| In the present study, the aseptic seedlings of Potamogeton crispus L. were selected as the experimental material and the Cr6+was used as the stress factor. These aseptic seedlings of P. crispus were cultivated in solution containing different concentrations of Cr6+under lab condition. The changes of physiological characteristics in the aseptic seedlings of P. crispus were investigated by applying technology of transmission electron microscope, liquid chromatography and atomic emission spectroscopy. Meanwhile, the physiological and biochemical mechanisms of pH value in affecting Cr6+toxicity of P. crispus was invested.The results are as follow:The effects of different concentrations of Cr6+(0、0.01、0.05、0.09、0.13mmol/L) on P. crispus were investigated.The results indicated that:electron microscopy observation indicated that Cr6+imposed serious injury on ultrastructure (especially chloroplast and mitochondria) of P. crispus, such as swelling of chloroplast and mitochondria, vacuolation of thylakoid and disorderly arrangement of cristae under lower Cr6+level. The extent of ultrastuctural damage was much more serious with the increase of Cr6+concentration,(as under0.13mmol/L Cr6+treatment):the disappearing of the outer membranes of chloroplast, vacuolization and further disintegration of mitochondria and chloroplast, appearing a lot of liposomes. Cr6+stress caused various effects on antioxidant system. The generation rate of O2·-and the content of H2O2and MDA increased which induced the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD). Synchronously the contents of the antioxidants [ascorbate (AsA), glutathione (GSH)] increased under Cr6+treatment. The up-regulation of antioxidant enzymes activities and the higher content of GSH, ASA were good for aseptic seedlings of P. crispus to resist Cr6+stress. They can alleviate the accumulation of O2·-, H2O2and MDA. With the increase of Cr6+concentration,The soluble protein content increased initially and then decreased. At0.13mmol/L Cr6+, the soluble protein content of P. crispus droped sharply, even lower than the control group. The effects of different concentrations of Cr6+on the metabolism of polyamine (PAs) of aseptic seedlings of P. crispus were investigated.The ratio of free (Spd+Spm)/Put and total PAs contents increased significantly.With higher value of ratio of free (Spd+Spm)/Put and contents of total PAs, P. crispus acquired stronger resistance to Cr6+stress. In addition, the activity of arginine decarboxylase (ADC) enhanced as the Cr6+concentration increased and it reached the maximum value at0.13mmol/L Cr6+; Differently the activity of ornithine decarboxylase (ODC) increased after a transient decrease; Polyamine oxidase (PAO) and diamine oxidase (DAO) activities declined under certain Cr6+treatment which promoted a further accumulation of polyamines.In sum, our results suggested that Cr6+stress disturbed metabolic balance of polyamines.The up-regulation of Spm content, the changes of Put into Spd and Spm as well as the conversion between different forms of polyamines played a key role in the adaptation mechanism of aseptic seedlings of P. crispus under Cr6+stress.(2) Sterile seedlings of Potamogeton crispus were cultured by tissue culture technique which was used as experimental material. In this study, we investigated contents of element, photosynthetic pigments, reactive oxygen species (ROS), malondialdehyde (MDA) and proline, polyamines as well as activities of antioxidant enzymes in Cr6+-treated P. crispus at different pH levels so as to approach the physiological and biochemical mechanisms of pH value in affecting Cr6+toxicity of aquatic plants. The results indicated that,①Acidic conditions enhanced the accumulation of Cr while pH7.0treatment helped P. crispus maintain the balance of nutrient elements;②Chlorophyll (ChI) a, chlorophyll b and carotenoid contents showed the same trend, which increased first and was followed by a rapid decrease.The minmimum value appeared when the pH was4.0. In addition, pH6.0, pH7.0and pH8.0treatments could retard the chlorosis and promoted the biosynthesis of photosynthetic pigments;③Contents of reactive oxygen species (ROS) and MDA increased when the pH was pH4.0, pH5.0and pH9.0. Activities of superoxide dismutase (SOD) and catalase (CAT) reached the minimum when the pH value was6.0,7.0; However, activity of peroxides (POD) decreased with the increase of pH value;±In response to Cr6+stress, plants showed sharp increase in proine content when the pH was6.0or7.0;⑤An improved level of Put was noted in pH4.0, pH5.0treatments while Spd and free (Spd+Spm)/Put ratio increased in pH6.0, pH7.0treatments; For Spm content, it generally increased with pH value and it was lower than the control group when the pH value was4.0or5.0. Our results demonstrated that resistance of P. crispus to Cr6+stress could be enhanced by reducing the excessive accumulation of Cr, promoting photosynthetic pigments, removing ROS efficiently, maintaining the balance of nutrient elements and keeping high levels of proine, polyamines as well as free (Spd+Spm)/Put ratio when pH value was within the range from6.0to7.0. |
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