Whole-plant hydraulic conductance, shoot growth, and leaf photosynthetic properties were measured on kiwifruit vines with four clonal rootstocks to examine the relationship between plant hydraulic conductance and leaf stomatal conductance (gs) and to test the hypothesis that reduced hydraulic conductance can provide an explanation for reductions in plant vigour caused by rootstocks. The rootstocks were selected from four species of Actinidia and grafted with Actinidia chinensis var. chinensis 'Hort16A' (yellow kiwifruit) as the scion. Total leaf area of the scion on the least vigorous Actinidia rootstock, A. kolomikta, was 25% of the most vigorous, A. hemsleyana. Based on shoot growth and leaf area, the selections of A. kolomikta and A. polygama are low-vigour rootstocks, and A. macrosperma and A. hemsleyana are high-vigour rootstocks for A. chinensis. Whole-plant hydraulic conductance, the ratio of xylem sap flux to xylem water potential, was lower in the low-vigour rootstocks, reflecting their smaller size. However, leaf-area-specific conductance (Kl) and gs were both higher in the low-vigour rootstocks, the opposite of the expected pattern. Differences in Kl were found in the compartment from the roots to the scion stem, with no difference between rootstocks in the conductance of stems or leaves of the scion. There was no evidence that the graft union caused a significant reduction in hydraulic conductance of vines with low-vigour rootstocks. Leaf photosynthetic capacity did not vary between rootstocks, but photosynthesis and carbon isotope discrimination (Delta13C) under ambient conditions were higher in the low-vigour rootstocks because gs was higher. gs and Delta13C were positively correlated with Kl, although the mechanism for this relationship was not based on stomatal regulation of a similar xylem water potential because water potential varied between rootstocks. For Actinidia rootstocks, changes in Kl do not provide a direct explanation for changes in vigour of the scion. However, depending on the rootstock in question, changes in hydraulic conductance, biomass partitioning, and crown structure are involved in the response.

Indirect evidence suggests that water supply to fleshy fruits during the final stages of development occurs through the phloem, with the xylem providing little water, or acting as a pathway for water loss back to the plant. This inference was tested by examining the water balance and vascular functioning of ripening kiwifruit berries (Actinidia chinensis var. chinensis ‘Hort16A’) exhibiting a pre-harvest ‘shrivel’ disorder in California, and normal development in New Zealand. Dye labelling and mass balance experiments indicated that the xylem and phloem were both functional and contributed approximately equally to the fruit water supply during this stage of development. The modelled fruit water balance was dominated by transpiration, with net water loss under high vapour pressure deficit (Da) conditions in California, but a net gain under cooler New Zealand conditions. Direct measurement of pedicel sap flow under controlled conditions confirmed inward flows in both the phloem and xylem under conditions of both low and high Da. Phloem flows were required for growth, with gradual recovery after a step increase in Da. Xylem flows alone were unable to support growth, but did supply transpiration and were responsive to Da-induced pressure fluctuations. The results suggest that the shrivel disorder was a consequence of a high fruit transpiration rate, and that the perception of complete loss or reversal of inward xylem flows in ripening fruits should be re-examined.

Root pressure was measured continuously over spring in eight clonal kiwifruit rootstocks selected from seven Actinidia species (A. chrysantha, A. deliciosa, A. eriantha, A. hemsleyana, A. kolomikta, A. macrosperma, A. polygama), using pressure transducers and miniature compression fittings. Rootstocks that promoted scion vigour developed root pressures up to 0.15 MPa before or during scion budburst, whereas those that reduced scion vigour developed root pressure up to 0.05 MPa only after scion shoot expansion. When several seasons were compared, the date of onset of root pressure and the magnitude of pressure achieved were consistent for each rootstock. Root pressure was first recorded between late July and early September in vigour-promoting rootstocks, while scion budburst and initial shoot growth were in late August and early September. Vigour-reducing rootstocks did not develop significant root pressure until October. The date of onset was similar for the grafted rootstock and ungrafted plant of the same clone, but was not clearly related to the timing of shoot growth by the ungrafted plant. In the grafted plants the leaf and xylem water potentials of the scion were more negative, midday turgor was 0.3-0.5 MPa lower, and wilting was sometimes observed in developing shoots growing on low-vigour rootstocks, indicating that water stress was contributing to reductions in growth. Leaf turgor was correlated with average root pressure but not pressure measured during the day, suggesting that root pressure was not supporting transpiration during peak flows and was, instead, indicative of higher root hydraulic conductance. The rapid temporal rise in root pressure observed each spring in the various rootstocks was not accompanied by changes in xylem sap solute potential, but when rootstock clones were compared those that developed higher root pressures had higher sap solute potentials. Xylem sap solute potential varied between rootstocks from -0.07 MPa to -0.15 MPa, while root pressures measured at the same time varied between 0.0 MPa and 0.09 MPa, suggesting that an osmotic mechanism could account for the observed root pressure. Differences in phenology between the rootstocks and scion appeared to account for the rootstock effects on shoot growth, and changes in root pressure provided a useful indication of seasonal changes in root hydraulic properties and solute transport behaviour.

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Patterns of shoot development and the production of different types of shoots were compared with scion leaf area index (LAI) to identify how eight clonal Actinidia rootstocks influence scion development. Rootstocks selected from seven Actinidia species (A. chrysantha Merri., A. deliciosa (A. Chev.) C. F. Liang et A.R. Ferguson, A. eriantha Benth., A. hemsleyana Dunn, A. kolomikta (Maxim. et Rupr.) Maxim., A. kolomikta C.F. Liang and A. polygama (Sieb. et Zucc.) Maxim.) were grafted with the scion Actinidia chinensis Planch. var. chinensis 'Hort16A' (yellow kiwifruit). Based on an earlier architectural analysis of A. chinensis, axillary shoot types produced by the scion were classified as short, medium or long. Short and medium shoots produced a restricted number of preformed leaves before the shoot apex ceased growth and aborted, resulting in a 'terminated' shoot. The apex of long shoots continued growth and produced more nodes throughout the growing seasons. Mid-season LAI of the scion was related to the proportion of shoots that ceased growth early in the season. Scions on low-vigor rootstocks had 50% or less leaf area than scions on the most vigorous rootstocks and had a higher proportion of short and medium shoots. On low-vigor rootstocks, a higher proportion of short shoots was retained during pruning to form the parent structure of the following year. Short parent shoots produced a higher proportion of short daughter shoots than long parent shoots, thus reinforcing the effect of the low-vigor rootstocks. However, overall effects of rootstock on shoot development were consistent regardless of parent shoot type and nodal position within the parent shoot. Slower-growing shoots were more likely to terminate and scions on low-vigor rootstocks produced a higher proportion of slow-growing shoots. Shoot termination also occurred earlier on low-vigor rootstocks. The slower growth of terminating shoots was detectable from about 20 days after bud burst. Removal of a proportion of shoots at the end of bud burst increased the growth rate and decreased the frequency of termination of the remaining shoots on all rootstocks, indicating that the fate of a shoot was linked to competitive interactions among shoots during initial growth immediately after bud burst. Rootstock influenced the process of shoot termination independently of its effect on final leaf size. Scions on low-vigor rootstocks had a higher proportion of short shoots and short shoots on all rootstocks had smaller final leaf sizes at equivalent nodes than medium or long shoots. Only later in the development of long shoots was final leaf size directly related to rootstock, with smaller leaves on low-vigor rootstocks. Thus, the most important effect of these Actinidia rootstocks on scion development occurred during the initial period of shoot growth immediately after bud burst.