experiments indicated, at T₆ treatment, Al can be detected at 2 h treatment. The Al content differed among different root segment, the sequence was 0 - 0.3 cm＞0.6 - 0.9 cm＞0.3 - 0.6 cm.The root parameters and some physiological responds soybean roots under long - term and short - term Al³⁺ treatments were also studied. The results showed that the influence of 7 d Al³⁺ treatment to the root growth was higher than 12 h treatment. At 90 mg·L^-1 Al³⁺ treatment, the total root length achieved its minimal value, and the permeabity of plasma membrane got the maximal value. However, no significant decrease was observed in the shoots growth. The inhibition to root growth of 14 d Al³⁺ treatment did not differ from 7 d Al³⁺ treatment. However, 14 d Al³⁺ treatment greately decreased shoots growth when compared with 7 d treatment. By comparison of the activities of the root tips and root hair zone under different Al³⁺ treatment, it could be indicated that the action sites of Al on plant was root tips and the harm to the soybean was happened after 12 hours Al³⁺ treatment. And no detectable txoxicity respond was observed in root hair activity before 14 d Al³⁺ treatment.Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetry (TG) and Differential Thermal analysis (DTA), as effective chemical methods, were introduced to identify Al toxicity in soybean. The results showed that the Al contents of soybean roots in the 60, 90 mg·L^-1 Al³⁺ concentration is higher than the other Al³⁺ treatments. And the Al content reached the level of Al toxicity, significantly. In the same wavenumber range, the character, wavenumber and absorption peak of infrared spectrogram had different value under different Al³⁺ treatments. The absorption peaks of 3300 cm^-1, 2930 cm^-1, 1542 cm^-1 and 721 cm^-1 were the characteristic peaks as identifications that can used to evaluate the Al toxicity of soybean. Detailed description as follow: At the high Al³⁺ treatment(60, 90 mg·L^-1), the value of characteristic peaks of 3300 cm^-1, 2930 cm^-1 were higher than other Al³⁺ treatment. There is no absorption at the 1542 cm^-1 under 90 mg·L^-1 Al³⁺ treatment, as the means of characteristic peak of Amideâ…¡. However, The characteristic peak of 721cm^-1 is clearly at 60 and 90 mg·L^-1 and no value in CK, 10, 30 mg·L^-1 Al³⁺ concentration. Curves of TG and DTA revealed the greatest difference with different Al³⁺ treatment in the range of temperature of 400 - 500℃. Under the Al³⁺ treatment of above 30 mg·L^-1, TG curve showed less weight than CK, which indicated that the root system may be lignification, produced some material that different duing burning. DTA curve had double peaks at 60, 90 mg·L^-1 Al³⁺ treatments, which may also used as identification mark of Al toxicity.Through Transmission Electron Microscope (TEM) and Energy Dispersive X - ray (EDS), the distribution of Al³⁺ in root tips affected by Al³⁺ were examined. We found that Al³⁺ stresses resulted in irregularly thickened cell wall, increased number of mitochondria, expanded nuclear membrane, and densified precipitates of vacuole. Under the highest Al³⁺ treatment, the mitochondria and other organelles disappeared and cell wall was intact. Al was detected in cell wall, mitochondria and electron - dense precipitates in vacuole of root tip cell under the 10 mg·L^-1Al³⁺ stresses by EDS. With the increase of external Al³⁺ concentration treated, the weight and atomic percentage of Al in the organelles increased. The Al³⁺ was found in nuclei when the external Al³⁺ was over 60 mg·L^-1. And there was no Al³⁺ in mitochondrion under 60 mg·L^-1 and 90 mg·L^-1Al³⁺ treatments and electron - dense precipitates of vacuole under the 90 mg·L^-1 Al³⁺ stresses. The 14 days Al³⁺ stresses significantly inhibited the growth of root system. The content of Al³⁺ in cell wall was most significantly impacted by the external Al³⁺ concentration. The atomic number of P / Al in cell wall and mitochondria decreased with increased Al³⁺ content. EDS can be used to determine the subcellular location of Al³⁺. As the treatment concentrations of Al³⁺ increased, Al³⁺ primarily accumulated in the cell wall, gradually gathered in part of the organelles and nuclei. The Al³⁺ concentrations also decreased from out layer to insider in the cell.