## 2 Results
High Temperature Negatively Impacts the Root System Distribution, but PsJN Buffers This Effect through Improvements to Individual Root Types  
The two temperature regimes showed clear differences in the root system distribution, with the ambient-grown plants showing greater depths to them than those of the heat-stressed plants did (Figure 3a). In general, the high temperature prevented the downward elongation of the roots, causing an average reduction of 28% from those of the ambient temperature condition. The difference between the two temperature regimes, i.e., Amb-Ctl and HT-Ctl, decreased after 16 DAI, which can be attributed to the faster growth rates of the primary and 1 st-order lateral roots under the ambient temperature (Figure 2g,h) that became constant towards the last two timepoints, presumably due to the limitation of the plate. In addition, the slower but continuous growth, particularly the linear growth, of the 1 st-order lateral roots under the high temperature (Figure 2g,h) towards the end of the growing period also reduced the difference in the depths of the heat-stressed and ambient-grown roots (Figure 3a). The high temperature also reduced the convex hull area and the branching angle of the root system by an average of 69% and 27%, respectively (Figure 3c,d). The reduction in the root system width due to the high temperature may be attributed to the responses of the 1 st- and 2 nd-order lateral roots (Figure 2c,d,h,i), which influence the horizontal extension of the root system. Moreover, the decrease in the number (Figure 2e,j), length (Figure 2c,d), growth rate (Figure 2h,i), and the angle of the growth (Figure 3d) of these lateral roots may have contributed to the huge difference between the root system width of the heat-stressed and ambient-grown plants (Figure 3b). In general, the effect of bacterial inoculation only manifested at 16 DAI under ambient temperature on the overall root system depth (Figure 3a), width (Figure 3b), and branching angle (Figure 3a,b,d). Whilst bacterial effects in these cumulative root traits were not found, it could be because there was compensation between the various root types and root traits  
Figure 3: **Root system morphological traits. ** Traits were measured from plants with or without bacteria PsJN inoculation under ambient or high temperature conditions from 5 to 21 days after inoculation (DAI) in the GrowScreen-Agar II platform. (**a**) Root system depth, (**b**) root system width, (**c**) convex hull area, and (**d**) branching angle of 1 st-order lateral roots. Temperature—black and circle symbol (ambient temperature), red and triangle symbol (high temperature); Bacterial application—empty symbol (control), filled symbol (PsJN-inoculated). Treatments: Amb-Ctl (control plants under ambient condition), Amb-PsJN (PsJN-inoculated plants under ambient condition), HT-Ctl (control plants under high temperature), and HT-PsJN (PsJN-inoculated plants under high temperature). All of the points are the mean \(\pm\) standard error of \(t\)= 12 (Amb-Ctl), 15 (Amb-PsJN), 12 (HT-Ctl), 8 (HT-PsJN) samples within each treatment. Asterisks “**”: Black—significant difference between mean of PsJN-inoculated and control plants under ambient condition, red—significant difference between mean of PsJN-inoculated and control plants under high temperature condition; these were based on the Student’s \(t\)-test with \(p<0.05\). (Figure 2). The convex hull area showed a significant bacterial effect under ambient at 7, whereas this effect was evident under high temperature from 5 to 9 DAI (Figure 3c).