Effects of Exogenous Melatonin on Growth and Cadmium Accumulation of Tobacco Seedlings under Different Nitrogen Levels and Cadmium Stress
In order to determine the effects of foliar application of melatonin(MT)on cadmium(Cd)accumulation and stress resistance of tobacco seedlings under different nitrogen levels,the experiment was con-ducted by using hydroponic method and setting sodium nitrate(NaNO3)levels as 0.1,5 and 10 mmol/L,MT concentration as 100 µmol/L,and Cd concentration as 20 µmol/L.The biomass,physiological characteristics and Cd accumulation and transport of tobacco seedlings under Cd stress and different nitrate nitrogen levels were analyzed after foliar application of MT.The expression differences of Cd transport-related genes(NtIRT1,NtNRAMP1,NtNRAMP3,NtHMA2 and NtHMA3)were also detected and analyzed.The results showed that there were significant differences in the relieving ability of MT on Cd stress under different nitrate nitrogen lev-els.Compared with the normal nitrogen supply level(5 mmol/L),high nitrogen treatment(10 mmol/L)could promote the growth of tobacco seedlings,and spraying MT could further effectively increase the tobacco seedlingbiomass,the superoxide dismutase(SOD),peroxidase(POD),catalase(CAT)and ascorbate perox-idase(APX)activities and the ascorbic acid(AsA)and reduced glutathione(GSH)contents,but reduce the malonaldehyde(MDA)content;while under low nitrogen treatment(0.1mmol/L),spraying MT signifi-cantly decreased the SOD,POD and APX activities and the AsA and GSH contents,and significantly in-creased the MDA content.The Cd absorption capacity of tobacco seedlings was the strongest at high nitrogen level.MT application could decrease the expression level of NtIRT1,NtNRAMP1,NtHMA2 and NtHMA3 in the root of tobacco seedlings,and the decrease of NtNRAMP1 and NtHMA2 reached the significant level,so as to inhibit the Cd absorption.The absorption capacity of Cd was the weakest at low nitrogen level,and the miti-gation capacity to Cd was weak after MT application;the expression of NtIRT1,NtNRAMP1 and NtHMA3 were up-regulated,and the increase of NtNRAMP1 and NtHMA3 reached a significant level,so as to improve the transport capacity of Cd.In conclusion,nitrate nitrogen playedthe significant role in promoting exogenous MT to alleviate the influence of Cd stress on growth and development of tobacco seedlings and cadmium accumula-tion.With the increase of nitrogen level,MT application could significantly change the physiological character-istics of tobacco,reduce the absorption and accumulation of Cd,and improve the tolerance of tobacco seed-lings to Cd stress.
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