Revisiting paradigms related to root hydraulic limitation under drought

In vascular plants, the long-distance water transport between organs occurs under tension. Because it is difficult to study a system under negative pressure without measurement artefacts, the field of plant hydraulics has been afflicted with various controversies. One such debate relates to the relative vulnerability of roots versus aboveground organs within woody angiosperms; specifically, whether coarse roots are far more vulnerable to drought-induced dysfunction than stems or branches. It has been proposed that the wider and longer xylem vessels observed in roots compared with aboveground organs are associated with higher vulnerability to xylem embolism, suggesting a trade-off between xylem safety and hydraulic efficiency at the whole-plant level. Since roots were thought to embolize first and to recover steadily from embolism, they were assumed to act as ‘hydraulic fuses’ that limit tension and embolism in proximal parts of the xylem network. Moreover, the ‘hydraulic fuse’ concept was consistent with the idea that roots were the sole source of abscisic acid and that chemical root-shoot signals were required to induce stomatal closure. Here, we re-evaluate several paradigms that still persist in the literature and show that they are outdated and require reinterpretation based on recent evidence. The surveyed paradigms include (i) a high root vulnerability to embolism formation, (ii) a within-tree trade-off between xylem safety and hydraulic efficiency, (iii) the assumption that roots act as hydraulic fuses, (iv) the occurrence of embolism repair in roots, and (v) the root-to-shoot abscisic acid signalling. Based on available evidence, we show that none of these classic hypotheses hold, except for an updated hydraulic fuse hypothesis, which could be attributed to a loss of contact at the soil-root interface when the cortex shrinks during moderate soil drought.