Physiological and biochemical mechanisms related with the nitrogen use efficiency in tobacco genotypes

Abstract

Knowing the nitrogen use efficiency (NUE) of tobacco is crucial to minimize environmental pollution, although NUE is rarely known for numerous genotypes for this crop. In the first experiment, through the growth of different genotypes in nutritive solutions contrasted in nitrogen (N) availability, the aims were to characterize five components of the NUE of 28 genotypes and to classify them according to their efficiency and responsiveness to N availability. On average, the physiological N use efficiency (ratio between dry mass and N concentration of the plant), N harvest index (ratio between leaves N accumulation by plant N accumulation), and N uptake efficiency (ratio between plant N accumulation by root dry mass) decreased by 16%, 4%, and 57%, respectively, under N-deficient conditions, while N utilization efficiency (ratio between dry mass and plant N accumulation) decreased by 43% at adequate N supply. The relative efficiency of N use (ratio between dry mass produced under deficiency and adequate N supply) varied from 35% to 59% among genotypes. All genotypes of the Virginia and Maryland varietal groups were efficient, and those of the Burley, Comum, and Dark groups were inefficient. The responsiveness varied among genotypes within the varietal groups, except for Maryland genotypes, being non-responsive. In the second experiment, we studied four efficient and responsive (ER) genotypes and four non-efficient and non-responsive (NENR) genotypes selected of the first experiment. The aim was to evaluate the responses in gas exchanges and of the N metabolism enzymes of tobacco genotypes under different N availabilities. Principal component analysis (PCA) revealed that the intercellular CO2 concentration (Ci), flavonoids contents and anthocyanins and glutamine synthase (GS) activity were parameters related to N deficiency, whereas net photosynthetic rate (Pn), activities of nitrate reductase (NR), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH), total dry mass (DM), total N accumulation (NA), instantaneous carboxylation efficiency (k), total chlorophyll, N balance (ratio between the chlorophyll and flavonoids contents), water use-efficiency (WUE), transpiration (E) and stomatal conductance (gs) were associated with adequate N supply. Generally, genotypes ER showed lower anthocyanins content, higher chlorophyll and flavonoids content, higher k, photosynthetic N use efficiency (PNUE), NR, GDH and GOGAT activities compared to genotypes NENR. Under N deficiency, ER genotypes revealed generally higher flavonoids content, PNUE, NR and GDH activities when compared to genotypes NENR under this same condition. The greater total DM and total NA of the genotypes ER is due to the greater pairing of diverse parameters of both C and N metabolisms here investigated. The findings reported here are useful in indicating genotypes with different efficiency and responsiveness to the N supply, which can be chosen according to the level of N in the soil or the availability of nitrogen fertilizers in tobacco crops worldwide. In general, the data obtained in this thesis can lead to a more sustainable use of nitrogen and can support tobacco breeding programs for NUE.