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- Author or Editor: James N. Cummins x
Rootstock influence on tree architecture may be seen in a variety of expressions. Above ground effects include canopy volume and shape, crotch angles, branch display angles, relative distribution of long shoots and spurs, internode length, relative distribution of fruit buds and spurs, and trunk taper. Below the graft union, effects include relative distribution of fine vs. coarse roots, total root mass, and numbers, nature and distribution of burrknots. Many of these phenomena are indirect effects that stare from induction of fruiting by the rootstock, e.g., early fruit production induced by the rootstock will result in reduced canopy volume, reduced aboveground total mass, flatter branch display angles, and reduced root mass. The rootstock also plays a major role in the duration of shoot extension growth; by influencing the production of growth regulators in the shoot tip, the rootstock indirectly influences the inhibit ion of lateral buds and therefore the production of feathers.
Spininess is characteristic of many Malus species, especially American crabapple and Malus baccata L. Spininess often is present on rooted stoolshoots of commercial apple rootstocks (M.9, M.26, MM.111, and MM.106) and some rootstocks from the Geneva Breeding Program. This undesirable characteristic makes liner production costly and laborious. It is estimated that the cost of manual removal of spines amounts to ≈20% to 25% of total production costs. To counteract spininess, the stoolshoots of excessive spiny rootstock selections [74R5M9-760 (T/1), 74R5M9-707 (T/2), and 75R5M9BR-521 (T/3)] were chemically treated while growing at stoolbed. Chemical treatments consisted of single sprays of nontranslocated growth regulators Tamex (a.i. butralin) or Tamex AG (a.i. butralin + fatty alcohols C8- C10), and commercial auxin formulation (Tre-Hold A-112). Tamex AG and A-112 at applied rates brought about some phytotoxicity effects while Tamex did not. On average, Tamex application (1000, 2000, and 4000 ppm) reduced spine number to ≈80%, 68%, and 84% of T/1, T/2, and T/3 control plants, respectively. However, Tamex at 4000 ppm reduced the number of spines to 57% and 60% of control T/2 and T/3 plants, respectively, without any detrimental side effects. A parallel greenhouse experiment is being performed using commercial M.26 and Geneva 30 apple rootstocks.
Isozymes of six enzyme systems extracted from 13 apple (Malus domestica Borkh.) rootstocks were separated electrophoretically on a horizontal starch gel. Each rootstock was clearly distinguished by its unique isozyme banding patterns. All the rootstocks were distinguishable using only two of the enzyme systems, phosphoglucomutase and 6-phosphogluconate dehydrogenase, both of which exhibited considerable isozyme polymorphism.
Dormancy patterns throughout the season were studied in more than 90 apple (Malus ×domestica Borkh.) cultivars and related Malus spp. The seasonal apple bud dormancy pattern resembles a normal curve: it starts to intensify soon after bud formation and reaches maximum intensity by the time of leaf fall/senescence. Genotypes were grouped into three general classes based on maximum dormancy intensity. Maximum intensity of bud dormancy measured in cold winters is inversely related to adaptation to the subtropics. Low-chilling requirement (CR) cultivars have a shallow depth of dormancy with very little alteration throughout the year. High-CR cultivars have intense bud dormancy, the first stage of which can be induced by growing these cultivars at temperatures above 20C. Genotypes differed in their rates of dormancy dissipation. The efficiency of chilling unit (CU) accumulation to break dormancy was negatively correlated with CR, which indicates the importance of factors other than CU accumulation in terminating bud dormancy in low-CR cultivars. The inherent length of bud dormancy plays a major role in determining the time of budbreak in the spring. Deviations may be related to the genotypic efficiency in which chilling modifies dormancy and possibly the basal temperatures to which buds respond. Chill unit requirement and heat unit requirement are dependent factors. Heat requirement comparisons may be meaningless if the dormancy intensities of the genotypes are not taken into consideration.
