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ABSTRACT: Vulnerability curves (VCs) generally can be fitted to the Weibull equation, however, a growing number of VCs appear to be recalcitrant, i.e., deviate from a Weibull but seem to fit dual Weibull curves. We hypothesize that dual Weibull curves in Hippophae rhamnoides L. are due to different vessel diameter classes, inter-vessel hydraulic connections, or vessels versus fiber-tracheids. We used dye staining techniques, hydraulic measurements, and quantitative anatomy measurements to test the hypotheses. The fibers contribute 1.3% of the total stem conductivity, which eliminates the hypothesis that fiber tracheids account for the second Weibull curve. Nevertheless the staining pattern of vessels and fiber-tracheids suggested us that fibers might function as a hydraulic bridge between adjacent vessels. We also argue that fiber bridges are safer than vessel to vessel pits and put forward the concept as a new paradigm. Hence we tentatively propose that the first Weibull curve may be accounted by vessels connected to each other directly by pit fields, while the second Weibull curve is associated with vessels that are connected almost exclusively by fiber bridges. Further research is needed to test the concept of fiber bridge safety in species which have recalcitrant or normal Weibull curves.
Plant Cell and Environment 04/2013; · 5.22 Impact Factor
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ABSTRACT: Vulnerability curves (VCs) measure the ability of vessels to retain metastable water without embolisms that lower the hydraulic conductivity of stems. The fastest method of measuring VCs is the centrifuge technique and the Cochard cavitron is a method that allows measurement of hydraulic conductivity of stems while they are spinning. This paper describes the pattern of embolism that results after spinning the stems of hybrid aspen (Populus tremula×P. tremuloides) and two hybrid cottonwoods (P38P38 P. balsamifera×P. simonii and Northwest, which is a hybrid of P. deltoides×P. balsamifera). It is recognized that the pattern of embolism induced in a centrifuge ought to differ from the pattern during natural dehydration of plants because the profiles of tension vs distance greatly differ under the two modes of inducing stress. The pattern of embolism was visualized by a staining technique and quantified by traditional measurements of percentage loss conductivity (PLC) performed on subsample segments excised from spun stems. We found a pattern of embolism approximating that expected from theory: (1) PLC near the axis of rotation exceeded the average; (2) PLC was quite high near the ends of the stems, even though tension ought to be zero; (3) large vessels cavitated before small vessels; (4) more embolism occurred near the base than near the apex of the stems. However, we could not always scale up from subsample conductivity and PLC to whole-stem conductivity. This pattern of embolism is interpreted in terms of vessel diameter and vessel length.
Physiologia Plantarum 12/2010; 140(4):311-20. · 3.11 Impact Factor
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ABSTRACT: The objective of this method paper was to examine a computational algorithm that may reveal how vessel length might depend on vessel diameter within any given stem or species. The computational method requires the assumption that vessels remain approximately constant in diameter over their entire length. When this method is applied to three species or hybrids in the genus Populus, vessel length is sometimes a linear function of vessel diameter and sometimes an exponential function of vessel diameter within a stem, based on R(2) values. Our results give within-species variation of vessel length versus diameter, and we compare this to between-species variation of mean diameter versus mean length.
Plant Cell and Environment 03/2010; 33(7):1234-8. · 5.22 Impact Factor
