## 3 Results
### Response of Bioristor to Water Flux Density  
The main driving force of the flux density of water (Jw) is the _VPD_, hence it is expected a reduction of Jw upon a reduction of _VPD_. To test the influence of Jw on R, a group of trees was bagged to bring the air surrounding the canopy close to the saturation point and in turn the _VPD_ close to zero. The bagging method has been documented to effectively reduce _VPD_ at plant or branch and fruit scale by reducing RH with negligible impact on air temperature [30; 34; 40].  
The _VPD_ promptly responded to the bag application (19 h, 5 DASI) and removal (19 h, 15 DASI) mainly through variation of relative humidity which approximated 90% RH (not shown). On average, the daily _VPD_ of the air surrounding the bagged olive canopy was reduced by 80% compared to that of control trees (Figure 7). This induced a consistent reduction of daily water consumption in bagged trees compared to control ones (Figure 8).  
Throughout the 10-day bagging experiment an 80% reduction of the _VPD_ was recorded which induced a 60% reduction in Jw (Figure 7).  
Under no stomatal limitation, the causal chain _VPD_\(\rightarrow\)Jw \(\rightarrow\) R anticipates the indirect influence of the _VPD_ on R as mediated by Jw because Jw is proportional to \(\Delta\)P (hydrostatic soil-plant-atmosphere pressure gradient) generated by the leaf-to-air _VPD_. Figure 9 shows diurnal oscillations of R recorded in some succeeding days with different _VPD_ and consequently different Jw. It is confirmed that reducing _VPD_ reduced Jw and in turn increased R. In addition, R increased by 8-12% during the time when Jw was negligible (from late afternoon until early morning of the following day). The increasing R signal at negligible Jw conceivably reflects the absence of sap dilution induced by water flux and  
Figure 6: Correlation between R signal and the water flux density (Jw) hourly measured in olive trees during (top panel) “morning” (\(\Circle\)) and (bottom panel) “afternoon” (\(\bullet\), grey filled) and “night” (\(\bullet\), black filled). The \(n+1\) h indicates the time of the day (between 13 and 15 h) corresponding to that when Jw become lower than that measured at time n. Note that scatters have been fitted using the model y = a + b/x consistent with that of the ions’ concentration expected in the xylem, lines are illustrative only.  
the increasing root pressure usually occurring overnight when plants have almost null transpiration [22; 23; 41].  
Figure 8: Cumulated diurnal plant transpiration measured in trees under control (\(\bullet\)) and reduced _VPD_ (\(\bigcirc\)) by means of bag application. Each point is the average of 6 trees, bars (\(\pm\) SE) are visible when larger than symbol. Data relates to day 8 th of experiment.  
Figure 7: Values of _VPD_ and Jw (at trunk level) measured throughout the experiment in olive trees under control and low _VPD_ (grid gray filled). The dashed line refers to the tree enclosed in a transparent plastic bag to induce the low _VPD_. Time of bag on and off were indicated by \(\uparrow\) and \(\downarrow\) arrows, respectively.  
Figure 9: Oscillation of diurnal bioristor signal R (continuous line), water flux density Jw (blue filled area) and _VPD_ (grid filled area) recorded over 4 consecutive days separated by vertical dashed lines.  
Figure 9 shows also the general correspondence of R and Jw patterns during the morning daytime and the alignment between maximum pick of Jw and the minimum one of R. The plot highlights that the Jw lags in advance to _VPD_ (Figure 9) confirming a known aspect of water relations [18; 42; 43] suggesting that Jw would be a robust parameter to consider for OECT-based sensors analysis.