Apple seedlings have a shallow dormancy, as has been observed in many other species. The length of bud dormancy in high-chilling-requirement seedlings does not reflect their genetic constitution well if dormancy is induced before they are 200 days old. Seedling populations sprayed with paclobutrazol and/or ethephon displayed bud dormancy periods resembling those of older populations of similar genetic constitution. Terminal bud formation and dormancy could not be induced by continuously exposing apple seedlings to low temperature (8 ± 1C) and short photoperiod, even after extended periods. Stomate operation may not be completely functional under these conditions. Terminal bud formation was induced by holding apple seedlings above 20C. Additional exposure to low temperature (8 ± 1C) induced leaf fall. These findings suggest the existence of an active regulatory factor that induces terminal bud formation and dormancy and is either turned on or synthesized above 15 to 17C. Chemical names used: β- [(4-chlorophenyl)methyl]- α -(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol(paclobutrazol);(2-chloroethyl)phosphoric acid (ethephon).
The chilling requirements (CR) to break bud dormancy in a broad range of apple cultivars (Malus ×domestica Borkh.) and related Malus spp. were assessed by periodic sampling and forcing of field-grown shoots as a function of chill unit (CU) accumulation and/or by the total growing degree hours (GDH) accumulated from leaf fall until the time of budbreak under a simulated subtropical winter. The mean number of CU required to break dormancy of field overwintered shoots varied between 218 ± 113 for `Anna' and 1516 ± 113 for `Wright #1'. However, most genotypes had CR between 800 and 1200 CU. Much wider variation for the length of bud dormancy was observed in plants growing under simulated subtropic winter conditions. Genotypes that had shown the lowest CR values under Geneva, N.Y., winters generally had the highest year-to-year variation in CR estimates. Cultivar bud CR values obtained under cold winters are related to field-observed CR estimates in a subtropical environment, but absolute values may differ markedly. Furthermore, several genotypes that show reasonable adaptation to the subtropics have similar or higher CR than apple cultivars with standard CR under Geneva conditions. In addition, enough CU accumulated under the simulated subtropic winters to break dormancy of standard apple cultivars. However, complete dormancy removal was observed only in cultivars well-adapted to a subtropical environment. This result indicates that in addition to CU accumulation, there are important interactions among cultivars and environmental factors that are responsible for terminating bud dormancy. Several cultivars and wild species have shown resistance to delayed foliation. Among the species, M. brevipes, M. rockii, M. spectabilis, and M. turesii are more tolerant than M. baccata and its hybrids, which are recognized for their adaptation to the subtropic environment.
In a study of chilling requirement in Malus, broad-sense heritability estimates for the length of vegetative bud dormancy in 43 clones growing under simulated subtropical winter conditions were 0.76 ± 0.04 in 1986 and 0.81 ± 0.04 in 1987. Narrow-sense heritability estimates were 0.66 ± 0.13 in 1986 and 0.69 ± 0.13 in 1987. Seedlings with low chilling requirements (CR) were not observed in crosses where both parents had high bud-chilling requirements. `Koningszuur' did not transmit its long CR to its seedlings. Open-pollinated (OP) seedling populations from the Malus × domestics Borkh. cultivars Anna, Dorsett Golden, Ein Shemer, Khashabi, Winter Banana, and Zabaoani, and the species and interspecific hybrids M. baccata L. DE#98, M. brevipes Rehd., M. ×robusta (Carr.) Rehd. DE#485, M. × robusta No. 5 (`R5'), M. rockii, M. turesi Rehd. PI 34143, and `Rosedale' had at least 5% of their descendants in the lower CR classes. In all but one instance, 50% or more of `Anna' descendants had low CR. Many of these seedlings were within a few classes of the extreme low CR. It is postulated that the low-CR character present in `Anna' is controlled by at least one major dominant gene and that minor genes interact to modulate its effects. Very low-CR cultivars have a shallow bud dormancy. This highly heritable component for low bud CR is related to a failure to develop a deep dormancy state, rather than to acceleration of the termination of the dormancy process.