Reducing nitrate and tobacco‐specific nitrosamine level in burley tobacco leaves through grafting on flue‐cured tobacco rootstock

Nitrosation of pyridine alkaloids in tobacco generates tobacco‐specific nitrosamines (TSNAs), which are notable toxicants in tobacco products and smoke. Burley tobacco, a chloroplast‐ and nitrogen (N)‐deficient phenotype that accumulates high levels of nitrate‐nitrogen (NO(3)‐N) in its leaves, is particularly susceptible to TSNAs formation. In this study, reciprocal pot and field grafting experiments were conducted using burley tobacco Eyan No.1 and flue‐cured tobacco K326 to investigate whether grafting burley tobacco scions on flue‐cured tobacco rootstocks could enhance pigment biosynthesis and photosynthesis, while reducing the NO(3)‐N level in burley tobacco leaves. Grafting burley tobacco scions on flue‐cured tobacco rootstocks significantly increased the total pigment content, photosynthetic rate, biomass, nitrate reductase and glutamine synthetase activities, as well as ammonium‐nitrogen (NH(4)‐N), total soluble and reducing sugar, and soluble protein levels in burley tobacco leaves compared with burley tobacco self‐rooting, while decreasing the NO(3)‐N level and nitrate‐N to total N ratio. Transcriptomic analysis revealed that grafting resulted in upregulated expression of genes involved in starch, sucrose, porphyrin, chlorophyll, and N metabolism, as well as carbon fixation and carotenoid biosynthesis. The findings suggest that grafting on high N use efficiency rootstock is an exceptionally promising means of decreasing NO(3)‐N accumulation by improving photosynthesis and N metabolism in the scion, thereby reducing the levels of harmful TSNAs.