## 1 Introduction  
The management of mineral nutrition in agricultural systems contributes to the adequate nutrients availability for biochemical and structural functions in planta. Matching plant nutrient demand with nutrient soil supply (and availability) at short interval time (days or even hours) might help to avoid excessive (supply>>demand) or deficient (supply<<demand) nutrition and in turn contributing to healthy plant and environment. A number of in-vivo sensors and non-touch image-based technologies are increasingly proposed within innovative agriculture to monitor plant traits including water status, diseases and minerals content [1, 2, 3, 4]. Real time monitoring of nutrient availability in xylem sap is pivotal for that purpose and to minimize agriculture dependency on mineral fertilization and/or face nutrition stress. A relatively new set of organic electrochemical transistors (OECT) devices is emerging for the determination of physical and chemical characteristics of liquid samples and biological system including the ionic content [5; 6; 7]. Through mathematical models the change of the OECT device bulk conductivity allows the detection of the concentration of ions in solution making them increasingly used for bio-interfacing, bio-sensing, and electrophysiological recording [7; 8; 9]. Due to their ions' sensitivity and selectivity (e.g., K, Mg, Na) [10; 11], OECT-based biosensors are promising tool within a smart plant nutrition management context. For example, current open-source application includes leaf nutrient content (sourced by foliar analysis) for the implementation of the monthly fertigation plan in olive [12]. Having a real time monitoring of ions carried in the sap as input data would strength that kind of app. An OECT sensor (hereafter named bioristor) has been proposed for the in vivo real time monitoring of some physiological traits (including concentration of ions in the xylem sap) under variable environmental conditions and drought stress in herbaceous crops (e.g., tomato) [13; 14; 15]. However, due to the differences in the vascular system between herbaceous and trees [16], testing similar sensors for perennials is highly desirable. The sensitivity of bioristor to positively charged ions (e.g., Ca, K) [15] led to a close relationship between the molar concentration of aqueous solutions and the sensor response (R). Moreover, previous work reported R and _VPD_ and stomatal conductance highly correlated (\(R^{2}\) = 0.82) in tomato [14; 15] under a _VPD_ ranging within 0-0.8 kPa. The _VPD_ is a well-known driver of transpiration (and in turn of sap flow) which often lags behind (or occurs in advance) of _VPD_ particularly under high _VPD_[17; 18]. In addition, values of _VPD_ in cultivated areas (e.g., Mediterranean-type) usually peak at 3-4 KPa [19], hence testing an OECT sensor in a perennial crop under Mediterranean growing conditions and in relation to plant transpiration would integrate current knowledge on OECT sensor. Concentration of xylem sap (Ci, mol m\({}^{-3}\)) at any instant is satisfactorily approximated by the ratio of the flux density of solutes (Js, mol m\({}^{-2}\) s\({}^{-1}\)) and that of (volumetric) water (Jw, m\({}^{3}\) m\({}^{-2}\) s\({}^{-1}\)) entering the xylem [20]. Values of Js depend on several factors including nutrients uptake by roots, xylem tissues loading-unloading, nutrients delivery and utilization along the transportation pathway [21]. In addition, roots might passively (diffusion) and/or actively uptake ions [22]. A mass flow of ions into root also occurs because they are dragged in by Jw (activated by transpiration flux) and/or because Jw induces a continuous removal of the ions at membrane interface after they have been released into the xylem lumen [23; 24].