Are needles of Pinus pinaster more vulnerable to xylem embolism than branches? New insights from X-ray computed tomography

Plants can be highly segmented organisms with an independently redundant design of organs. In the context of plant hydraulics, leaves may be less embolism resistant than stems, allowing hydraulic failure to be restricted to distal organs that can be readily replaced. We quantified drought-induced embolism in needles and stems of Pinus pinaster using high-resolution computed tomography (HRCT). HRCT observations of needles were compared with the rehydration kinetics method to estimate the contribution of extra-xylary pathways to declining hydraulic conductance. High-resolution computed tomography images indicated that the pressure inducing 50% of embolized tracheids was similar between needle and stem xylem (P-50 needle xylem=-3.62MPa, P-50 stem xylem=-3.88MPa). Tracheids in both organs showed no difference in torus overlap of bordered pits. However, estimations of the pressure inducing 50% loss of hydraulic conductance at the whole needle level by the rehydration kinetics method were significantly higher (P-50 needle=-1.71MPa) than P-50 needle xylem derived from HRCT. The vulnerability segmentation hypothesis appears to be valid only when considering hydraulic failure at the entire needle level, including extra-xylary pathways. Our findings suggest that native embolism in needles is limited and highlight the importance of imaging techniques for vulnerability curves. The main goal of this paper is to test the vulnerability segmentation hypothesis in Pinus pinaster, which suggests that needles are more vulnerable to drought-induced cavitation than stems. Results based on high resolution X-ray computed tomography (HRCT) provide unambiguous evidence that xylem tracheids in needles of P. pinaster are not more vulnerable to cavitation than stem xylem. However, comparison of vulnerability curves based on HRCT with the rehydration kinetics method show that the vulnerability segmentation hypothesis is valid when hydraulic conductance is considered at the whole needle level. This paper illustrates the importance of visualizing embolism at the tissue level to interpret vulnerability curves of stems and leaves, and raises questions about earlier reports of daily embolism formation and refilling in gymnosperm xylem.