The P-V curve has been used to estimate symplastic water volume, ψS, pressure potential, turgor loss point, and elastic modulus of plant cells, tissue, and organs (Holbrook and Sinclair, 1992; Richter, 1978; Scholander et al., 1965; Tyree and Hammel, 1972). It is considered to be genetically determined (Lenz et al., 2006) and reflects drought-response properties of plant species (Dichio et al., 2003; Joly and Zaerr, 1987; Saito and Terashima, 2004). From analysis of the P-V curve, a Höfler diagram can be derived to display the components of water potential as a function of relative water content of living plant materials (Joly and Zaerr, 1987; Nobel, 2005; Richter, 1978). Traditionally, a pressure chamber has been used to obtain the P-V curve and Höfler diagram of leaves (Dichio et al., 2003; Jones and Higgs, 1979; Ruíz-Sánchez et al., 1993; Saito and Terashima, 2004) and stems (Joly and Zaerr, 1987; Tyree and Hammel, 1972). However, little information is available regarding the P-V curve and Höfler diagram of fruit because of the difficulty in measuring fruit water relation parameters by the pressure chamber technique (Gelly et al., 2004).
It is generally accepted that fruit water status is closely related to physiological processes of fruit growth and development and plays an important role in fruit quality and responses to environmental stresses (Behboudian et al., 1994; Mills et al., 1996, 1997; Mpelasoka et al., 2001; Steudle and Wieneke, 1985; Wada et al., 2008; Yamada et al., 2005). Nevertheless, monitoring fruit water status remains a challenge using a pressure chamber (Gelly et al., 2004) or pressure probe (Steudle and Wieneke, 1985). The psychrometer technique used to measure fruit water parameters is time-consuming to reach equilibrium and prone to produce errors (Behboudian et al., 1994; Gelly et al., 2004).
Rotational motion by centrifugation creates known negative tensions, which are able to pull water out of the centrifuged sample. Thus, a centrifuge method has been used to determine water retention properties of soils (Reatto et al., 2008) and xylem water potential in plants (Holbrook et al., 1995). In this study, we tried to apply a series of centrifugal force to apple cortex samples to obtain a P-V curve of cortex tissue by calculating rotation-created water potential corresponding to the measured volume of extracted water. Determining the water relation parameters of fruit by the centrifuge method may lead to further understanding of the mechanism by which fruit grow and develop in relation to water status.
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