Winter embolism, mechanisms of xylem hydraulic conductivity recovery and springtime growth patterns in walnut and peach trees

Abstract

Xylem vessels of Prunus persica Batsch (peach) and Juglans regia L. (walnut) are vulnerable to frost-induced embolism. In peach, xylem embolism increased progressively over the winter, reaching a maximum of 85% loss of hydraulic conductivity (PLC) in early March. Over winter, PLC in walnut approached 100%, but the degree of xylem embolism varied during the winter, reflecting the ability of walnut to generate positive xylem pressures in winter and spring. In contrast, positive xylem pressures were not observed in peach. Controlled freeze-thaw experiments showed that frost alone is insufficient to increase embolism in peach; evaporative conditions during thawing are also required. However, when both species were protected from frost, PLC was zero. At bud break, there was complete recovery from embolism in walnut, whereas PLC remained high in peach. Three mechanisms responsible for the restoration of branch hydraulic conductivity were identified in walnut: the development of stem pressure, the development of root pressure and the formation of a new ring of functional xylem, whereas only one mechanism was observed in peach (new functional ring). The climatic conditions necessary for the manifestation of these mechanisms were investigated.

Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences

“…For tree species elaborating positive xylem sap pressures in the roots during spring like Fagus sylvatica, the recovery of conductivity is partially achieved by flushing embolised vessels with pressurized sap; full recovery of the transport ability occurs usually only after the new year ring has been developed [30]. Recovery of xylem conductivity after embolism can also occur during spring due to xylem pressure following starch hydrolysis [2]. It may also happen during transpiration, as has been reported for Laurus nobilis [125], which is able to recover despite predawn leaf water potential remaining as low as -1 MPa.…”

“…Two questions will be addressed: (1) what are the factors that could contribute to stand vulnerability or resilience under extreme drought and (2) what is the degree of diversity in the responses to drought among tree species. We begin with processes involved at soil-root and canopy-atmosphere interfaces that will be illustrated with results gained from water and carbon flux measurements.…”

Annual patterns of xylem embolism in high-yield apple cultivars

“…Thus the extent to which a tree's hydraulic system is impaired during winter strongly depends on anatomical parameters of the xylem (Sperry and Sullivan 1992;Choat et al 2011), the protection against evaporative water losses (leaf shedding, cuticular and peridermal conductance (g p ) ;Larcher 2003;) and, with regard to climatic parameters, air and soil temperature, vapour pressure deficit and soil water availability. Previous studies have shown that trees in temperate regions can suffer a considerable loss of hydraulic conductance in winter (Sperry et al 1988b(Sperry et al , 1994Cochard and Tyree 1990;Sperry and Sullivan 1992;Hacke and Sauter 1996;Cochard et al 2001;Ameglio et al 2002;Mayr et al 2003;ChristensenDalsgaard and Tyree 2014). In deciduous species, though, a full water transport capacity is not required in winter and even a high level of native embolism is not particularly risky for the plant.…”

“…This changes suddenly in spring, when transpirational water demand increases in response to warmer temperatures, increased daylength and bud break (e.g. Sperry et al 1994;Ameglio et al 2002;Hao et al 2013). In spring, the functionality of the water transport system has to be restored rapidly to avoid growth limitation or desiccation-related dieback of shoots.…”

“…Restoration of the hydraulic system in spring is related to the differentiation of new xylem conduits and, in many diffuse-porous angiosperms, to the refilling of embolised ones (Sperry et al 1988b(Sperry et al , 1994Sperry and Sullivan 1992;Hacke and Sauter 1996;Utsumi et al 1998;Cochard et al 2001;Cobb et al 2007;Christensen-Dalsgaard and Tyree 2014). One possible mechanism for hydraulic recovery is the generation of positive xylem pressures in roots or stems that is sufficient to dissolve embolisms before leaf flushing (Sperry et al 1987(Sperry et al , 1994Cochard et al 2001;Ewers et al 2001;Ameglio et al 2002;Cobb et al 2007;Brodersen and McElrone 2013;Hao et al 2013). Refilling has also been observed in the absence of root or stem pressure (Tyree et al 1999;Cochard et al 2001;Salleo et al 2006;Nardini et al 2011;Brodersen and McElrone 2013;Christensen-Dalsgaard and Tyree 2014).…”