Xylem refilling and ionic effect as effective strategies adopted by three Mediterranean woody plants during summer drought to alleviate xylem embolism

Recent experimental evidence suggests that plants face drought-induced xylem embolism through both ion-mediated enhancement of the xylem hydraulic conductivity (Kx) of still functioning conduits (“ionic effect”) as well as by refilling embolised xylem conduits. Although both mechanisms have been recorded in different species, the diurnal and seasonal patterns of both phenomena had not been thoroughly investigated before. This study reports diurnal and seasonal measurements of percentage loss of hydraulic conductivity (PLC) , sap xylem [K+], ion-mediated changes in Kx, xylem hydraulic conductivity values (as measured in planta), vessel grouping index and starch content of vessel-associated parenchyma cells (VAC) of three typical Mediterranean species (Ceratonia siliqua L., Olea europeae L. and Laurus nobilis L.) co-occurring in the same habitat. Summer drought induced an increase of PLC in all 3 species under study. The recorded magnitude of ionic effect as well as Kx measurements in planta clearly demonstrated that an increase in xylem sap [K+] induced a significant enhancement of Kx both on a diurnal and a seasonal time scale. Hence, the ionic effect alleviated xylem embolism, substantially. Moreover, in all three species under study xylem refilling was recorded both on a diurnal and a seasonal time scale. Data of xylem refilling and ionic effect are discussed in terms of VAC starch content values and vessel grouping index, respectively.