Water Transport Properties of the Grape Pedicel during Fruit Development: Insights into Xylem Anatomy and Function Using Microtomography

Xylem flow of water into fruits declines during fruit development, and the literature indicates a corresponding increase in hydraulic resistance in the pedicel. However, it is unknown how pedicel hydraulics change developmentally in relation to xylem anatomy and function. In this study on grape (Vitis vinifera), we determined pedicel hydraulic conductivity (k_h) from pressure-flow relationships using hydrostatic and osmotic forces and investigated xylem anatomy and function using fluorescent light microscopy and x-ray computed microtomography. Hydrostatic k_h (xylem pathway) was consistently 4 orders of magnitude greater than osmotic k_h (intracellular pathway), but both declined before veraison by approximately 40% and substantially over fruit development. Hydrostatic k_h declined most gradually for low (less than 0.08 MPa) pressures and for water inflow and outflow conditions. Specific k_h (per xylem area) decreased in a similar fashion to k_h despite substantial increases in xylem area. X-ray computed microtomography images provided direct evidence that losses in pedicel k_h were associated with blockages in vessel elements, whereas air embolisms were negligible. However, vessel elements were interconnected and some remained continuous postveraison, suggesting that across the grape pedicel, a xylem pathway of reduced k_h remains functional late into berry ripening.In grape (Vitis vinifera), fruit growth by water accumulation follows a double sigmoid pattern and is influenced by the diurnal and developmental changes in water flow between fruit and the parent plant (Matthews and Shackel, 2005). Until the onset of fruit ripening (i.e. veraison), water enters the fruit predominantly via the xylem and thereafter mainly through the phloem (Greenspan et al., 1994, 1996). Choat et al. (2009) showed that the hydraulic conductance (i.e. 1/resistance) of the grape berry and pedicel declines substantially at later ripening stages predominantly due to a decline in pedicel conductance. Significant developmental changes in pedicel hydraulic properties were also reported for tomato (Solanum lycopersicum) and were found to be associated with xylem anatomical changes (Lee 1989; Van Ieperen et al., 2003; Rancić et al., 2008, 2010). Due to its position along the vascular transport pathway between fruit and the parent plant, the pedicel can play an important role in affecting fruit growth, as in kiwi (Actinidia deliciosa; Mazzeo et al., 2013). However, for grape, it needs to be elucidated how pedicel hydraulic properties change developmentally in relation to xylem anatomy and function.The location and nature of the loss in hydraulic conductance between the parent plant and the fruit is unclear and may differ among fruits. For tomato, Malone and Andrews (2001) showed that most of the loss of hydraulic conductance occurs in the fruit per se, but Van Ieperen et al. (2003) reported important and decreasing hydraulic conductance in the pedicel abscission zone over fruit development. For Citrus spp., Garcia-Luis et al. (2002) reported that xylem vessels in the pedicel remain largely functional late into fruit ripening. For grape, although vessel breakage in the berry was thought to lead to xylem dysfunction (Coombe and McCarthy 2000), several studies and methods have shown that xylem vessels in the fruit remain functional (Rogiers et al., 2001; Bondada et al., 2005; Chatelet et al., 2008a, 2008b). In line with these findings, data by Keller et al. (2006) suggest that the pedicel xylem also remains at least partially functional in ripening grape berries and can conduct water to and from the parent plant. Nevertheless, a reduction in the ability to transport water during ripening has been reported for grape (Tyerman et al., 2004; Choat et al., 2009) and other fleshy fruits, such as apple (Malus domestica; Lang and Ryan, 1994) and kiwi (Mazzeo et al., 2013), and it still remains unclear what causes this loss in xylem hydraulic conductance. For the grape pedicel, Choat et al. (2009) detected higher concentrations of xylem solutes postveraison and proposed that this is related to the deposition of gels into the xylem vessel lumen. However, direct evidence for the presence of xylem blockage and/or embolism formation in the grape pedicel is missing.This study of the grape ‘Cabernet Sauvignon’ pedicel was conducted with the goal to obtain a comprehensive understanding of how changes in hydraulic properties relate to changes in xylem structure and function over fruit development. Over the course of fruit development from 20 to 90 d after anthesis (DAA), water transport properties of pedicels were investigated under osmotic and hydrostatic driving forces using a modified pressure-probe system. This was combined with analyses of spatial and temporal changes in pedicel xylem anatomy and function using fluorescent light microscopy and x-ray computed microtomography (microCT; Brodersen et al., 2010, 2013; Rancić et al., 2010).