Comparative Vegetative Growth Responses of Two Peach Cultivars Grown on Size-controlling versus Standard Rootstocks

Vegetative growth of two peach (Prunus persica L. Batsch) cultivars Flavorcrest and Loadel growing on six different rootstocks (‘Nemaguardʼ, ‘Hiawathaʼ, K-146-43, K-146-44, P-30-135, and K-119-50) was analyzed during the third season of growth in an experimental orchard at the University of California Kearney Agricultural Center near Parlier, California. Seasonal trunk cross-sectional area, shoot and internode growth, diurnal stem extension growth rate and summer and dormant pruning weights were measured to determine extent of size-control imparted by the experimental rootstocks compared to the trees on the ‘Nemaguard ̓control and to characterize the nature of the sizecontrolling response. Trunk cross-sectional area growth of trees on the two smallest rootstocks (K-146-43 and K-146-44) was only 25% to 37% of the growth of trees on ‘Nemaguardʼ, while trees on the other three rootstocks provided an intermediate level of size control. Generally, the seasonal patterns of shoot growth did not vary substantially among trees on the different rootstocks, but average shoot and internode lengths did correspond with tree size. Vigorous watersprout growth was decreased by more than 80% in the trees on the least vigorous rootstocks compared to trees on ‘Nemaguard ̓resulting in major reductions in the extent of summer and winter pruning weights. Variations in vegetative shoot growth appeared to correspond to variations in daily shoot extension growth rates but more research is needed to explore these relationships. region near the scion, may reduce the amount of water, growth regulators and nutrients transported to the scion, and has also been suggested to cause a reduction in size or vigor (Childers, 1983; Glenn and Scorza, 1992; Jones, 1984; Soumelidout et al., 1994; Ussahatanonta and Simons, 1988; Zimmermann, 1983). Olien and Lakso (1984) studied water relations of ‘Empire ̓apple on fi ve rootstocks and suggested that differences in mean midday stem potential were due to high resistances to fl ow at the graft union or to high root hydraulic resistance. Rootstocks can markedly affect the vegetative growth of the scions. However, the vegetative growth can be defi ned by several parameters. Trunk cross-sectional area, number of terminal and lateral shoots, total shoot length, and internode length, are the most conspicuous. Trunk cross-sectional area (TCA) is correlated to total above-ground mass of apple trees (Forshey and Elfving, 1989; Heinicke, 1922; Khatamian and Hilton, 1977; Murray, 1927; Tustin et al., 1997; Westwood and Roberts, 1970) and it is commonly used to compare differences in growth among fruit trees on different rootstocks. Shoot growth has a large infl uence on the fruitfulness of trees and the amount of vegetative growth removed in summer and dormant pruning (Childers, 1973; Westwood, 1978). In addition, rootstocks can also affect the seasonal pattern and time of cessation of shoot growth (Barlow, 1964; Forshey and Elfving, 1989; Maggs, 1957; Preston, 1958, 1968; 1957; Swarbrick, 1929; Vyvyan, 1935). There have been relatively few studies on seasonal vegetative growth patterns of peach scion varieties (DeJong et al., 1987) and fewer related to peach rootstock effects. Internode length may also be affected by dwarfi ng rootstocks. Peach trees grafted on dwarfi ng rootstocks had shorter internodes compared to trees on the more vigorous rootstocks (Murase et al., 1990). In addition, dwarfi ng apple rootstocks appear to have wider crotch angles with the uprightgrowing ‘Delicious ̓variety (Warner,1991). Received for publication 28 May 2002. Accepted for publication 1 Apr. 2003. 1Corresponding author. A grafted fruit tree is a combination of two different genotypes, the scion and the rootstock. The interaction between the two genotypes is very complex. The transported nutrients and other compounds crossing the bud union infl uence the scion and rootstock and produce changes that affect tree size, fertility, longevity, pathogen resistance, fruit quality, yield, etc. During the past few decades, apple researchers and fruit growers introduced spur scion varieties and dwarfi ng rootstocks to control tree size and reduce hand labor costs (Webster, 2001). The increasing density of the orchard plantings to obtain high yields and the accompanying high costs of the hand operations emphasized the necessity of size-controlling rootstocks (DeJong et al., 1999). Unlike apples, there are no widely acceptable size-controlling rootstocks for peaches. The need for such rootstocks to improve the management effi ciency of high-density peach orchards induced the University of California Pomology Department to develop a rootstock evaluation program from 1986 to 1994 (DeJong et al., 1997). In this project, rootstocks were selected on the basis of rooting capacity, size controlling performance and compatibility with peach. The resulting selections were the basis of the present study. There are several theories about the dwarfi ng mechanisms of size controlling rootstocks (Crabbé, 1984; Lockard et al., 1982; Rom and Carlson, 1987), but none of these theories has been defi nitively proven. Theories on the dwarfi ng effect have come from various research approaches, and hormones appear to be related to the dwarfi ng mechanism in some way (Crabbé, 1984; Kamboj et al., 1997; Richards et al., 1986). Other theories involve bark tissues, which could be the key for explaining the dwarfi ng mechanism in apples (Lockard et al., 1982; McKenzie, 1961; Simons and Chu, 1984). The graft union, especially the