The recurrent evolution of extremely resistant xylem

Abstract

Key message: Highly resistant xylem has evolved multiple times over the past 400 million years. Context: Water is transported under tension in xylem and consequently is vulnerable to invasion by air and the formation of embolism. A debate has raged over whether embolism formation is non-reversible occurring at low water potentials or a regular diurnal occurrence that is non-lethal because of a capacity to refill embolised conduits. Aims: This commentary is on a recent article, which utilised new non-invasive imaging techniques for assessing the formation of embolism in xylem, finding that the xylem of Laurus nobilis was highly resistant to the formation of embolism. Methods: The recent results of this discovery are placed in the context knowledge from a diversity of species that has so far been identified with xylem similarly highly resistant to embolism formation. Results: The discovery that L. nobilis has xylem highly resistant to embolism formation adds to a body of literature suggesting that the resistance of xylem to embolism formation is a key adaptation utilised by many species native to seasonally dry environments. Highly resistant xylem has evolved numerous times across the angiosperm clade. Conclusion: With more studies utilising similar observational and direct methods of assessing embolism resistance, further insight into the ecological and evolutionary relevance of this trait is imminent.