Study On The Alleviating Effect Of Exogenous Dopamine On Alkali Stress Of Apple Seedlings And Its Mechanism

The northwest region of China is the best and main producing area of apple.The temperature difference between day and night is large and the light is sufficient,which lays the foundation for the excellent quality of apple.However,the annual precipitation in this area is less and the evaporation is large,which leads to the prominent phenomenon of alkali fication and the increasingly serious soil salinization.The pH value of soil is as high as 8.0,which is contrary to the fact that apples are suitable to grow in slightly acidic soil and is not conducive to the sustainable development of apple industry in China.In addition to drought stress,saline-alkali stress has become an important factor restricting the yield and quality of apples in this region.Alkali stress not only inhibits the growth and development of plants,but also restricts crop production and ecological environment construction.At present,it has been found that many exogenous substances,such as betaine,humic acid,?-aminobutyric acid?GABA?,melatonin,spermidine?Spd?and so on,have the capability to improve the stress tolerance of plants and alleviate the inhibitory effect on plants caused by stress.However,there is few studies on the alleviating effect of exogenous dopamine on apple adaptation to alkali stress.In this study,the seedlings of M.hupenensis Rehd were used as the material.The suitable concentration of exogenous dopamine to alleviate plant growth under alkali stress and the effects of exogenous dopamine on growth,nutrient metabolism,oxidative activity and organic acid of apple seedlings under alkaline stress were investigated by hydroponic experiment.The results are as follows:1.After 15 days treatment of alkali stress,the growth of apple seedlings was inhibited,but the damage of alkali stress to apple seedlings was obviously alleviated after exogenous dopamine.For apple seedlings,different concentrations of dopamine had different effects alleviating effects on alkali stress.The results of dry and fresh weight,net growth,net photosynthetic rate?Pn?,intercellular carbon dioxide concentration?Ci?,chlorophyll and carotenoid showed that 0.1 mM dopamine concentration had the best alleviating effect,the net photosynthetic rate was 2.3 times higher than that of alkali stress group.Therefore,the concentration of 0.1 mM dopamine was determined to be the most suitable concentration to alleviate the inhibitory effect of alkali stress.2.Effects of exogenous dopamine on growth of apple seedlings under alkali stress.Compared with the control,alkali stress decreased the plant height,chlorophyll level and chlorophyll fluorescence values of apple seedlings,inhibited the growth and development of seedling roots,and increased the relative conductivity and root-crown ratio of leaves and roots of apple seedlings.However,the inhibitory effect of alkali stress on seedling growth was significantly alleviated by 0.1 mM dopamine.Compared with alkali stress,the contents of chlorophyll a,chlorophyll b,total chlorophyll and carotenoid increased by 68.5%,75.5%,70.6%and 74.3%,respectively.The results of relative conductivity analysis showed that under the alkali stress dopamine could increase the permeability of seedling tissue membrane.3.Effects of exogenous dopamine on nutrient uptake and utilization of apple seedlings under alkali stress.Compared with the control,alkali stress reduced the contents of N,P,K,S,Ca,Mn,Zn and Mo in the roots,stems and leaves of apple seedlings.the decrease of Mg and Fe contents in leaves may be related to weakened photosynthesis and leaf yellowing.However,under the alkali stress,the contents of N,P,S and Mo in the roots,stems and leaves increased significantly,the contents of K,Zn,Ca,Mg and Mn increased significantly only in leaves,the contents of K,Zn,Fe,Ca and Mg in stems decreased after application of dopamine instead.The contents of Na and B elements were significantly higher than those of the control under alkali stress,but decreased significantly after the application of dopamine.Alkali stress decreased the activities of nitrate reductase?NR?,glutamate synthase?GS?and acid phosphatase?ACP?in apple seedling leaves,and down-regulated the expression of some genes related to nitrogen metabolism?e.g.MdNRT1.7,MdNRT2.7,MdNRT2.4?.Exogenous dopamine increased the enzyme activities of NR,GS and ACP.The expression of MdNRT1.7,MdNRT2.7 and MdNRT2.4genes in seedling leaves was increased.All these indicated that dopamine could alleviate the inhibitory effect of alkali stress on nutrient absorption in seedlings and promote the absorption and effective utilization of mineral elements in seedlings.4.Exogenous dopamine affected the antioxidative activity of apple seedlings under alkali stress.Compared with the control,the contents of endogenous dopamine,malondialdehyde?MDA?and hydrogen peroxide?H₂O₂?in roots and leaves of seedlings increased under alkali stress.The activities of catalase?CAT?,peroxidase?POD?and superoxide dismutase?SOD?in leaves decreased.However,the activities of endogenous dopamine,CAT,POD and SOD in leaves increased significantly,while the contents of MDA and H₂O₂ in leaves decreased significantly after external application of dopamine under alkali stress.These results suggest that exogenous dopamine could increase the antioxidant capacity of apple seedlings under alkali stress and improve the stress tolerance in order to adapt to alkaline environment quickly.5.Exogenous dopamine affects the content of organic acid in apple seedlings under alkali stress.Compared with the control,the contents of malic acid and citric acid in leaves and roots of apple seedlings were significantly increased under alkali stress,and the content of malic acid continued to increase under alkali stress.But there was no significant difference between the two treatments,while the accumulation of citric acid decreased significantly.The mechanism remains to be further studied.In summary,exogenous dopamine can promote the growth of apple seedling and absorption and utilization of nutrients in alkaline environment,improve the antioxidant capacity of apple seedlings,and increase the accumulation of organic acids in seedlings.