| Stevia rebaudiana is new sugar source plant widely planted in the world. Its leaves contain a large amount of steviol glycosides which possessed potent sweetness and low calorie. These glycosides can be used to take the place of artificial sugar and other sugars owning high calorie. Its promising market has attracted a lot attention. China is the country that possesses the largest plantation area and exports the greatest quantity of stevia extract. In China, the planted districts cover the provinces such as Anhui, Jiangsu, Shandong, Jiangxi, Gansu and Xinjiang. However, with the climate change and global warming, parts of these districts become salty. In general, salt stress has significant negative effects on the growth, biomass and quality of plant. Previous studies showed that the application of adequate concentration of sodium nitroprusside (SNP, nitric oxide donor) could alleviate the stress symptoms and promote the growth and development and adaptation ability of plants under stress conditions. Therefore, to understand the effects of salt stress, exogenous nitric oxide and their combination application on the growth, physiological responses and glycoside contents of S rebaudiana can not only provide theory basis for further breeding salt-tolerant S. rebaudiana strains but also enhance the availability of saline-alkali soil in China. This study was carried out by hydroponics in controlled room, aiming to explore the effects of different concentrations of NaCl and SNP, and their combination treatment on the growth, physiological responses and glycoside contents of S. rebaudiana. Main results obtained showed as follows:1. Four salt concentrations designed as0,60,90and120mM were carried out on S. rebaudiana. With the increase of NaCl dose, chlorophyll content decreased significantly and the lowest total chlorophyll content was only30%of the control. However, the activities of antioxidant enzymes and proline content increased several fold. S. rebaudiana planted in nutrient solution contationed above60mM NaCl for28d showed significant lower height, leaves number, leaf fresh/dry weight and shoot dry weight than control. In60and90mM NaCl, leaves not only showed K+/Na+higher than1but also displayed two-fold higher K+content than stem and root. However, the total dry weight kept unchanged in60and90mM NaCl while decreased significantly by40%in120mM NaCl and could not survive in120mM NaCl for a month. According to these results, it was suggested that S. rebaudiana was moderate salt tolerant and its positive physiological responses were beneficial to its adaptation to stress conditions. Furthermore, the notably increased rebaudioside A (RA) content and RA ratio under60mM NaCl suggested the taste of S. rebaudiana leaves could be modulated by planting S. rebaudiana in hypohaline soil.2. Five SNP concentrations designed as0,40,80,120and160μM were carried out on S. rebaudiana. Plants treated with40μM SNP showed significant higher values of leaves fresh/dry weight (40%increased), chlorophyll content (12%increased), activities of superoxide dismutase and peroxidase (2-3fold), proline content (8fold) and K+/Na+in leaves than control. Based on this result, it was suggested40μM SNP was good for the leaves growth and was beneficial to promote the efficacy of physiological defense system of S. rebaudiana. However, the leaves number and chlorophyll content decreased significantly in160μM SNP, which indicated that high SNP concentration could induce the oxidation degradation of chlorophyll. In addition,40μM SNP improved the ratio of rebaudioside A and stevioside (ST). According to these results, we concluded that nitric oxide had dual roles in the growth of S. rebaudiana. Adequate exogenous nitric oxide can be put into field practices to improve the taste of S. rebaudiana leaves and the adaptation ability to stress conditions.3. Four treatments including60mM NaCl (T1),60mM NaCl+40mM SNP (T2),60mM NaCl+80mM SNP (T3) and60mM NaCl+160mM SNP (T4) were carried out on S. rebaudiana. Compared to T1, T2increased the leaves fresh/dry weight, shoot fresh weight and total fresh weight slightly. Moreover, proline content in T2was twice as much as control. In T3and T4, leaves fresh/dry weight, shoot fresh weight and total fresh weight decreased significantly while proline content remained unchanged. In terms of the activities of antioxidant enzymes, T2, T3and T4significantly increased the activities of SOD and POD but CAT. Moreover, K+/Na+, Ca2+/Na+and RA content reduced obviously in T2, T3and T4. The lowest RA content was obtained in T4. ST content was promoted by T2, which showed35%increase when compared to T1and higher than that in T3and T4. Based on these results, the combination treatment of low concentration of NaCl and high concentration SNP could induce the oxidative stress and ion stress response of S. rebaudiana leaves. Furthermore, the combination treatment of low concentration of NaCl and low concentration of SNP did not improve the RA content as expectation while increased the ST content markedly.In all,60mM NaCl was beneficial to the RA accumulation in S. rebaudiana leaves after28days’treatment. However, plants under above60mM NaCl had to face the physiological stress, growth inhibition and even death. The effects of28days’SNP treatment on S. rebaudiana varied among SNP concentrations.40μM SNP not only increased the leaves biomass and promoted the growth of leaves and even the whole plant, but also improved the RA/ST ratio of leaves of S. rebaudiana and was conducive to improve the level of physiological protection. In contrast,160μM SNP induced the oxidative degradation of chlorophyll, the physiological stress responses and the reduction of ST content. The16days’combination treatment of60mM NaCl and different SNP concentrations all inhibited the growth of S. rebaudiana. Even though60mM NaCl+40μM SNP did not improve the RA content as expected, it still increased the ST content. Besides,60mM NaCl+160μM SNP induced the physiological stress responses in S. rebaudiana as160μM SNP. |
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