Loss and recovery of leaf hydraulic conductance: Root pressure, embolism, and extra-xylary resistance

Authors: OCHELTREE, TROY - Colorado State University; Gleason, Sean; JIANG, GUO-FENG - Guangxi University; CAO, KUN-FANG - Guangxi University

Submitted to: Journal of Plant Hydraulics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2020
Publication Date: 7/23/2020
Citation: Ocheltree, T.W., Gleason, S.M., Jiang, G., Cao, K. 2020. Loss and recovery of leaf hydraulic conductance: Root pressure, embolism, and extra-xylary resistance. Journal of Plant Hydraulics. 7. Article e001. https://doi.org/10.20870/jph.2020.e-001.
DOI: https://doi.org/10.20870/jph.2020.e-001

Interpretive Summary: Water transport through plant leaves must be both efficient and safe from failure. It is well-understood that the thin conduits that transport water through leaves are susceptible to failure during drought, such that the liquid water in these conduits can undergo sudden phase change and convert to water vapor (embolization). Once this happens, the conduits become useless for transporting liquid water until they can be refilled. It is not currently known if embolism can be readily reversed in some species, nor what conditions are necessary for refilling to occur. We measured the abilities of a tall grass species (Bambusa vulgaris) and a broad-leafed tree species (Bauhinia blakeana) to refill embolized conduits in leaves. We found that although both species experienced meaningful amounts of embolization in leaves through the day, that only the grass species, Bambusa vulgaris, was able to refill embolized conduits through the generation of root pressure (simulated in this case by a pressure chamber). In contrast, the broad-leafed tree species, Bauhinia blakeana, was not able to refill embolized conduits when simulated root pressure was applied. The absence of embolism reversal in Bauhinia blakeana suggests that embolization may be permanent in species without the capacity to generate root pressure.

Technical Abstract: Vascular networks in plant leaves must provide for the safe and efficient transport of water, nutrients, and energy; however, the conditions whereby these networks lose and regain conductive capacity are still poorly understood. We measured the loss and recovery of leaf hydraulic conductance (Kleaf) in a tropical monocotyledon (Bambusa vulgaris) and dicotyledon species (Bauhinia blakeana) using Rehydration Kinetics Method (RKM) as well as a recently developed optical method. We found that both species lost ca 88% of their maximal Kleaf (measured by RKM) before any embolization was detected in their conductive elements (via optical observation). This suggests that the majority of loss in Kleaf, as measured with RKM, was associated with resistances other than embolization. Furthermore, embolism in B. vulgaris, a species known to generate root pressure, was reversed when rehydrated under positive pressure (120 kPa), but not under atmospheric pressure. In contrast, embolism was not reversed in B. blakeana under either elevated or atmospheric pressure. However, reductions in Kleaf that was not associated with embolization was recovered by this species when rehydrated under atmospheric conditions, whereas B. vulgaris did not exhibit any recovery under the same conditions. We suggest that root pressure is an adaptive mechanism allowing for the reversal of embolism and the recovery of extraxylary conductance. The absence of embolism reversal in B. blakeana suggests that embolization may be permanent without neutral or positive xylem pressure, but that the recovery of extraxylary conductance can be regained routinely in this species in the absence of root pressure.