Borage extracts affect wild rocket quality and influence nitrate and carbon metabolism

Abstract

Market is increasingly demanding vegetables with high quality and nutraceutical characteristics. It was demonstrated that leafy vegetables can get benefit from biostimulants, for the reduction of nitrate concentration and the increment of antioxidants, with potential benefit for human health. The research purpose was to investigate on the role of a novel plant-based biostimulant in affecting nitrogen and carbon metabolism in wild rocket (Diplotaxis tenuifolia L.). Foliar spray treatments were performed with extracts obtained from borage (Borago officinalis L.) leaves and flowers. To evaluate the treatments effect, in vivo determinations (chlorophyll a fluorescence and chlorophyll content) were performed. At harvest, nitrate concentration, sucrose, total sugars, chlorophyll, and carotenoids levels were measured in leaves. In order to characterize the mechanism of action also at molecular level, a set of genes encoding for some of the key enzymes implicated in nitrate and carbon metabolism was selected and their expression was measured by qRT-PCR. Interesting results concerned the increment of sucrose, coherent with a high value of Fv/Fm, in addition to a significant reduction of nitrate and ABA than control, and an enhanced NR in vivo activity. Also, genes expression was influenced by extracts, with a more pronounced effect on N related genes.

Keywords: Biostimulant; Borago officinalis L.; Diplotaxis tenuifolia L.; Gene expression; Nitrate.

Fig. 1

Chlorophyll a fluorescence parameters, Fv/Fm (a), PI (b), and RC/CSm (c), measured in vivo in rocket leaves treated with water (control), 10 mL L⁻¹ borage LE or FE. Values are means ± SE (n = 3). Data were subjected to one-way ANOVA with Bonferroni’s post-test (P < 0.05). Different letters, when present, indicate statistical differences among treatments.

Fig. 2

Chlorophyll content determined in vivo (a), chlorophyll a + b (b), and carotenoids concentration (c) in rocket leaves treated with water (control), 10 mL L⁻¹ borage LE or FE. Values are means ± SE (n = 3). Data were subjected to one-way ANOVA with Bonferroni’s post-test (P < 0.05)

Fig. 3

Abscisic acid (ABA) concentrations in rocket leaves treated with water (control), 10 mL L⁻¹ borage LE or FE. Values are means ± SE (n = 3). Data were subjected to one-way ANOVA with Bonferroni’s post-test (P < 0.05). Different letters, when present, indicate statistical differences among treatments

Fig. 4

Sucrose (a) and total sugars (b) concentration of rocket leaves treated with water (control), 10 mL L⁻¹ borage LE or FE. Values are means ± SE (n = 3). Data were subjected to one-way ANOVA with Bonferroni’s post-test (P < 0.05). Different letters, where present, represent significant differences among treatments

Fig. 5

Nitrate concentration of rocket leaves treated with water (control), 10 mL L⁻¹ borage LE or FE. Values are means ± SE (n = 3). Data were subjected to one-way ANOVA with Bonferroni’s post-test (P < 0.05). Different letters represent significant differences among treatments

Fig. 6

Nitrate reductase in vivo activity measured in rocket leaves treated with water (control), 10 mL L⁻¹ borage LE or FE at three different time points (0, 2, and 4 h of light exposure). Values are means ± SE (n = 3). Data were subjected to two-way ANOVA (P < 0.0001) with Bonferroni’s post-test. Different letters represent significant differences among treatments and time

Fig. 7

Heat map illustrating variations in the expression (Log fold change, compared to control) of genes involved in nitrogen metabolism, in rocket leaves treated with 10 mL L⁻¹ borage LE or FE. For each gene, high expression is depicted as intense red color, and low expression as intense blue color. Data were compared by using two-way ANOVA, with Bonferroni’s post-test. Asterisks represent significant differences compared to untreated control

Fig. 8

Heat map illustrating variations in the expression (Log fold change, compared to control) of genes involved in carbon metabolism, in rocket leaves treated with 10 mL L⁻¹ borage LE or FE. For each gene, high expression is depicted as intense red color, and low expression as intense blue color. Data were compared by using two-way ANOVA, with Bonferroni’s post-test. Asterisks represent significant differences compared to untreated control