Fruit water volume is supplied by xylem.
This idea of reproductive prioritisation depends on the assumption that the water reaching fruit is supplied by xylem and thus exposed to the tension and cavitation risk experienced by the broader plant in drought. The proportion of water proposed to be supplied to the tomato fruit by the xylem ranges from 10 to 90% (L. C. Ho et al., 1987; Windt et al., 2009). However, we found that phloem girdled fruit continued growing at the same rate as ungirdled fruit under water stress indicating that in this species fruit water is predominantly supplied by the xylem. This result is supported by previous studies in tomato (Van De Wal et al., 2017; Windt et al., 2009). By demonstrating that the xylem is the main source of water supplying reproductive tissues even during periods of water stress, these data emphasize the special relevance of the cavitation resistance of xylem in the peduncle for understanding fruit growth and survival in water stress. In this case, identifying xylem cavitation characteristics in reproductive tissues is highly relevant to understanding the ecology and reproductive behaviour of plant species.
We anticipated that the capacitance from shrinking tissue or early cavitation of vulnerable organs would make water stored in these tissues available to more resistant tissues, provided a xylem connection remains intact between them (Bourbia et al., 2020; Cochard & Tyree, 1990; Hölttä et al., 2009; Johnson et al., 2016). Given the relative vulnerability of leaves compared with reproductive tissues, we argue that the collapse and early cavitation of non-prioritised leaves liberates water from these tissues, while a strong xylem connection remains intact under powerful dehydration stress allowing an intact passage of water from the vulnerable leaves to the hydraulically prioritised fruit (Hölttä et al., 2009). Organ hydraulic architecture may play a role in facilitating the variation in xylem vulnerability between reproductive and vegetative organs. Here, the small vessels of peduncles appeared more resistant to cavitation, while larger petiole vessels support delivery of the larger leaf water requirements for photosynthesis but cavitate more easily under drought stress. This observation supports literature indicating a relationship between vessel size and cavitation vulnerability (Cochard & Tyree, 1990; Gauthey et al., 2020; Hacke, Sperry, Pockman, Davis, & McCulloh, 2001; Jacobsen, Ewers, Pratt, Paddock, & Davis, 2005; Lens et al., 2011; Petit et al., 2009; Scoffoni et al., 2017).