Effects of Exogenous Potassium (K+) Application on the Antioxidant Enzymes Activities in Leaves of Tamarix ramosissima under NaCl Stress

Saline soil is a worldwide distributed resource that seriously harms plants’ growth and development. NaCl is the most widely distributed salt in saline soil. As a typical representative of halophytes, Tamarix ramosissima Lcdcb (T. ramosissima) is commonly grown in salinized soil, and halophytes have different abilities to retain more K⁺ under salt stress conditions. Halophytes can adapt to different salt environments by improving the scavenging activity of reactive oxygen species (ROS) by absorbing and transporting potassium (K⁺). In this study, electron microscope observation, hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents determination, primary antioxidant enzyme activity determination and transcriptome sequencing analysis were carried out on the leaves of T. ramosissima under NaCl stress at 0 h, 48 h and 168 h. The results showed that H₂O₂ and MDA contents increased in the 200 mM NaCl + 10 mM KCl and 200 mM NaCl groups, but the content increased the most in the 200 mM NaCl group at 168 h. In addition, the leaves of T. ramosissima in the 200 mM NaCl + 10 mM KCl group had the most salt secretion, and its superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities were all higher than those of the 200 mM NaCl group and significantly higher than those of the control group. According to the results of transcriptome sequencing, it was found that the expression of 39 genes related to antioxidant enzyme activity changed significantly at the transcriptional level. Among them, 15 genes related to antioxidant enzyme activities were upregulated, and 24 genes related to antioxidant enzyme activities were downregulated in the leaves of T. ramosissima when exogenous potassium (K⁺) was applied under NaCl stress for 48 h; when exogenous potassium (K⁺) was used for 168 h under NaCl stress, 21 antioxidant enzyme activity-related genes were upregulated, and 18 antioxidant enzyme activity-related genes were downregulated in T. ramosissima leaves. Based on the changes of expression levels at different treatment times, 10 key candidates differentially expressed genes (DEGs) (Unigene0050462, Unigene0014843, Unigene0046159, Unigene0046160, Unigene0008032, Unigene0048033, Unigene0004890, Unigene0015109, Unigene0020552 and Unigene0048538) for antioxidant enzyme activities were further screened. They played an important role in applying exogenous potassium (K⁺) for 48 h and 168 h to the leaves of T. ramosissima in response to NaCl stress. Their expression levels were dominated by upregulation, which enhanced the activity of antioxidant enzymes, and helped T. ramosissima mitigate NaCl poison and resist NaCl stress. Particularly, Unigene0048538 in glutathione S-transferase (GST) activity had the largest log₂ fold-change in the comparison groups of 200 mM NaCl-48 h vs. 200 mM NaCl + 10 mM KCl-48 h and 200 mM NaCl-168 h vs. 200 mM NaCl + 10 mM KCl-168 h. Its expression level was upregulated and played an important role in NaCl toxicity. At the same time, the results of the phylogenetic tree analysis showed that Unigene0048538 had the closest genetic distance to Prunus persica in the evolutionary relationship. In summary, with the increase of exogenous potassium (K⁺) application time under NaCl stress, T. ramosissima can resist high NaCl stress by enhancing antioxidant enzymes’ activity and maintaining the growth of T. ramosissima. Still, it is not enough to completely eliminate NaCl poison. This study provides a theoretical basis for the molecular mechanism of salt tolerance and K⁺ mitigation of NaCl poison by the representative halophyte T. ramosissima in response to NaCl stress.