No Evidence for Light-Induced Embolism Repair in Cut Stems of Drought-Resistant Mediterranean Species under Soaking

(1) Recent studies suggested that stem photosynthesis could favor bark water uptake and embolism recovery when stem segments are soaked in water under light conditions, but evidence for this phenomenon in drought-resistant Mediterranean species with photosynthetic stems is missing. (2) Embolism recovery upon immersion in water for 2 h-4 h under light was assessed (i) via a classical hydraulic method in leafless Fraxinus ornus and Olea europaea branch segments stressed to xylem water potentials (psi(xyl)) inducing ca. 50% loss of hydraulic conductivity (PLC) and (ii) via X-ray micro-CT imaging of the stem segments of drought-stressed potted F. ornus saplings. Hydraulic recovery was also assessed in vivo in intact drought-stressed F. ornus saplings upon soil re-irrigation. (3) Intact F. ornus plants recovered hydraulic function through root water uptake. Conversely, the soaked stem segments of both species did not refill embolized conduits, although psi(xyl) recovered to pre-stress levels (between -0.5 MPa and -0.2 MPa). (4) We hypothesize that xylem embolism recovery through bark water uptake, even in light conditions, may not be a common phenomenon in woody plants and/or that wounds caused by cutting short stem segments might inhibit the refilling process upon soaking.