## 3 Results
### Sensitivity of the Bioristor to Diurnal Change of Environmental Conditions
The vertical dashed line separates the two consecutive days (8 and 9 April). xylem and phloem as per installation procedure, however a specific experiment is needed to test it. Values of xylem sap concentration depend on the ratio between the solutes flux density (Js) and that of water (Jw) [38], hence factors affectingJs and Jw would be influential on output of any device measuring xylem sap concentration. This study was undertaken to test whether the bioristor response R (which is an OECT-based proxy for xylem sap concentration) changes according to Jw. Particularly, considering that in the xylem there is no membrane selectivity and that at high Jw it dominates upon other parameters influencing the xylem ions load (e.g., temperature, carriers) [20,24], it is expected that R would change (decline) with increasing Jw. Accounting for the diurnal variation of R (Figure 2), the \"morning\" and the \"afternoon-night\" daytime parts were identified. The \"morning\" was characterized by a fast increase of Jw from negligible values early morning (~6 a.m.) to maximum ones recoded at around midday which induced a consistent decay of R (Figure 4). Incidentally, Figure 4 shows again the hysteretic behavior of R between a.m. and p.m. data. The Jw values detected during the \"afternoon-night\" stage were in general lowest than that measured \"morning\" and follow an exponential pattern, however the highest and often sharp increase of R was detected late in the afternoon (e.g., after 18:00 h) when Jw < 0.5 mol m\({}^{-2}\) s\({}^{-1}\) (Figure 4). This result is difficult to discuss due to very limited data existing on this specific issue, however it is line with the non-linear correlation between Jw and xylem sap concentration reported in the literature for soybean plant [39]. Under increasing transpiration (as during \"morning\") the load of solutes into the stele of xylem at root region is dominated by ions dragged in by the water flux density Jw [21,24] Hence, the increasing Jw during the \"morning\"  
Figure 3: Example of diurnal variation of R in tree #5 (**A**,**C**) and #10 (**B**,**D**) plotted against _VPD_ recorded during days with maximum high _VPD_ (**C**,**D**) and low (**A**,**B**) highlighting the hysteresis of R between a.m. (\(\circ\)) and p.m. (\(\bullet\)) data. Note that a.m. and p.m. values in each panel were pooled before fitting (continuous mild line) and \(R^{2}\) determination. For panel (**C**,**D**) additional fittings were performed separately for a.m. (bold continuous line) and p.m. (bold dashed line) data. The numbers close to the symbol indicate the day hour when data have been recorded. Arrows indicate the time course of the day from 0 to 23 h.  
(Figure 4) induced a consistent reduction of R because of a conceivably \"dilution\" of the sap concentration dependent on water moving with increasing transpiration stream [24]. By contrast, when Jw is null or negligible (because of low \(\Delta\)P) (e.g., during night) solutes are loaded mainly by passive diffusion or even active transport [32]. However, active transport requires availability of newly synthesized sugars/carbohydrates which are poorly available during night ([PERSON] et al., 2012). Hence, the increase of R observed when transpiration is negligible (i.e., approx. from 18-19 p.m. to 4-5 a.m. of the the next day) (Figure 4) might reflect the increase of xylem sap concentration due to osmotic root pressure and unload of nutrients from saturated xylem tissues [23]. Data collected in this study allow to generalize the partitioning of R to Jw according to the daytime. Correlative information collected over an 8-day period reported in Figure 5 confirms a non-linear decay of R with increasing Jw during the \"morring\" (Figure 4; [38]). Figure 4: Example of the influence of Jw on the diurnal variation of R recorded during (\(\Circle\)) “morning” and (\(\bullet\)) “afternoon–night”. Labels indicate the hour of the day, note that 1–5 h refer to the following day.