Alleviating Effects Of Exogenous Nitric Oxide, Indoleacetic Acid And Their Interaction On Alkaline Stress Of Cucumber Seedlings

Cucumber (Cucumis sativus L.) is one of the most widely cultivated vegetable in the world, and it often suffers from alkaline stress in both greenhouse and outside production, resulting in yield decrease dramatically. In this study, the effects of different concentrations of IAA and the interaction between NO and IAA in alleviating alkaline stress on cucumber seedlings were investigated with using Sodium nitroprusside (SNP) and Indole-3-acetic acid (IAA) as exogenous NO and IAA donor, respectively; cPTIO and NPA as a specific scavenger of NO and an auxin efflux blocker accordingly, NaHCO3treatment as alkaline condition, and nutrient solution was used as growth medium. The results could provide theoretical basis for exogenous NO and IAA in enhancing cucumber tolerance to alkali stress. The main results as follows:1. NaHCO3treatment significantly induced cucumber leaf chlorosis, reduced photosynthetic pigment contents, net photosynthetic rate, photochemical efficiency of PSII, K+content of leave, root activity and activities of antioxidant enzymes of cucumber root, enhanced the Na+content, reactive oxygen species (ROS) content and scavenging capacities of ROS in cucumber leaves, resulting in the growth inhibiton of cucumber plant. In addation, Alkaline stress could induced the production of endogenous NO in cucumber root and significantly improve the content of endogenous ABA in cucumber, while showed different effect on endogenous IAA in different organs, and reduced the content of endogenous IAA in root and increased the content of endogenous IAA in stem and leaf.2. Application of exogenous SNP and IAA could significantly alleviate the harm of alkali stress on cucumber seedlings. Application of different concentrations (0.01-1μmol·L-1) of IAA resulted in higher biomass in alkali-stress cucumber seedlings, reduced Na+content and increased K+content. The effect was dependent on IAA concentration, and the alleviating effect of1umol L-1IAA was the best. Exogenous SNP treatment with100μ. mol L"’SNP had the best mitigation effect. Exogenous IAA could induce NO production in cucumber seedling roots, this effect became more significant under alkali stress. Exgenous NO and IAA could improve cucumber plants tolerance to sodic alkaline stress, which depended on their roles in regulating ROS scavenging, antioxidant enzymes activities and Na+accumulation, protecting PSII from damage. Further investigation indicated that the function of IAA could be abolished by NO scavenger (cPTIO). On the contrary, IAA transport inhibitor (NPA) showed no significant effects on abolishing the function of NO. Therefore, nitric oxide, as a downstream signal, plays vital roles in auxin induced cucumber tolerance to sodic alkaline stress. Meanwhile, exogenous IAA could improve the content of IAA and decrease content of ABA in cucumber seedlings, while application of SNP had a modest effect on IAA content, and could decrease content of ABA in cucumber seedlings. The effect of exogenous IAA on content of endogenous ABA could be abolished by NO scavenger (cPTIO) and cPTIO have different effects on different organs of cucumber. While IAA transport inhibitor (NPA) could only abolish the effect application of SNP on content of endogenous ABA and had no significant effect on endogenous IAA in SNP-treated cucumber seedlings under alkaline stress.