Control of scion vigour by kiwifruit rootstocks is correlated with spring root pressure phenology

M. J. Clearwater 0 P. Blattmann 0 Z. Luo 0 R. G. Lowe 0 0 Horticulture and Food Research Institute of New Zealand, Te Puke Research Centre , RD 2 Te Puke, New Zealand 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 lowvigour 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 20.07 MPa to 20.15 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. Introduction Rootstocks are used for the control of vigour and fruit quality in many horticultural crops (Webster, 1995). Despite their economic importance, the mechanism for scion vigour control by rootstocks is poorly understood. Previously, the effects of experimental kiwifruit (Actinidia species) rootstocks on scion shoot growth (Clearwater et al., 2006) and vine hydraulic architecture (Clearwater et al., 2004) have been described. Rootstock vigour was defined according to the leaf area index produced by the scion, which was itself a function of the proportion of shoots that grew rapidly without self terminating (Clearwater et al., 2006). Rootstocks that reduced scion vigour were found to exert an effect on shoot growth during the earliest period of shoot development in spring, starting immediately after budburst. Low-vigour rootstocks reduced shoot growth and increased the proportion of shoots that terminated (abortion of the shoot apex) and stopped growth early in the season (Clearwater et al., 2006). Vine hydraulic conductance and canopy physiology were measured later in the season and did not provide a direct explanation for the effects of rootstock on growth (Clearwater et al., 2004). In this study, the properties of the roots and their effects on shoot physiology during the important spring period when the first cluster of leaves emerged from the bud were measured. The development of root pressure, defined as positive xylem pressure that arises in the roots (Kramer and Boyer, 1995), is a noticeable feature of kiwifruit vines in spring (Davison, 1990). Root pressure usually rises before budburst and results in copious bleeding (exudation) from the xylem if pruning cuts are made during this time. Exudation from a cut stem can occur for many days with no apparent negative effects for the vine. Spring root pressure has often been used to collect sap for compositional analysis and the diagnosis of nutritional disorders (Ferguson et al., 1983; Clark et al., 1986). Wang et al. (1994a, b) proposed that the capacity for root pressure or water supply in spring might be associated with Actinidia rootstock effects on levels of floral abortion by the scion. Although it is not widely recognized, root pressure occurs in kiwifruit throughout the summer (not only before budburst), but only develops at night during humid weather when night-time transpiration is prevented (kiwifruit stomata do not close completely at night, resulting in significant night-time transpiration if the humidity does not rise to saturation; Green et al., 1989). Observations of root pressure have been made for centuries in a wide variety of other plant species, with the most parsimonious explanation that it is an osmotic process resulting from the accumulation of apoplastic solutes in the xylem of the roots (Kramer and Boyer, 1995). Preliminary observations of exudation and root pressure for this study suggested that there were differences in the timing and magnitude of root pressure between the rootstocks. A method for reliably monitoring xylem pressure was therefore developed, and root pressure and shoot water relations monitored over the 2-month period between budburst and flowering. These observations relate to our earlier studies of shoot growth and vine hydraulic conductance, and their significance for rootstock effects in other perennial fruit crops is discussed. Materials and methods Plant material The rootstock trial used for this study has been described previously (Clearwater et al., 2004, 2006). Briefly, shoots from eight rootstock clones encompassing a range of Actinidia species and growth forms were taken in 1995 and rooted in a nursery. A. hemsleyana Dunn Kaimai (formerly known as TR2) is a registered rootstock cultivar known to promote flowering and vigour in green kiwifruit (Wang et al., 1994b). A. deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson var. deliciosa Hayward is the common green kiwifruit, grown internationally. The other six clones, of unknown potential as rootstocks, were selected from five species held in the Actinidia germplasm collection at the HortResearch Te Puke Research Orchard, New Zealand. The species were A. eriantha Benth., A. macrosperma C.F. Liang, A. chrysantha Merr., A. polygama (Sieb. et Zucc.) Maxim., and A. kolomikta (Maxim. et Rupr.) Maxim. Hereafter each clonal selection will be referred to by its species nam (...truncated)