where d is vessel diameter (m), p is the fluid density, η is the viscosity of water (1.002 × 10–9 MPa at 20°C) (Tyree & Ewers, 1991; Bourbia et al., 2020). This metric was used to compare organ total theoretical capacity to conduct xylem water per unit pressure gradient.
Statistical analysis
The cavitation resistance of the leaf lamina, petiole, petal, and peduncle (all organs were measured on each intact individual) was compared using two-way ANOVA to test the effect of organ type and individual on P50 values in R. A log transformation was applied to fulfil assumptions of homoscedasticity and normality. Post hoc tests were computed using Tukey’s HSD.
The effect of disabling phloem water delivery by heat girdling on fruit expansion was tested using a two-way repeat measures ANOVA. This model compared differences in growth between treatments at decreasing water potentials. The interaction term between treatment and water potential was determined to be non-significant and was removed from the model. Assumptions were tested for normality and no transformations were necessary. A qq plot could not be fitted given the small data set, however, the consistency of results indicated that model assumptions could be met.
The cavitation resistance (quantified as the P12, P50 and P88) of peduncles and stems (stems measured on independent individuals) was compared using a student’s T-test (unpaired samples of equal variance). The mean diameter of the largest numerically ranked 10% of vessels in peduncles and petioles, and the mean theoretical hydraulic conductivity of these organs, were also assessed using a student’s T-test (paired samples of equal variance).
Results:
Shrinkage and growth under water stress
A common pattern of growth and shrinkage was observed amongst individuals as a severe water stress was imposed (fig. 1). As whole plants dehydrated from 0 to -3MPa, peduncles shrunk by a mean 20% (±4.99% SE) of their original hydrated diameter, while petioles shrunk by a mean of 46% (±3.39%SE) (fig. 2). At the same time, fruits continued to grow during plant dehydration, showing a mean increase of 26% (±4.17%SE) above their original diameter. This growth stopped immediately after the vascular connection with the stem was manually severed when water potential reached -1MPa (fig. 2).