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.