Abstract
Salinity stress severely inhibits the growth of plant via ionic toxicity and osmotic constraint. Exogenous silicon (Si) can alleviate salinity stress, but the mechanisms behind remain unclear. To investigate the role of Si in alleviating ionic and osmotic components of salinity, rice (Oryza sativa L.) seedlings were grown hydroponically in iso-osmotic stress conditions developed from NaCl or polyethylene glycol (PEG). The effects of Si on the growth of shoot and root of rice under salinity and PEG-derived osmotic stress were evaluated and further compared using principal coordinate analysis (PCoA). We also analyzed the concentrations of Na, K, and compatible osmolytes, tissue sap osmotic potential, antioxidant enzymes activities, and the expression of aquaporin genes. Generally, Si significantly promoted shoot and root growth in rice exposed to both NaCl and PEG. PCoA shows that the Si-induced distance change under NaCl treatment was larger than that under PEG treatment in the shoot, while the Si-induced distance changes under NaCl and PEG treatments were at an equal level in the root. Under salinity, Si decreased Na concentration and Na/K ratio in the shoot but not in the root. However, Si decreased net Na uptake and increased root Na accumulation content. Osmotic potential was increased in the shoot but decreased in the root by Si addition. Si decreased soluble sugar and proline concentrations in the shoot but increased soluble sugar and soluble protein concentrations in the root. Besides, Si promoted shoot transpiration rate and root morphological traits. Although both NaCl and PEG treatments upregulated aquaporin gene expression, Si addition maintained the expression of OsPIPs under NaCl and PEG treatments at same levels as control treatment. Furthermore, Si alleviated oxidative damages under both NaCl and PEG by regulating antioxidant enzyme activities. In summary, our results show that Si improves salt stress tolerance in rice by alleviating ionic toxicity and osmotic constraint in an organ-specific pattern. Si ameliorates ionic toxicity by decreasing Na uptake and increasing root Na reservation. Si alleviates osmotic constraint by regulating root morphological traits and root osmotic potential but not aquaporin gene expression for water uptake, and promoting transpiration force but not osmotic force in shoot for root-to-shoot water transport.
Highlights
Silicon (Si) is the second most abundant element in soil and a widely recognized beneficial element in plant (Epstein, 1994, 1999; Liang et al, 2015)
The MDA concentration and Electrolyte leakage (EL) of rice shoot were higher under NaCl treatment than under PEG treatment
There was no significant difference between NaCl- and PEG-induced decrement of photosynthesis rate (Pn), transpiration rate (Tr), chlorophyll concentration, and Relative water content (RWC) in rice shoot (Table 1)
Summary
Silicon (Si) is the second most abundant element in soil and a widely recognized beneficial element in plant (Epstein, 1994, 1999; Liang et al, 2015). The alleviation effect of Si on salinity stress, one of the major environmental problems, has been found in different kinds of plant species including rice, barley, wheat, sorghum, tomato, and cucumber (Liang et al, 2003; Zhu et al, 2004; Gong et al, 2006; Hattori et al, 2007; Saqib et al, 2008; Shi et al, 2016). The situation is getting worse due to global warming and excess chemical fertilization (Munns and Gilliham, 2015). Under such a background, the application of Si fertilizer would be a promising approach with the advantages of high efficiency and low economic cost (Liang et al, 2015; Yan et al, 2018). The veiled mechanism behind Siinduced salt stress tolerance impedes the extensive application of Si fertilizer in agriculture
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