| Potassium is one of the essential macroelements for plant growth. Lack of potassium resources, and increasing demand for potassium fertilizer in agricultural production, this contradiction has become one of the important factors limiting the further development of agricultural production. Watermelon is an important horticultural crop and has great demand for potassium throughout its growth and development process. In our present study, the experimental materials were two cultivars of watermelon (wild watermelon cv.Yongshi and cultivated watermelon cv. Zaojia8424), which were different in potassium efficiency. The present study was aimed to compare the physiological mechanisms of two watermelon genotypes with contrasting potassium efficiency in response to short-term (6h) and long-term (120h) potassium shortage. The normal K. concentration was3mM. The main results are as follows:1. Effects of potassium shortage on growth and physiological characteristics of different genotypes of watermelon seedlings. The results showed that compared with plants grown under3mM K, Zaojia8424showed significant decrease in shoot dry weight, root dry weight, root morphology parameters, root vigor, leaf area and chlorophyll content under0mM K for120h. Potassium shortage significantly inhibited the growth of Zaojia8424seedlings, and no obvious effect was observed on Yongshi.2. Effects of potassium shortage on the plant mineral nutrition (K+, Ca2+, Mg2+concentration) and the relative transcript abundance of gene ClaO12281that encoded watermelon potassium ion channel and gene C1a02760that encoded high-affinity potassium transporter, the root potassium uptake kinetics parameters were also analyzed. The results showed that Yongshi had the ability to make better use of Ca2+and Mg2+instead of K+, and had higher expression levels of genes ClaO12281and Cla02760, as well as K+affinity and capacity in the rhizosphere than Zaojia8424.3. Effects of potassium shortage on the hormones (IAA, GA3, ZR, ABA, ETH) contents. The results showed that after long potassium shortage, plant growth-related hormones (IAA, GA3) concentrations were significantly decreased, and the aging-related hormone (ABA, ETH) concentrations were significantly increased in Zaojia8424. ZR content of Yongshi roots increased significantly. The IAA/ABA、GA3/ABA、ZR/ABA were significantly declined after long potassium shortage, but the hormones concentrations were declined higher in Zaojia8424than in Yongshi. 4. Effects of potassium shortage on the ROS-generating and ROS-scavenging systems were studied. The results showed that long-term potassium shortage significantly increased H2O2and MDA content in Zaojia8424plants. A substantial increase in Yongshi on the content of antioxidants (AsA, GSH) was observed, while Zaojia8424showed only slightly increased. The APX and GR were increased obviously in Yongshi, but others showed little change compared with control, while Zaojia8424had an overall increased in antioxidant enzyme activity.The above results indicated that potassium shortage significantly inhibited the growth of Zaojia8424, which was more sensitive to low potassium stress. Yongshi was less affected by potassium shortage. The larger root volume, better ion substitution capability, as well as K+high affinity in the rhizosphere, which made Yongshi have better absorption and utilization of potassium and better adapt to potassium shortage. Meanwhile, the hormonal balance steady in Yongshi was not been broken basically, which were very favorable for maintaining normal physiological metabolism in plants. In addition, Yongshi owned a highly efficient ROS scavenging mechanism, when subjected to potassium shortage, Yongshi could promptly remove the excess reactive oxygen species, and which made Yongshi less affected by membrane lipid peroxidation injury, resulting in strong tolerance to low potassium stress of Yongshi. |
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