In vivo magnetic resonance imaging of xylem vessel contents in woody lianas

By M. Clearwater, C. J. Clark. 2003

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ABSTRACT Previous reports suggest that in some plant species thereﬁlling of embolized xylem vessels can occur while nega-tive pressure exists in the xylem. The aim of this experimentwas to use non-destructive nuclear magnetic resonanceimaging (MRI) to study the dynamics of xylem cavitationand embolism repair in-vivo. Serial 1 H-MRI was used tomonitor the contents of xylem vessels in stems of two dicot-yledonous ( Actinidia deliciosa and Actinidia chinensis ,kiwifruit) and one monocotyledonous ( Ripogonum scan-dens , supplejack) species of woody liana. The conﬁgurationof the horizontal wide bore magnet and probe allowed theimaging of woody stems up to 20mm in diameter. Testsusing excised stems conﬁrmed that the image resolution of78 m m and digital image subtraction could be used to detectthe emptying and reﬁlling of individual vessels. Imagingwas conducted on both intact plants and excised shootsconnected to a water supply. In the case of Ripogonum theexcised shoots were long enough to allow the distal end ofthe shoot, including all leaves, to be exposed to ambientconditions outside the building while the proximal end wasinside the MRI magnet. In total, six stems were monitoredfor 240h while the shoots were subjected to treatments thatincluded light and dark periods, water stress followed byre-watering, and the covering of all leaves to prevent tran-spiration. The sudden emptying of water-ﬁlled vesselsoccurred frequently while xylem water potential was low(below -0.5MPa for Actinidia, -1.0MPa for Ripogonum),and less frequently after xylem water potential approachedzero at the end of water-stress treatments. No reﬁlling ofempty vessels was observed at any time in any of the speciesexamined. It is concluded that embolism repair under neg-ative pressure does not occur in the species examined here.Embolism repair may be more likely in species with nar-rower xylem vessels, but further experiments are requiredwith other species before it can be concluded that repairduring transpiration is a widespread phenomenon.Key-words: Ripogonum; Actinidia; cavitation; embolismrepair; negative pressure; transpiration; water transport.

Subfields: Global and Planetary Change, Mechanical Engineering, Plant Science

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