Effects Of Molybdenum Application On Root Architecture, Photosynthesis And Nitrogen Metabolism Of Winter Wheat Under Different Nitrogen Sources

Wheat（Triticum aestivum L.）is the second most widely grown crop in the world.Wheat has been reported to have a specific preference for NO₃^--N source and shows toxicity symptoms under sole NH₄⁺source.Molybdenum（Mo）is an essential micronutrient for higher plants and involved in many physiological and biochemical processes.Especially,Mo has an important role in N metabolism.Here hydroponic experiment was carried out to comprehensively investigate the effects of Mo supply（0and 1μM）on root system architecture,photosynthetic apparatus,NH₄⁺-induced oxidative stress,N metabolism enzymes and uptake of mineral elements under NO₃^-,NH₄NO₃,and NH₄⁺sources in two winter wheat cultivars,Mo-efficient‘97003’and Mo-inefficient‘97014’respectively.The main findings are summarized as follows:（1）The results revealed that Mo application improved the root growth of winter wheat plants under all N sources.However,the Mo-induced increases in the root dry weight and morphological traits,NR activities,NO contents,NO3assimilation,total N contents,and transcripts of NR and nitrate transporter genes among different N sources followed the order of NH₄NO₃>NO3>NH₄⁺,suggesting that Mo has more complementary effects to nitrate nutrition than sole NH₄⁺in wheat plants.These findings imply that Mo has a significant role in shaping the root system architecture of wheat plants through NO production,efficient N uptake and assimilation,and regulation of nitrate transporter genes expressions,especially under NH₄NO₃ nutrition.（2）The results showed that Mo application improved the plant dry weight,photosynthetic efficacy,chlorophyll contents and chloroplast structure of winter wheat plants under all N sources,showing that Mo has a key role in photosynthetic process.Our study results also highlighted that Mo-induced enhancements in the photosynthetic characteristics（such as Pn,Tr,Gs,Ci）,NR activities,NO3assimilation,transcripts of Ta NR and Ta NRT1.1 genes and total N contents followed the same order as NH₄NO₃>NO₃^->NH₄⁺sources.Interestingly,Mo-inefficient‘97014’cultivar showed more pronounced alterations in Mo-dependent parameters in+Mo treated plants than Moefficient‘97003’cultivar,indicating that‘97014’cultivar is more dependent on external Mo supply.In short,these findings imply that Mo supply increased Pn not only through improved chlorophyll contents and chloroplast configuration but also by N uptake and assimilation which may represent a strategy of Mo fertilizer to strengthen the photosynthetic machinery.（3）Effects of Mo application on N metabolism enzymes activities and expressions and the elemental profile in leaf tissues in winter wheat under different nitrogen sources.The results revealed that the expressions and activities of nitrate reductase（NR）and nitrite reductase（Ni R）followed the order of NH₄NO₃>NO₃^->NH₄⁺sources while glutamine synthetase（GS）,and glutamate synthase（GOGAT）as NH₄⁺>NH₄NO₃>NO₃^-in both winter wheat cultivars under different nitrogen sources.However,Mo application increased both expressions and activities of all studied enzymes under all N sources except NR and Ni R in sole NH₄⁺source.Moreover,the concentrations of macro-elements（K,Ca,and Mg）and micro-elements（Cu,Zn,Fe,except Mn）under different N sources,respectively,increased and decreased in the order of NO₃^->NH₄NO₃>NH₄⁺in both the winter wheat cultivars.However,generally Mo presence or absence had no significant effects on the mineral elements contents in winter wheat leaves under all N sources.These findings highlight that both Mo and different N sources have significant effects on the expressions and activities of N metabolism enzymes,while elemental profiles are mainly affected by different N forms in winter wheat plants.（4）The observed results demonstrated that,among different N sources,wheat plants supplied with sole NH₄⁺source accumulated the highest superoxide anions and malonaldehyde（MDA）contents,than NO₃^-fertilizers.However,Mo-induced enhancement in the antioxidant enzymes activities,abscisic acid（ABA）contents,and expressions of Ta AO and Ta Aba3 genes followed the order of NH₄⁺>NO₃^->NH₄NO₃,suggesting that Mo has a significant role in improving the antioxidant defense system of plants under sole ammonium stress conditions.These results indicate that Mo application alleviates the NH₄⁺toxicity via ABA productions and the ROS-scavenging enzymes among different N sources in the order of NH₄⁺>NO₃^->NH₄NO₃ in winter wheat cultivars.