Abstract
Stevia (Stevia rebaudiana Bertoni) is an important industrial crop, and its leaves contain steviol glycosides (SGs), which are valuable compounds because they are characterized by a high sweetness and low caloric content. SGs yield depends on nitrogen (N) nutrition; however, the role of N uptake in SGs synthesis needs clarification, especially in organic farming systems that depend on organic and biofertilizers. Plant growth-promoting bacteria (PGPB; i.e., Azospirillum brasilense and Bacillus circulans) and compost were used to study the effect of N nutrition on SGs yield. The experiment consisted of five N treatments with or without PGPB inoculation. The fertilization treatments were: C = without N, T1 = urea, T2 = compost, T3 = urea and compost (75% of N from urea and the rest from compost) and T4 = urea and compost (50% of N from urea and the rest 50% from compost). Compost and PGPB increased the soil quality indicators and enhanced the synthesis of chlorophyll and nutrient uptake by stevia plants. Mixtures of compost and urea under inoculation with PGPB enhanced soil microbes and enzymatic activity. The highest significant value of nitrogen use efficiency (NUE) resulted from urea alone, while the maximum phosphorus (P) and potassium (K) uptake came from compost. Although urea led to an increase in stevia N uptake, it led to a clear decrease in SGs yield. Urea decreased the total soluble sugar and leaf C/N ratio by 29% and 31%, respectively, compared to those of C. T4 increased the SGs concentration by 74% and 46%, respectively, compared to that of urea alone in the presence and absence of PGPB. PGPB enhanced the soil quality and promoted nutrient uptake, resulting in significant increases in SGs yield. Regulating N uptake enhanced synthesis of soluble sugars and increased the C/N ratio, which caused an increase in SGs production. The use of compost and plant growth-promoting bacteria is a suitable strategy to produce stevia plants with low rates of inorganic nitrogen without any yield loss.
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•Compost and plant growth-promoting bacteria (PGPB) enhances stevia growth.•Compost and PGPB reregulate N-uptake in stevia plants.•Urea decreased the total soluble sugar and leaf C/N ratio.•Regulating N-uptake increased the synthesis steviol glycosides (SGs).•The findings might be an aid for sustainable N management in stevia production.