| Nitrogen and water are two important environmental limiting factors for plant growth,especially in the desert ecosystem limited by nutrients and water.The effects of nitrogen deposition and precipitation changes on desert plants has been a hot topic in recent years.In this study,Reaumuria soongorica and Salsola passerina were used as the research objects.A pot experiment was used in this research.In this experiment,different nitrogen(ammonium nitrogen and nitrate nitrogen),different nitrogen deposition levels(0 g·(m2·a)-1?4 g·(m2·a)-1?8 g·(m2·a)-1)and different precipitation treatments(W-30%?W?W+30%)were used.The changes of photosynthetic fluorescence parameters,non-structural carbohydrates,nitrogen metabolism enzymes,antioxidant enzyme activities and osmotic adjustment substances of the two species were determined in order to further understand the adaptability of desert plants to environmental changes under different growth patterns.The main results were showed as follows:1.The interaction of water increasing and nitrogen application increased the photosynthetic pigment content,net photosynthetic rate,stomatal conductance,intercellular carbon dioxide concentration,transpiration rate,Fv/Fo and Fv/Fm values of R.soongorica and S.passerina,and reached the maximum when the precipitation increased by 30%and nitrogen application rate was 8 g·(m2·a)-1.The chlorophyll(a+b)and net photosynthetic rate of R.soongorica can be increased by 1.65 times and1.19 times respectively,and S.passerina can be increased by 97.44%and 1.32 times respectively.The instantaneous water use efficiency of the two plants increased with the decrease of precipitation and the increase of nitrogen application,and reached the maximum when the precipitation decreased by 30%and the nitrogen application increased by 8 g·(m2·a)-1.The maximum increase of R.soongorica was 39.84%,and the maximum increase of S.passerina was 35.77%.The photosynthetic pigment content and photosynthetic rate of R.soongorica and S.passerina under the nitrate nitrogen treatment were higher than those under ammonium nitrogen,but there was no significant difference in chlorophyll fluorescence parameters.The photosynthetic pigment content,photosynthetic gas exchange parameters and chlorophyll fluorescence parameters R.soongorica were significantly increased by mixed cultivation(P<0.05),while S.passerina decreased.From July to September,photosynthetic parameters and chlorophyll fluorescence parameters of R.soongorica and S.passerina decreased under different growth patterns.The interaction of nitrogen and water significantly increased the contents of soluble sugar,starch and non-structural carbohydrate(NSC)in R.soongorica and S.passerina,while the mixed growth promoted the increase of non-structural carbohydrate contents in both plants.2.Nitrate reductase activity of R.soongorica was significantly increased by applying ammonium nitrogen under natural precipitation and nitrate nitrogen under precipitation increase 30%.Nitrogen and water interaction significantly increased the nitrate reductase activity of S.passerina under different growth patterns in July,while the activity of nitrate reductase of solitary S.passerina decreased in September,and mixed growth could slow down the decline of enzyme activity.Glutamate synthase activity of R.soongorica and S.passerina was higher under the interaction of precipitation and nitrogen application.The glutamine synthetase activity of S.passerina decreased under the treatment of decreasing precipitation and adding nitrogen fertilizer.The nitrate reductase activity of R.soongorica and S.passerina in mixed growth is higher than that in solitary growth.The activity of glutamine synthase in the mixed treatment was significantly higher than that of the solitary treatment in July,while the mixed treatment significantly reduced the glutamate synthase activity.Under different growth patterns,The nitrate-nitrogen content of R.soongorica and S.passerina in two months increased with the increase of nitrogen application,and decreased with the increase of precipitation.Both were reached the maximum when precipitation reduced 30%and nitrogen application rate was 8 g·(m2·a)-1,and the nitrate nitrogen treatment was higher than the ammonium nitrogen treatment.Mixed growth significantly increased the nitrate-nitrogen content of R.soongorica(P<0.05),but had no significant effect on the S.passerina.3.The single factor,double factor and three factor treatments of nitrogen application,precipitation and growth mode had extremely significant effects on the activities of catalase,peroxidase and superoxide dismutase in the leaves of R.soongorica and S.passerina(P<0.01).Increased application of nitrogen fertilizer under reduced precipitation or normal precipitation can significantly increase the antioxidant capacity of R.soongorica and S.passerina and reduce the content of malondialdehyde.The activities of antioxidant enzymes of R.soongorica and S.passerina under nitrate nitrogen treatment were higher than those under ammonium nitrogen treatment,while the content of malondialdehyde under nitrate nitrogen treatment was lower than that under ammonium nitrogen treatment.The mixed growth of R.soongorica and S.passerina significantly increased the catalase activity of the two,while the activities of peroxidase and superoxide dismutase decreased.However,the malondialdehyde content of R.soongorica increased,while that of S.passerina decreased.The contents of soluble protein and proline in leaves of R.soongorica and S.passerina increased with the increase of nitrogen application rate.The contents of soluble protein increased with the decrease or increase of precipitation,while the contents of proline decreased with the increase of precipitation.And the contents of soluble protein and proline reached the maximum when the precipitation decreased by30%and the nitrogen application rate was 8 g·(m2·a)-1.R.soongorica increased by2.75 times and 2.38 times,and S.passerina increased by 2.53 times and 3.89 times.The mixed growth of R.soongorica and S.passerina significantly increased the soluble protein content of R.soongorica,but the content of proline was significantly decreased.The content of proline in S.passerina was increased. |
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