## 3 Results
### Sensitivity of the Bioristor to Diurnal Change of Environmental Conditions  
The sensor response (R) showed a typical circadian pattern confirming previous studies employing the bioristor in annual crops [13; 14]. In the present study, R trends have been analyzed against the changes of Jw and transpiration. It was observed that R quickly declined from early morning (\(\sim\)6:00 h) concomitantly with the beginning of plant transpiration flux (Figure 2). As soon as plant transpiration rate begun to decline (approx. 1300-1500 h), R trend reversed and continuously increased until approx. 19:00-20:00 h when the transpiration reached minimum values (Figure 2). Thereafter, while transpiration continued to sit at the minimum, R further increased and highest values were recorded around midnight or just before the beginning of the transpiration cycle of the next day (Figure 2). To explain increasing R values observed at null or very low transpiration (Figure 2) the main mechanisms of ions and water load into xylem should be evoked. Although the weight of active and passive loading mechanism(s) of ions (and other solutes) into the xylem has not yet clearly defined [32], the xylem loading of ions might be uncoupled from that of water because plant's membrane is differentially permeable to water and ions [20; 32]. Xylem conduits are non-selective structures, hence water and solutes flow in the xylem at the same velocity through the mass flow driven by the \(\Delta P\) independently by the osmotic pressure, excepting under saturating conditions when transpiration ceases (e.g., night or at very low _VPD_), or in case of low-transpiring organs (e.g., fruit,) [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. Under non-limited water condition and according to the constancy of water flow through the plant [34], almost the whole volumetric water flux density taken up by root is transpired by leaf and in turn the same volume of water is loaded into xylem. Hence, variation in the Jw calculated through plant transpiration is a reliable proxy for volumetric water uptake from soil and loaded into xylem. Values of plant transpiration (E) (Figure 2) were consistent with those of sap flow (referred per unit of leaf area) measured in potted olive trees under similar _VPD_[35]. Variations of R appeared to be proportional to those of plant transpiration particularly during the first part of the day (from \(\sim\)6 h to \(\sim\)13 h, hereafter referred as \"morning\") (Figure 2). Figure 2 also shows a substantial late \"afternoon\" and \"night\" variation (increase) of R which was independent of E (and thus of _VPD_). Considering that E depends on leaf-to-air _VPD_ and canopy conductance [20], R would correlate with _VPD_ as suggested by previous reports [15]. Such a correlative link between _VPD_ and R might be influenced by the eventual hysteretic pattern of R. That is, diurnal R might move in a hysteresis loop when plotted against _VPD_ with the hysteretic magnitude differing day-by-day depending on _VPD_ level (Figure 3). Results show that in a warm day with _VPD_ peaking at approx. 3 KPa, data of R collected a.m. had different slope compared to that measured p.m. (Figure 3C-D). While in a mild day (max _VPD_ at \(\sim\)1.5 KPa) the slopes of R data measured a.m. and p.m. were more comparable (Figure 3A-B). This might be explained considering the possible time lag effect of _VPD_ on diurnal pattern of certain traits (e.g., sap flow, transpiration) [17; 36; 37] related to sap ionic concentration and in turn to R. It appears that disentangling the diurnal pattern of R in relation to Jw (which is triggered by _VPD_), would contribute to expand previous knowledge on the response of OECT sensors to the surrounding environment (see below). The _VPD_-R linear correlations and the evidence that _VPD_ triggers the xylem stream [20] suggest that the OECT would monitor the xylem tissue although it was in contact with  
Figure 2: Diurnal variations of R (\(\Circle\)) and plant transpiration (E) (continuous line) recorded over two consecutive days.