Differential Growth Responses of Apple Species to Chilling and Root Pruning

Malus dometica Borkh. , M. anis, M. prunifolia Borkh. , M. × robusta Rehd. , M. antonovka, M. borwinkw, and M. ranetka bare-root seedlings were chilled at 5C for 0, 400, 800, 1200, or 1600 hours. After chilling treatments, one-half of the seedlings were root-pruned and all seedlings were placed in a greenhouse for 15 days. Quantitative differences between species in the timing and magnitude of new root and shoot growth responses to chilling were observed. Root pruning decreased and delayed the production of roots 0.6 mm in diameter) root growth criteria, interactions between chilling hours and species were apparent. Chilling requirements and growing degree hour requirements for vegetative budbreak of each species were estimated. Recent studies indicated an enhancement of root regeneration potential in M. domestica following exposure to chilling temperatures (Young and Werner, 1984b, 1985b, 1986). Similar enhancements of root growth following artificial chilling or exposure to winter conditions have been reported for other species (DeWald et al., 1985; Farmer, 1975; Lathrop and Mecklenburg, 1971; Stone and Norberg, 1979; Webb, 1976, 1977). For M. domestics (Young and Werner, 1985b) and Taxus cuspidata Sieb. and Zucc. (Lathrop and Mecklenburg, 1971), this enhancement is thought to be perceived by the root system. However, quantitative data on chilling enhancement of M. domestics root growth are lacking. Differences among species in root regeneration capacity following exposure to various levels of chilling have been reported (Webb, 1977). Differences in root regeneration capacity in response to exposure to chilling temperatures have also been reported between commercial apple rootstock (Young and Werner, 1984b, 1985b) and between half-sib families of Pinus taeda L. (DeWald et al., 1985). Such differences in root and shoot growth responses to chilling might be useful in apple cultivar and rootstock breeding. While root pruning has often been shown to reduce subsequent new shoot growth (Arnold and Struve, 1989a, 1989b; Geisler and Ferree, 1984) and sometimes root regeneration capacity (Arnold and Struve, 1989b), we found no information on the interaction of root pruning and chilling on root growth or on the effect of root size on the chilling response. The objectives of this experiment were to: 1) quantify the effect of chilling on root and shoot growth, 2) determine if differences in root regeneration potential exist between species of Malus, 3) determine if root pruning alters root and shoot growth responses to chilling, and 4) estimate the chilling requirements and growing degree hour requirements for vegetative budbreak of the species included in this study. d for publication 31 Mar. 1989. Paper no. 12122 of the Journal Series orth Carolina Agricultural Research Service, Raleigh, NC 27695-7601. dy will be included as part of a dissertation to be written in partial nt of the requirements for the PhD degree by M.A.A. The use of trade n this publication does not imply endorsement by the NCARS of the named, nor criticism of similar ones not mentioned. The cost of pubhis paper was defrayed in part by the payment of page charges. Under gulations, this paper therefore must be hereby marked advertisement indicate this fact. te Research Assistant. or. Materials and Methods One-year-old seedlings of M. domestics, M. anis, M. antonovka, M. prunifolia, M. x robusta, M. borwinka, and M. ranetka were lifted bare-root in early November from the nursery (Lawyer’s Nursery, Plains, Mt.) after receiving 0.6 mm in diameter (large roots), and number of roots <0.6 mm in diameter (small roots). Diameters of a random sample of large roots per plant were measured under a dissecting microscope. New root : new shoot ratios, percent budbreak (number of buds J. Amer. Soc. Hort. Sci. 115(2):196-202. 1990. Table 1. Least squares means significance levels for hours of chilling at SC (C), species (S), root pruning (P), and interactions for various growth characteristics of apple seedlings.