Effects Of Exogenous NO On Physiological Characteristics Of Pinus Koraiensis Seedlings Under UV-B Stress

The principal ultraviolet radiation absorbance occurs in the stratospheric ozone layer in earth's atmosphere. The destruction of this layer, which is caused by ozone depletion, has lead to an increase in solar UV-B radiation (280-320 nm) reaching the Earth's surface and produced potentially biological effect for plants in the earth. Nitric oxide (NO) as the second messenger of the UV radiation, has been the most activity research hotspot in plant stress physiology and signal transduction fields.In this research we studied the effects of enhanced UV-B radiation, different concentrations of exogenous NO and increased exogenous NO under enhanced UV-B radiation on Pinus koraiensis seedlings, respectively. Gas exchange parameters, the concentrations of photosynthetic pigments, contents of free radicals, activities of antioxidant enzymes, concentrations of flavonoids and other physiological indicators were determined. We revealed the adapted mechanism of Pinus koraiensis to enhanced UV-B radiation, sprayed different concentrations of exogenous NO and optimized the best UV-B radiation dose and concentration of exogenous NO to promote the growth of Pinus koraiensis seedlings. The main results are as follows:The T1, T2, T3 UV-B treatments were increased UV-B radiation by 1.40,2.81 and 4.22 kJ-m-2·d-1 compared to the control treatment (CK), respectively. Gas exchange parameters, the concentrations of photosynthetic pigments, the contents of secondary metabolites, epicuticular wax, free radical, malondialdehyde (MDA), and the activities of antioxidant enzymes were determined after 40 days exposures. Our results showed that the contents of hydrogen peroxide (H2O2) and MDA were significantly increased by enhanced UV-B radiations (P<0.05). The supplemental UV-B radiation induced the activities of catalase (CAT), ascorbate peroxide (APX), and superoxide dismutase (SOD), but significantly decreased the activity of peroxidase (POD) (P<0.05). Compared to CK, the concentration of soluble protein was increased with the exposure of UV-B radiation. The contents of flavonoids and carotenoid were significantly decreased by all levels of supplemental UV-B radiations (P<0.05). After 40 days UV-B radiation the striped uplift of wax in needle of Pinus koraiensis seedlings was broken, the number of stripe-like uplift was reduced and wax particles were increased. Compared with the CK, the wax contents were significantly decreased with the exposure UV-B radiations. The long-term treatment of high doses of UV-B radiation was inhibited the accumulation of wax in needle of Pinus koraiensis. The most prominent substance of Pinus koraiensis was alkanes. The alkanes represented more than 50% of the compounds. The percentage of alkanes and phenols in Pinus koraiensis needles were decreased by enhanced UV-B radiation. In contrast, the percentage of long chain esters was increased by enhanced UV-B radiation.The Pinus koraiensis seedlings were treated with sodium nitroprusside (SNP), a NO donor, at 5 different concentrations ranging from 0 mM to 1 mM and 0.01 mM of SNP had the most satisfactory effect. The result showed that at different concentrations exogenous NO had different effects on the photo synthetic parameters, the concentrations of photosynthetic pigments, the contents of free radical and the activities of antioxidant enzymes and photosynthetic pigments. The content of chlorophyll was maximal in the treatment with SNP at 0.01 mM. The different concentrations of SNP treatment significantly increased the content of H2O2 and MDA. Moreover, the 0.1 mM SNP treatment had the maximal concents of H2O2 and MDA. Meanwhile, the different concentrations of SNP treatment had different effects on antioxidant activity. The activities of CAT, APX, POD were maximum in the concentration of 0.1 mM SNP. The different concentrations of SNP treatments significantly decreased the activity of SOD (P＜0.05). Our results showed that high concentration of exogenous NO may be a stress factor for Pinus koraiensis seedlings.The appropriate concentration SNP treatment could significantly alleviate the damage of UV-B stress on Pinus koraiensis seedlings. The results showed that exogenous SNP, a NO donor, significantly increased activities of SOD,PQD,CAT and APX of Pinus koraiensis seedling, reduced the contents of H2O2 and MDA, and increased content of flavonoids. The results of NO content, nitric oxide synthase (NOS) and nitrate reductase (NR) activity assay showed that the NO production in vivo of Pinus koraiensis seedlings may was mainly dependent NOS pathway, but not relying on NR pathway.Our results demonstrate that Pinus koraiensis can increase of the CAT, APX and SOD activities to prevent oxidative stress by supplemental UV-B radiation, but the defense mechanism is not efficient enough to prevent the UV-B induced damage. The 0.1 mM SNP treatment significantly promoted the growth and regulated the physiology of Pinus koraiensis seedlings. The experiment proved that NO influenced the photosynthesis of Pinus koraiensis seedlings. Morevoer, the research showed that high concentrations of exogenous NO may be a stress factor for Pinus koraiensis seedlings. The appropriate concentration SNP treatment could significantly alleviate the damage of UV-B stress on Pinus koraiensis seedlings.