Abstract
Gas embolisms formed during drought can disrupt long-distance water transport through plant xylem vessels, but some species have the ability to remove these blockages. Despite evidence suggesting that embolism removal is linked to the presence of vessel-associated parenchyma, the underlying mechanism remains controversial and is thought to involve positive pressure generated by roots. Here, we used in situ x-ray microtomography on excised grapevine stems to determine if embolism removal is possible without root pressure, and if the embolism formation/removal affects vessel functional status after sample excision. Our data show that embolism removal in excised stems was driven by water droplet growth and was qualitatively identical to refilling in intact plants. When stem segments were rehydrated with H2O after excision, vessel refilling occurred rapidly (<1 h). The refilling process was substantially slower when polyethylene glycol was added to the H2O source, thereby providing new support for an osmotically driven refilling mechanism. In contrast, segments not supplied with H2O showed no refilling and increased embolism formation. Dynamic changes in liquid/wall contact angles indicated that the processes of embolism removal (i.e. vessel refilling) by water influx and embolism formation by water efflux were directly linked to the activity of vessel-associated living tissue. Overall, our results emphasize that root pressure is not required as a driving force for vessel refilling, and care should be taken when performing hydraulics measurements on excised plant organs containing living vessel-associated tissue, because the vessel behavior may not be static.
Highlights
Gas embolisms formed during drought can disrupt long-distance water transport through plant xylem vessels, but some species have the ability to remove these blockages
The goal of this study was to investigate if vessel refilling exists in excised grapevine stems, and in turn reveal whether (1) embolism removal can occur in the absence of root pressure, (2) requires a long-distance signaling mechanism delivered from distal organs, and/or (3) is driven predominantly by vessel-associated tissue
No differences in temporal dynamics of vessel refilling and embolism formation were observed in excised stems coated with petroleum jelly or those coated with petroleum jelly and sealed in a plastic bag (Fig. 1A, gray diamond symbol; Fig. 1C, black and white circle symbols)
Summary
Gas embolisms formed during drought can disrupt long-distance water transport through plant xylem vessels, but some species have the ability to remove these blockages. Grapevines have long been used as a model system for studying processes associated with long-distance water transport including root pressure generation, xylem vulnerability to cavitation, and embolism repair Since the recognition of their ability to generate root pressure (Hales, 1727), it has been speculated that grapevines utilize root pressure to refill xylem vessels that have become air-filled during recovery from winter freeze-thaw events (Sperry et al, 1987) and drought (Holbrook et al, 2001). Two grapevine species (Vitis arizonica and V. riparia) were identified for their ability to remove embolism after generation of high root pressure following rewatering after drought
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