The metabolome of olive xylem sap can reflect plant physiological status in response to differential agricultural practices

Metabolomic analysis was carried out on the xylem sap (XS) of olive plants (Olea europaea L.) from different groves located in Southern Italy (Basilicata and Puglia regions). The treatments considered included different soil types, topologies, climates, soil, and plant management systems (organic, sustainable, or conventional). The XS was extracted from olive shoots at different sampling times during the year using a modified Sholander pressure-bomb chamber (Fig. 1), and its metabolome analyzed by gas chromatography-mass spectrometry (GC-MS) (primary metabolism) and by ultra-high-performance liquid chromatography (UHPLC) coupled to a hybrid quadrupole-time-of-flight mass spectrometer (QTOF-MS) (secondary metabolism). Significant differences, in terms of metabolite numbers and abundances, were found both for primary and secondary metabolites in plants subjected to different environmental conditions (Fig. 2). Among the identified metabolites of XS, more than half were involved in the primary metabolism, such as amino acids, organic acids, soluble sugars, and sugar alcohols, some of them with a well-known role as osmoprotectants or involved in plant growth and development. On the other side, the discriminating secondary metabolites found in XS were mainly involved in plant chemical defense (e.g., phenols, antibiotics, and phitoecdysones), growth regulation (e.g., cytokinins, abscisic acid, gibberellins, jasmonates, strigolactones, and brassinosteroids) and signal transduction (e.g. signaling lipids and precursors of phosphatidic acid), or they were pigments (e.g., retinols and carotenoids) or waste products derived from partially metabolized xenobiotics. The most prevailing classes of secondary compounds included terpenoids, phytohormones, alkaloids, sterols/steroids, retinols/retinoids, tocopherols, and carotenoids. The XS of a tree crop contains not only water and inorganic minerals but also a wide range of organic metabolites, whose number and abundance significantly respond to soil use changes. Our results could lead to a better understanding of the ability of plants to overcome environmental stressors and enhance their resistance and resilience against them.