Fruit is hydraulically prioritised through cavitation resistance.
The advantages of prioritising the water requirements of reproduction
over vegetative tissues observed here in tomato become particularly
apparent when considering the ancestry and evolution of this species.
Tomato belongs to the Solanum section Lycopersicon, in the Solanaceae
that includes the cultivated tomato and 12 wild relatives (Bergougnoux,
2014; Peralta, Knapp, & Spooner, 2007). While the specifics of tomato
domestication are uncertain, the wild species originate in Western South
America, growing from Northern Chile to Ecuador and the Galapagos
Islands, although domestication may have occurred in Mesoamerica
(Bergougnoux, 2014; Blanca et al., 2012). These ancestral Lycopersicon
occur in dry desert and pre-desert environments, including the Atacama
Desert of Northern Chile where rainfall is highly sporadic, making the
prioritisation of reproduction even under water stress a necessity for
species persistence in the landscape (Bergougnoux, 2014; Chetelat,
Pertuzé, Faúndez, Graham, & Jones, 2009; Fischer, Camus-Kulandaivelu,
Allal, & Stephan, 2011; Knapp & Peralta, 2016). In the case of annual
plants, it appears advantageous to ensure all available water is
directed to reproduction, safeguarding the potential for future
offspring above the survival of the parent plant. In an opportunistic
annual species such as tomato where reproduction often takes place under
conditions of ephemeral rain, there is good reason to expect that
prioritization of reproduction would enhance fitness, as the need to set
seed is paramount. We hypothesised that selection would favour
individuals where xylem supply to fruiting tissue (in the stem and
peduncle) was more resistant to cavitation than other organs, ensuring a
water supply to fruit for as long as possible in drought conditions. The
reasoning behind this prediction follows the hydraulic vulnerability
segmentation hypothesis which considers the energetic or fitness cost of
losing tissues to cavitation (Johnson et al., 2016; Melvin T. Tyree &
Ewers, 1991; Zimmermann, 2013). Under the likely scenario that water
becomes limited during reproduction (e.g. when growth was initiated
after an ephemeral rain storm), the plant would be able to ensure
reproductive completion, even if it required water to be extracted from
the rest of the vegetative plant body.