ABSTRACT
Apples, with their most commonly accepted ancestor as Malus sieversii Lebed., are believed to have originated in Central Asia from where they were disseminated to China and Europe through the ‘Silk Route’. They came as a result of domestication through interspecific hybridization. With the exception of a few triploid and tetraploid cultivars of little mercantile importance, most of the cultivars share the same chromosome number of 2n = 34. Apple breeding programs for varietal improvement started in the American state of Iowa followed by Czechoslovakia, Germany, England and Sweden. The parents used mostly included ‘Northern Spy’, ‘Cortland’, ‘Rome Beauty’, ‘Winesap’, ‘Wolf River’, etc. By the 1970s, the important varieties were all grown from ‘Delicious’, ‘Golden Delicious’, ‘McIntosh’, ‘Rome Beauty’, etc. The early years of 1990s saw the introduction of various important varieties like ‘Gala’, ‘Braeburn’, ‘Akane’ and ‘Fuji’. Nowadays, we seem to be entering into the era where buyers are more inclined toward buying better colored varieties that have caused the actual strains to become outdated. Propagation in apples is another operation which was initially performed by seeds mostly because of their abundant availability and ease of propagation. But due to their heterozygous nature, apple cuttings (to some extent) and rootstocks (largely) have been used for apple propagation for around 2000 years. Rootstocks hold a very important position in the plant systems of temperate fruits including apples. It is the machinery that works continuously to provide anchorage, conduct water and minerals and most importantly protect itself and the above ground portion against various biotic and abiotic stresses. A large number of factors in the scion are determined by the stock including the scion growth and habit, the flowering phenology, copious flowering, tendency of fruit set and yield potential. The study of the inheritance of important apple rootstock traits can be a rather difficult endeavor as some of the traits investigated are measured on a different genotype (the scion) and are collected over many years of the life of an orchard. Despite these difficulties, there have been some major advances in understanding the complexity and inheritance of inherent (belonging to the rootstock genotype) and induced (measured in a grafted scion) rootstock traits in recent years. Direct gene transfer is proving to be a very promising tool for the improvement of rootstocks in temperate fruit crops. Genetic transformation methods like transgrafting have already shown successful results while some other methods including cisgenesis, intragenesis, RNAi and genome editing can be the potential methods of rootstock improvement in the future.
