Relationship between nitrogen metabolic enzyme activity, gene expression and nitrogen use efficiency of flue-cured tobacco in maturing stage

Objectives Enzyme activities and gene expression represent the nitrogen metabolism of crops. Studying these gene expression and enzyme activities in leaves of mature tobacco cultivars will help to elucidate physiological and biochemical differences between caltivars, provide theoretical basis for breeding of nitrogen inefficient varieties.

Methods Three flue-cured tobacco varieties Yuyan 10, K326 and NC89 with different nitrogen efficiencies were used as tested materials, the abundance of glutamine synthetase and nitrate reductase gene in mature leaves was measured using the Real-time PCR, the leaf enzyme activities, NH₄⁺ concentration, total nitrogen, soluble protein, apoplast related indicators and ammonia volatilization were determined using the physiological and biochemical test techniques.

Results The NtGS1 gene expression abundance of all cultivars was significantly increased from 35 d, reaching the maximum at 55 d, and then down-regulated. The gene expression abundance of NtGS2 was continuously down regulated since 35 d. The overall trend of NtNit gene expression was similar to that of NtGS2. At the age of 45–65 d, the expression abundance of the above three genes all showed NC89 > K326 > Yuyan 10, and the differences among the cultivars reached the extremely significant level. The abundance of NtGS1 gene was significantly and negatively correlated with total nitrogen, leaf ammonium concentration, extracellular ammonium concentration and ammonia volatilization, and negatively correlated with the NR activity and apoplast pH. Very significant positive correlations were observed between the NtGS2 gene abundance and GS activity, total nitrogen, soluble protein content and apoplast pH, and it was significantly and positively correlated with the NR activity and leaf ammonium concentration. However, it had significant negative correlations with ammonium concentration and ammonia volatilization. The NR gene expression abundance was positively correlated with the total nitrogen, soluble protein content and leaf ammonium concentration, but negatively correlated with the ammonia volatilization.

Conclusions Nitrogen-inefficient flue-cured tobacco cultivars are weak in nitrogen transferring and reutilization, nitrogen would volatilize mainly in the form of ammonia, leading to faster leaf senescence. However, nitrogen-efficient cultivars are strong in nitrogen reutilization, nitrogen is thus less volatilized in ammonia, the aging is postponed as a result.