The past, present and future of walnut genetic improvement and propagation

Climate change and global warming have caused special attention to genetic improvement of fruit trees such as walnut. Lateral bearing, high nut weight and kernel percentage, thin sell thickness, light kernel color were the main walnut breeding objectives to achieve high quality and production. In addition to these traits, walnut breeders are now looking for late leafing, earlier harvest, disease resistance, drought tolerance and low chilling requirement cultivars and rootstocks. Walnut breeding started by selection of superior genotypes in its origin centers (Persia). The first walnut breeding program was started in France by selection superior genotypes which let to release 'Franquette', 'Mayette', 'Crone', 'Grandjean' and 'Parisienne'. After that, the walnut breeding program of the United States was started by introducing French cultivars and continued with targeted hybridization. Now, selection, hybridization and application of biotechnology are the most common walnut breeding strategy in the United States as well as other countries such as France, China, Iran, Spain, Italy, etc. 'Vina', 'Pedro', 'Chico', 'Serr', 'Chandler', 'Howard', 'Hartley', 'Sunland', 'Chandler' (as standard cultivar in the world), 'Tulare', 'Robert Livermore', 'Sexton', 'Gillet', 'Forde', 'Ivanhoe', 'Solani' and 'Durham' are some of the commercial cultivars obtained from the walnut breeding program in the United States. According to the long period of the walnut breeding cycle, walnut breeders have been able to bypass the long breeding cycle by using biotechnology alongside classic breeding programs. In addition, selection of superior genes and genotypes are most useful to reduce the breeding cycle. Morphological evaluations of walnut germplasm in its origin and diversity centers, long-term hybridization between promising genotypes and identify superior genotypes are some of the most important components of traditional walnut breeding programs. The development of new breeding techniques relies on molecular biology helped us to more easily evaluated genetic diversity and obtain targeted hybrids. In addition, walnut genome sequencing in recent years has greatly accelerate the world's walnut genetic improvement. In addition to walnut genetic improvement, walnut propagation techniques are also quickly developed and traditional walnut propagation technique (by seed) are obsolete. Although, walnut can be propagated by cuttings and layering, but the commercial methods of walnut propagation are grafting, budding and micropropagation. Whip and bark grafting as well as patch, I and T-budding are the most widely budding and grafting methods in walnut propagation.