Mechanisms of xylem embolism repair in woody plants: Research progress and questions

Abstract

To maintain long-distance water transport in woody plants is critical for their survival, growth and development. Water under tension is in a metastable state and prone to cavitation and embolism, which leads to loss of hydraulic conductance, reduced productivity, and eventually plant death. In face to water stress-induced cavitation, plants either reduce frequency of embolism occurrence through cavitation resistance with specialized anatomical struc- ture, or/and form a metabolically active embolism repair mechanism. For the xylem embolism and repair, how- ever, there are controversies regarding the occurring frequency, conditions and underlying mechanisms. In this review paper, we first examined the process, temporal dynamics and frequency of xylem embolism and repair. Then, we summarized hypotheses for the mechanisms of the novel refilling in xylem embolism repair, including the osmotic hypothesis, the reverse osmotic hypothesis, the phloem-driven refilling hypothesis, and the phloem unloading hypothesis. We further compared differences in xylem embolism and repair between conifers and an- giosperms tree species, and examined the trade-offs between cavitation resistance and xylem recovery perform- ance. Finally, we proposed four priorities in future research in this field: (1) to improve measuring technology of xylem embolism; (2) to test hypotheses for the mechanisms of the novel refilling in xylem embolism repair and the signal triggering xylem refilling; (3) to explore species-specific trait differences related to xylem embolism and repair and their underlying trade-off relationships; and (4) to enhance studies on the relationship between the involvement of carbon metabolism and aquaporins expression in xylem embolism and repair.