In Vivo Visualizations of Drought-Induced Embolism Spread in Vitis vinifera      

Abstract

Long-distance water transport through plant xylem is vulnerable to hydraulic dysfunction during periods of increased tension on the xylem sap, often coinciding with drought. While the effects of local and systemic embolism on plant water transport and physiology are well documented, the spatial patterns of embolism formation and spread are not well understood. Using a recently developed nondestructive diagnostic imaging tool, high-resolution x-ray computed tomography, we documented the dynamics of drought-induced embolism in grapevine (Vitis vinifera) plants in vivo, producing the first three-dimensional, highresolution, time-lapse observations of embolism spread. Embolisms formed first in the vessels surrounding the pith at stem water potentials of approximately -1.2 megapascals in drought experiments. As stem water potential decreased, embolisms spread radially toward the epidermis within sectored vessel groupings via intervessel connections and conductive xylem relays, and infrequently (16 of 629 total connections) through lateral connections into adjacent vessel sectors. Theoretical loss of conductivity calculated from the high-resolution x-ray computed tomography images showed good agreement with previously published nuclear magnetic resonance imaging and hydraulic conductivity experiments also using grapevine. Overall, these data support a growing body of evidence that xylem organization is critically important to the isolation of drought-induced embolism spread and confirm that air seeding through the pit membranes is the principle mechanism of embolism spread.

Acoustic Emissions to Measure Drought-Induced Cavitation in Plants

“…Although the AE activity may continue after the selected endpoint (Figure 3), the obtained P50 values (´2.30 to´2.73 MPa) corresponded well with values found in literature for grapevine (´2.17 tó 2.97 MPa; [82,83]), from which was concluded that the endpoint was accurate, and that the remaining AE activity was related to other sources than cavitation. Strong evidence for their approach was given by validation with X-ray micro-computed tomography (µCT), which showed quite similar patterns for both visually and acoustically detected cavitation [31] (Figure 4).…”

Mechanical Failure of Fine Root Cortical Cells Initiates Plant Hydraulic Decline during Drought

“…Vessel diameter (d) was derived by A ¼ pðd=2Þ 2 . Percentage theoretical loss of xylem hydraulic conductivity was determined using a modified Hagen-Poiseuille equation (Tyree and Ewers, 1991;Brodersen et al, 2013;Knipfer et al, 2015). The xylem hydraulic conductivity (k h ) of the population of embolized and water-filled vessels of the roots was calculated according to:…”

“…Using a semiautomated routine in Fiji (Schindelin et al, 2012; as described in Brodersen et al, 2013), vessel number and cross-sectional area (A) were determined by using the Analyze Particles tool. Vessel diameter (d) was derived by A ¼ pðd=2Þ 2 .…”

Physiological and Genomic Basis of Mechanical-Functional Trade-Off in Plant Vasculature

“…Monocots do not exhibit secondary thickening and ray cells thus the osmoticum and sucrose transport theory do not apply to monocots (Andre, 1998). Selection for root pressure in these species solves the embolism repair problem and negates the need for carbohydrate transport along the pathway common in woody angiosperms (Brodersen et al, 2013). However, Stiller et al (2005) showed the presence of "novel" refilling in rice in presence of high negative pressure and suggested that in upland or low-rainfed rice this mechanism can serve side by side of a positive root pressure.…”