The distribution of xylem hydraulic resistance in the fruiting truss of tomato

The distribution of hydraulic resistances in xylem throughout the pathway l eading to the tomato fruit was investigated. Previous work had indicated th at there were large resistances within the supporting sections of this path way (the peduncle and pedicel), perhaps associated with interruptions in th e xylem. These high resistances are believed to impede calcium flux into th e fruit and thus impair fruit development. It is shown here that fruit on i ntact plants do not shrink detectably during drought, even when the drought is sufficient to cause marked shrinkage of leaves and visible wilting of t he shoot. In explants, it is possible to induce back-flow from the fruit in to the stem (probably via the xylem) but this flow is small and very slow. These observations support the view that there is a large hydraulic resista nce in the pathway between fruit and stem, When pulses of water were made a vailable within explants, by scorching of one leaflet, there was a rapid sw elling of leaves and sepals. Such rapid fluxes indicate the presence of str ong hydraulic (xylem) connections throughout the pathway between leaf and c alyx. This shows that there are no significant hydraulic constrictions in t he xylem proximal to the calyx. This finding is contrary to some previous c onclusions but it is supported by experiments with dyes which showed contin uous, functional xylem throughout the peduncle and pedicel, Calculations sh ow that over 90% of the hydraulic resistance between stem and fruit must re side within the fruit pericarp. Implications for calcium nutrition are disc ussed.