Role of pH and nutrient source on the production and release of aucubin, acteoside and catalpol in hydroponically grown Plantago lanceolata

Nitrous oxide (N2O) emissions contribute c. 20% of total agricultural greenhouse gas (GHG) emissions in New Zealand. The incorporation of plantain (Plantago lanceolata) in pastures has reportedly reduced N2O emissions through the production of secondary compounds inducing biologically nitrification inhibition (BNI), although the mechanisms associated to their release are still unclear. The experiment aimed to understand the role of pH (4.2, 5.6 and 6.8) and nitrogen (N) source (N-NH4+, N-NO3-) on plantain’s BNI metabolites. For this, plantain and ryegrass (Lolium perenne, control) were sown in rockwool cubes and after 10 days the seedlings were transferred to a hydroponic system for a period of 45 days (30 L cubic tanks, 8 plants per tank), solution N concentration and pH were revised and adjusted weekly. An additional solution medium was used to collect root exudates. The concentration of aucubin, acteoside and catalpol in root exudates, leaf and root samples were determined by high-performance liquid chromatography (HPLC) and UV-Vis detector (leaf and root) or a mass spectrometer (root exudates), and data was analysed using SAS 9.6. All three metabolites were found in higher concentrations in plantain leaves and roots, in comparison to ryegrass (p<0.05). Higher concentrations of acteoside were found at pH 5.6 (0.51±0.008 mg/g DM for pH 5.6 and 0.45±0.013 mg/g DM for pH 4.2 and 6.8, on average, respectively; p<0.07), and when N-NH4+ was used as N source (0.73±0.007 mg mg/g DM for NH4+ and 0.40±0.03 mg mg/g DM for NO3-1 treatments, respectively; p<0.05), reflecting that weakly acid conditions may favour the release of this metabolite. No significant differences between treatments were found for aucubin concentrations in plant tissue (p>0.05), and catalpol was not detected in leaves or roots. Aucubin, catapol and acteoside were not detected in root exudates, with implications for the detection methodology.