Reconciling discrepancies in measurements of vulnerability to xylem embolism with the pneumatic method

Abstract

Chen et al. (2021) reported discrepancies between pneumatic and other methods for measuring embolism vulnerability in plant xylem tissue, leading them to caution against using the pneumatic method. We show that this discrepancy arises from faulty implementation: failing to measure air discharge to sufficiently negative water potentials, they under-estimated maximum air discharge (ADmax), and hence embolism resistance. Embolism vulnerability methods identify how dehydrated a stem must become (in water potential, P50) to cause a 50% loss in xylem conductivity. Pneumatic methods require measurements to water potentials roughly twice as dehydrated as P50. Yet Chen et al. often stopped at magnitudes even less than what other methods showed P50 to be, making it mathematically impossible for their pneumatic implementation to measure true P50. Our simulations confirmed that premature stopping causes large errors in derived P50, accounting for reported discrepancies. Further, literature studies show consistent agreement between pneumatic and other methods. Thus, Chen et al.’s pneumatic results are invalid, and should not be taken to undermine confidence in pneumatic measurements. We recommend best practices, including the ‘Pneumatron’ for automated measurements, that prevent the errors of Chen et al. and make pneumatic measurements of embolism vulnerability that are fast, replicable, and accurate.