Mechanism Of Rootstock Grafting Regulating NO₃^--N Absorption And Utilization In Watermelon Under Reduced Nitrogen Application

Watermelon is one of the important vegetable crops in the gourd family,and is also the most popular fruit in summer.However,in recent years,a large amount of nitrogen fertilizer has been blindly used in facility watermelon cultivation in China,which not only severely restricted the growth of watermelon,but also reduced its nitrogen use efficiency and fruit yield.Studies have shown that grafting can change the root system of plants that absorb nutrients,thereby effectively promoting their absorption and utilization of nutrients,and the rootstocks play a decisive role in the efficacy of grafting.Therefore,this study first conducted normal nitrogen and nitrogen fertilizer reduction treatments on watermelon and bottle gourd seedlings to clarify the response advantages of bottle gourd rootstock to nitrogen fertilizer reduction environment;after that,the watermelon were grafted onto bottle gourd to study the role of rootstock root system on nitrate nitrogen absorption of grafted watermelon seedlings under reduced nitrogen fertilizer application conditions,and the effects of nitrogen efficient rootstock grafting on nitrate nitrogen uptake,transport and utilization of watermelon were analyzed.Finally,by analyzing the dynamic expression differences of nitrate nitrogen absorption and metabolism related genes,the molecular mechanism of rootstock grafting regulating the efficient nitrate nitrogen absorption and utilization in watermelon seedlings at the transcriptional level.The main research results are as follows:1.Comparison of differences in response between watermelon and bottle gourd rootstock plants to reduced nitrogen application environments.The’Zaojia8424’watermelon seedlings and’Jingxin No.1’bottle gourd seedlings were treated with normal nitrogen（9 m M）and reduced nitrogen application（4 m M）.Differences of the changes in plant growth,root morphology,photosynthetic characteristics,nitrate nitrogen content,and cumulative amount of each plant were compared.The results showed that under reduced nitrogen fertilizer application conditions,the growth,root morphology,the content and accumulation of nitrate nitrogen,photosynthesis and transpiration rate of watermelon plants were significantly better than those of watermelon plants.The growth of watermelon plants was inhibited under nitrogen fertilizer reduction treatment,while the root length,root volume,and root tip branching number of bottle gourd rootstock seedlings were increased and its growth was promoted.This result indicates that bottle gourd plants have stronger growth advantages than watermelon plants,and can better adapt to nitrogen fertilizer reduction environments by changing root morphology to maintain normal plant growth.2.Study the effect of rootstock grafting on nitrate nitrogen absorption and utilization in watermelon seedlings under reduced nitrogen fertilizer application conditions.The self-grafted’Zaojia 8424’watermelon seedlings and the rootstock-grafted watermelon seedlings with’Jingxin No.1’bottle gourd were treated with normal nitrogen（9 m M）and reduced nitrogen fertilizer application（4 m M）.Differences of the changes in plant growth,root morphology,nitrate nitrogen content and accumulation,nitrogen content and nitrogen absorption and utilization efficiency,enzyme activities related to nitrogen metabolism in leaves,and photosynthetic characteristics of the plants were analyzed.The results showed that under different nitrogen treatments,the above index values of rootstock grafted seedlings were significantly better than those of watermelon self-grafted seedlings,and nitrogen fertilizer reduction treatment inhibited the growth of self-grafted watermelon seedlings,while the nitrogen utilization efficiency and growth of rootstock-grafted seedlings were enhanced.Correlation analysis showed a significant positive correlation between NUE and plant growth with root morphology,root NO₃^-uptake,photosynthesis,and nitrogen metabolism enzyme activity.This result indicates that under the condition of reduced nitrogen fertilizer application,the bottle gourd rootstock grafting can enhance the nitrate nitrogen absorption of watermelon plants by its well-developed root systems,and enhancing the photosynthetic capacity and nitrogen metabolism potential in the shoots of grafted watermelon plants,thereby improving its nitrogen utilization efficiency and promoting the plant growth.3.Study the regulation of watermelon NO₃^--N absorption and utilization by rootstock grafting under nitrogen reduction treatment on transcription level.The‘Zaojia 8424’watermelon self-rooted seedlings,‘Jingxin NO.1’bottle gourd self-rooted seedlings,and the self-grafted and rootstock-grafted watermelon seedlings were treated with normal nitrogen（9 m M）and reduced nitrogen fertilizer application（4 m M）.The dynamic expression differences of NO₃^-uptake related NRT genes in the roots and nitrogen metabolism enzyme related genes in the leaves of each plant were analyzed after 1,5,10,and 18 days of nitrogen treatment.The results showed that the NRT1.5gene in the root system of rootstock-grafted seedling at different time points with 4 m M nitrate nitrogen treatment,the NRT2.1 gene in the root system of bottle gourd self-rooted seedlings and rootstock grafted watermelon seedlings at 18 days treatment,and the NR2,NR3,Ni R,GS1,and GS2 genes in the leaves of rootstock grafted seedlings at18 days treatment were all significantly up regulated.Correlation analysis revealed a significant positive correlation between NUp E and NRT1.5 gene expression,root tip branching number and NRT2.1 gene expression,GS and NR activity and NR2,NR3,and GS1 gene expression.This result indicates that under nitrogen fertilizer reduction treatment,bottle gourd rootstock grafting may promote efficient nitrogen absorption and utilization in watermelon plants by regulating the expression of nitrate nitrogen absorption and metabolism related genes at the transcriptional level.Among them,the NRT1.5 gene plays a key regulatory role in NO₃^-absorption,the NRT2.1 gene participates in the regulation of rootstock roots by changing their morphology in response to nitrogen reduction environment,and the NR2,NR3,Ni R,GS1,and GS2genes regulate plant nitrogen metabolism.In summary,the study will provide a reference for revealing the nutritional and physiological mechanisms of grafting for increasing yield,and provide a new approach for optimizing fertilizer management,improving fertilizer utilization efficiency,and developing a grafting cultivation model for melon crops with reduced fertilizer application and increased efficiency.