Abstract
Melatonin has been regarded as a promising substance that enhances the abiotic stress tolerance of plants. However, few studies have devoted attention to the role of melatonin in improving salt tolerance in sugar beets. Here, the effects of different application methods (foliar application (100 μM), root application (100 μM), and combined foliar and root application) of melatonin on the morphological and physiological traits of sugar beets exposed to salt stress were investigated. The results showed that melatonin improved the growth of sugar beet seedlings, root yield and sugar content, synthesis of chlorophyll, photosystem II (PS II) activity, and gas exchange parameters under salt stress conditions. Moreover, melatonin enhanced the capacity of osmotic adjustment by increasing the accumulation of osmolytes (betaine, proline, and soluble sugar). At the same time, melatonin increased the H+-pump activities in the roots, thus promoting Na+ efflux and K+ influx, which maintained K+/Na+ homeostasis and mitigated Na+ toxicity. In addition, melatonin strengthened the antioxidant defense system by enhancing the activities of antioxidant enzymes, modulating the ASA-GSH cycle, and mediating the phenylalanine pathway, which removed superoxide anions (O2•−) and hydrogen peroxide (H2O2) and maintained cell membrane integrity. These positive effects were more pronounced when melatonin was applied by combined foliar and root application. To summarize, this study clarifies the potential roles of melatonin in mitigating salt stress in sugar beets by improving photosynthesis, water status, ion homeostasis, and the antioxidant defense system.
Highlights
More sugar must be produced to meet the demands of the growing population, but in recent years, due to limitations of farmland and deterioration of the ecological environment, sugar production has continued to decline [1]
Salt stress significantly inhibited the growth of sugar beet seedlings, while the application of melatonin alleviated this inhibition, especially the combined root and foliar applications
The current study demonstrated the positive role of exogenous application of melatonin in enhancing salt stress tolerance in sugar beets
Summary
More sugar must be produced to meet the demands of the growing population, but in recent years, due to limitations of farmland and deterioration of the ecological environment, sugar production has continued to decline [1]. Statistics from the Food and Agriculture Organization (FAO) show that nearly 21% of farmland is affected by salinization worldwide [2]. In addition to the primary salinization caused by natural processes, anthropogenic actions are accompanied by a rapid increase in secondary salinized farmland [3]. It is very important to increase sugar production by promoting the utilization of salinized farmland. The sugar beet, whose salt stress tolerance is better than that of most other crops, is one of the major sugar crops [4]. Growing sugar beets on salinized farmland may be an effective strategy to enhance the production of sugar and the utilization of salinized farmland. The seedling stage is most vulnerable to salt stress, and salt tolerance gradually increases during growth and development [6]
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