| Citrus is an evergreen fruit tree grown in tropical,subtropical,and temperate regions.In recent years,because of the unreasonable nitrogen（N）fertilization and the neglect of magnesium（Mg）fertilizer supplement,resulting in the phenomenon of N imbalance and Mg deficiency in citrus plants has been widely occurred,which has seriously restricted the green and sustainable development of citrus industry in China.Previously,our studies demonstrated that citrus has an ammonium sensitive plant,and Mg deficiency can also disrupt the physiological and biochemicalprocesses in citrus plants.However,there are few studies on citrus plants responding to N form coupled Mg supply.Based on the previous research results,a characteristic and dominant variety of Fujian Province,as‘Lugan（Citrus reticulata Blanco cv.Ponkan）’was selected for the experiment materials.A combined experiment of two N forms（Ammonium nitrogen,AN;Nitrate nitrogen,NN）and three Mg levels(0,0.5,and 2 mmol·L-1,as Mg0,Mg0.5,and Mg2)was designed under a given N concentration(4 mmol·L-1),with a total of 6 treatments.This study focuses on the effects of N form and Mg level on the physiological characteristics of growth and morphology,N and Mg uptake and distribution,gas exchange and photosynthetic electron transfer chain,and assimilates transport and antioxidant stress,which the results will help to reveal the physiological mechanism of N form regulating citrus growth and Mg uptake,and can also provide a theoretical basis for the optimized N and Mg management in citrus production.The main results are as follows:（1）N form coupled with Mg level significantly affected plant growth in citrus seedlings.Compared with AN treatment,NN treatment significantly increased plant biomass and promoted the establishment of leaf and root morphology.Also,the plant biomass was increased with increasing Mg supply across AN and NN treatment,and the increased degree of plant biomass under AN treatment was higher than that of NN treatment.（2）N form coupled with Mg level significantly affected plant N and Mg uptake in citrus seedlings.With increasing Mg level,plant N concentration was decreased under AN treatment but increased under NN treatment,while NN treatment greatly increased plant Mg concentration and N and Mg accumulation compared with AN treatment.Also,Mg concentration and accumulation of citrus plants were increased with increasing Mg supply across AN and NN treatment,and the increased degree of plant N and Mg accumulation under AN treatment was lower than that of under NN treatment.（3）N form coupled with Mg level significantly affected plant N assimilation in citrus seedlings.The activities of nitrate reductase（NR）,glutamine synthase（GS）,and glutamate synthase（GOGAT）in citrus seedlings were increased under NN treatment compared with AN treatment.Also,the activities of NR,GS,and GOGAT were increased with increasing Mg supply across AN and NN treatment,and the increased degree of these enzyme activities under AN treatment was lower than that of under NN treatment.（4）N form coupled with Mg level significantly affected leaf gas exchange and photosynthetic electron transfer chain in citrus seedlings.Compared with AN treatment,NN treatment significantly decreased fluorescence heat dissipation,resulting in increased photosynthetic electron transport performance and improved the photosynthesis capacity,whereas reduced non-structural carbohydrate（NSC）accumulation in the leaf.Also,the characteristics of photosynthetic electron transport performance,photosynthesis capacity,and NSC content were increased with increasing Mg supply across AN and NN treatment,and the increased degree of photosynthetic electron transport performance and photosynthesis in citrus plants under AN treatment was higher than that of under NN treatment.（5）N form coupled with Mg level significantly affected plant antioxidant properties in citrus seedlings.Compared with AN treatment,NN treatment significantly increased the activities of superoxide dismutase（SOD）,peroxidase（POD）,and catalase（CAT）but decreased the malondialdehyde（MDA）content in citrus seedlings.Also,the activities of SOD and POD and the MDA content were decreased with increasing Mg supply across AN and NN treatment,and the inhibition degree of antioxidant enzymes and MDA content of citrus plants under AN treatment was higher than that of under NN treatment.In conclusion,our results demonstrated that N form and Mg level significantly affected the plant growth,N and Mg uptake,and related physiological and biochemical characteristics of citrus seedlings,which further confirmed that citrus is an ammonium sensitive plant.Also,the negative effect of ammonium on citrus plants could be alleviated by an appropriate Mg supply,and showed an interaction between N and Mg.These results play a vital role in guiding the optimizing management of N and Mg nutrients for improved plant growth and N and Mg uptake in citrus production. |
