Comparison of isohydric and anisohydric Vitis vinifera L. cultivars reveals a fine balance between hydraulic resistances, driving forces and transpiration in ripening berries

The degree to which isohydric or anisohydric behaviour extends to the water balance of developing fruits has not previously been explored. Here, we examine the water relations and hydraulic behaviour of Vitis vinifera L. berries during development from two contrasting cultivars that display isohydric (cv. Grenache) or anisohydric (cv. Shiraz) behaviour. Hydraulic resistance normalised to the berry surface area of Grenache clusters was significantly lower and more constant during development, whereas that of Shiraz increased. Lower rachis hydraulic resistance in Grenache compared with Shiraz was inversely related to xylem vessel diameter. Berry transpiration and xylem water uptake measured on detached berries decreased alike during development. From veraison, detached berries of both cultivars showed a transition to a net imbalance between xylem water uptake and transpiration, with Shiraz showing a larger imbalance and berry dehydration towards the end of ripening. In planta, this imbalance must be counterbalanced by a larger phloem water influx in post-veraison berries. Concurrently, the calculated pressure gradients for xylem water uptake showed a decline, which broadly agreed with the measured values. Higher suction for xylem water uptake in pre-veraison berries was mainly generated by transpiration. We conclude that isohydric or anisohydric behaviour is reflected in the contrasting behaviour of fruit hydraulics and that a change from xylem water uptake to phloem import is correlated with the loss of the propensity to generate negative apoplastic pressure in the berry.