Borage extracts affect nitrate metabolism in rocket

Biostimulants are products capable to stimulate plant development, improve the nutrient use efficiency and counteract stress factors. These products may influence plant metabolism, for example acting on the regulation of enzymes involved in nitrate (NO3) metabolism. The present work was conducted by using aqueous extracts of leaves (LE) or flowers (FE) of Borago officinalis L. (500 g of plant material macerated in 1 L of water) to assess their effects as biostimulant on rocket plants (Diplotaxis tenuifolia L.) grown in floating system. Treatments were: water (control); 10 mL L-1 LE; 10 mL L-1 FE. Extracts were applied as spray (until run off) onto leaves twice, 35 days after sowing and 1 day before harvest (45 days after sowing). The experiment was mainly focused on the influence of treatments on nitrate levels; rocket is in fact a hyperaccumulator of nitrate and so it is very important to reduce this compound in leaves. To screen the effect of the extracts, several analyses (among which nitrate concentration, sugars, chlorophyll, and carotenoids levels) were done on leaves at harvest. From the biochemical point of view, the most interesting result was the substantial reduction of NO3 level caused by both extracts. This result was consistent with the increment of the nitrate reductase (NR) in vivo activity observed at harvest. Considering these results, a set of genes encoding for the key enzymes involved in nitrate metabolism were selected and studied, such as nitrate reductase (DtNR), nitrite reductase (DtNiR), glutamine synthetase (DtGS1), glutamate synthase (DtGLU), and nitrate transporter (DtNTR). Sampling was performed 2, 4, 6, 9 and 24 h after the second treatments, to evaluate the molecular response over time and to identify the peaks of expression. Borage treatments influenced the expression of these genes, confirming that these extracts have a role in the regulation of NO3 metabolism, at several levels.