Abstract Detail


Michaud, Joseph M. [1], Mocko, Kerri [2], De Guzman, Samantha T. [1], Nguyen, Khoi T. [1], Pieters, Vanessa N. [1], Garcia, Jessica [1], Zou, Yu [1], Schenk, H. Jochen [1].

Positive Nocturnal Stem Pressure in Avocado and Other Lauraceae.

Vascular plants move water through xylem conduits in two different ways: transpiration-driven passive transport via the cohesion-tension mechanism, largely under negative pressure, and movement under positive pressure in the absence of transpiration. Quantitatively, transpiration-driven transport far exceeds pressure-driven transport, but periods of positive pressure transport are common in angiosperms and in some ferns, likely playing an important role in water relations for a large number of plants. Here we report on newly discovered nocturnal stem pressure in avocado and other trees in the Lauraceae. Nocturnal stem pressure has been inferred from guttation from leaves of some basal angiosperm trees, but to our knowledge has never been directly measured in trees before. Historically, positive pressure in xylem conduits has been referred to mostly as “root pressure”, even though it can also manifest in stems, rhizomes, and leaves. Here, we use the term “positive stem pressure” to mean above atmospheric pressure measured in the non-living parts (apoplast) of a stem, not to be confused with turgor pressure in living cells. Stem pressure was measured with small board-mounted pressure sensors inserted into the sapwood. In avocado, we also measured xylem and bark diameter variations, sap flow, and xylem water potentials using stem psychrometers. Diurnal pressure fluctuations were found in stems of well-irrigated, mature avocado (Persea americana, vars. Fuerte, Hass, and Bacon), Machilus yunnanensis, and Cinnamonum camphora trees, all grown in the Fullerton Arboretum in Fullerton, California. These are evergreen trees originating from tropical and subtropical humid or seasonally dry environments. Nocturnal pressure in these species often exceeded 400 kPa above atmospheric. Positive stem pressure was not observed in three other Lauraceae: Cryptocarya alba, Laurus nobilis, and Umbellularia californica, all native to regions with Mediterranean climates. Pressure in avocado stems developed at more apical locations before it occurred at the trunk base, and no positive pressure was observed in woody roots. Avocado bark tended to swell before xylem, and xylem began to shrink when xylem pressure declined below zero. Positive pressure in all species subsided in the morning with a resumption of transpiration-driven sap flow, and daytime pressures often went well below zero. A recent review of the literature on positive xylem pressure had concluded that there was no direct evidence for nocturnal pressure in trees, possibly because the pressures required to push water up to the leaves and rehydrate the entire tree would have to be very high. Our findings show that some evergreen tropical trees are actually capable of creating such high pressures in stems. This pressure originates most likely in the stem bark and certainly not in roots.

1 - California State University Fullerton, Department of Biological Science, PO Box 6850, Fullerton, CA, 92834-6850, USA
2 - Trinity College, Biology Department, 300 Summit Street, Hartford, CT, 06106, USA

positive pressure
water relations

Presentation Type: Oral Paper
Number: PHYS1005
Abstract ID:889
Candidate for Awards:None

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