Dynamic changes in hydraulic conductivity in petioles of two savanna tree species: factors and mechanisms contributing to the refilling of embolized vessels

Abstract

Diel variation in specific hydraulic conductivity ( k s ) was recorded in petioles of two savanna tree species, Schefflera macrocarpa and Caryocar brasiliense , from central Brazil. These two species have compound leaves with long petioles (10-30 cm). In both species, petiole k s decreased sharply with increasing transpiration rates and declining leaf water potentials ( y L ) during the morning. Petiole k s increased during the afternoon while the plants were still transpiring and the water in the non-embolized vessels was still under tension. Dye experiments confirmed that in both species diel variation in k s was associated with embolism formation and repair. When transpiration was prevented in individual leaves, their petiole k s and water potential remained close to their maximum values during the day. When minimum daily y L on selected branches was experimentally lowered by 0.2-0.6 MPa, the rate of k s recovery during the afternoon was slower in comparison with control branches. Several field manipulations were performed to identify potential mechanisms involved in the refilling of embolized petiole vessels. Removal of the cortex or longitudinal incisions in the cortex prevented afternoon recovery of k s and refilling of embolized vessels. When distilled water was added to petiole surfaces that had been abraded to partially remove the cuticle, k s increased sharply during the morning and early afternoon. Evidence of starch to sugar conversion in the starch sheath cells surrounding the vascular bundles of the petioles was observed during periods of rapid transpiration when the abundance of starch granules in the starch sheath cells surrounding the vascular bundles decreased. Consistent with this, petiole sugar content was highest in the early afternoon. The most parsimonious explanation of the field observations and the experimental results was that an increase in osmotically active solutes in cells outside the vascular bundles at around midday leads to water uptake by these cells. However, the concurrent increase in tissue volume is partially constrained by the cortex, resulting in a transient pressure imbalance that may drive radial water movement in the direction of the embolized vessels, thereby refilling them and restoring water flow. This study thus presents evidence that embolism formation and repair are two distinct phenomena controlled by different variables. The degree of embolism is a function of tension, and the rate of refilling a function of internal pressure imbalances.

Diurnal and seasonal variation in root xylem embolism in neotropical savanna woody species: impact on stomatal control of plant water status

“…However, the present study appears to be the first showing evidence for short-term changes in root conductivity in woody species. Our results also confirm previous studies that embolism reversal can occur when the water in adjacent non-embolized xylem is under considerable tension (Salleo et al 1996;Bucci et al 2003). The refilling of embolized xylem vessels during the day under negative water potentials of stems presents an apparent paradox (Tyree et al 1999;Pickard 2003a).…”

“…Our findings are consistent with reports of short-term changes in hydraulic conductivity in stems, petioles and leaf blades of several woody species (Salleo et al 1996;Zwieniecki & Holbrook 1998;Bucci et al 2003;Brodribb & Holbrook 2004;Meinzer, James & Goldstein 2004) as well as with a report of daily change in k s in roots of non-woody plants (McCully et al 1998). However, the present study appears to be the first showing evidence for short-term changes in root conductivity in woody species.…”

“…Instead, we propose that this refilling mechanism only operates on a local scale, causing dynamic changes in root conductivity. Even though measurements of solute concentration in xylem sap during refilling have not shown an increase in osmotic concentration sufficient to drive refilling (Tyree et al 1999), to create a transient pressure imbalance, it would be critical to demonstrate that in roots the mechanism leading to xylem recovery is associated with an increase in osmotically active solutes (Milburn 1996;Bucci et al 2003). This increase in xylem solute concentration may have occurred in Q. parviflora in August, which would explain the increase in k s and the decrease in PLC in mid-afternoon compared with predawn (Fig.…”

Water Stress Deforms Tracheids Peripheral to the Leaf Vein of a Tropical Conifer

“…4, A and B). Closure of stomata after the onset of xylem dysfunction has been shown to occur in some tropical angiosperms but appears to require some sort of refiling process to repair embolized xylem (Bucci et al, 2003;Brodribb and Holbrook, 2004a). It was imagined that conifers might display more conservative stomatal regulation similar to ferns, which close their stomata before the onset of xylem dysfunction (Brodribb and Holbrook, 2004b).…”

Promotion of Dermal Wound Healing by Polysaccharides Isolated from Phellinus gilvus in rats

“…The recovery procedure of the polysaccharides solution from the fruiting body of PG was performed by the method of Kim et al [12]. The concentration of 0.025, 0.25, and 2.5% polysaccharides solutions was determined by total sugar content with the anthrone method [7] with glucose as the standard material. These solutions, formulated as a suspension in a small amount of aqueous gel (Biosonic ® , AMITIE Co., Korea) and Madecassol ® ointment (MC), were from Dongkook Pharmaceutical Co., Korea.…”