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).