Shoot growth and bud topology in 'Chandler' walnut

In walnut (Juglans regia L.), the distribution and the type of bud complex are important for the application of disbudding techniques aiming to shoot growth manipulation in the different training systems. To ascertain differences in bud topology related to the propagation technique, a study was carried on 1-year-old 'Chandler' trees, micropropagated (M) and grafted on walnut seedlings (G), in two orchards located in Emilia Romagna region (45°North), Italy. Bud complexes were classified as follows: 0 - blind node; 1 - secondary bud alone; 2 - primary bud alone; 3 - primary and secondary bud very close each other; 4 - primary and secondary bud well distanced each other; 5 - very short sylleptic shoot without secondary bud; 6 - very short sylleptic shoot with secondary bud; 7 - extended sylleptic shoot. The number, the position along the axis and the length of shoots developed in the 2017 growing season were also measured. The results evidenced that on average, micropropagated plants had a similar bud distribution along the shoot in comparison with grafted trees in the different experimental fields. Moreover, the internode length varied along the shoots with different fate of the buds, apparently according to shoot growth rate. The sylleptic growth was closely related with a longer internode while single primary buds were detected where the internode was shorter, in not dependent way. The bud topology was described also in 8-year-old micropropagated 'Chandler' trees, samples of 4 different branching structures were selected in mid-October 2017: 4-year-old branch, 2-year-old branch, 1-year-old shoot, sylleptic shoot. Shoots were classified according to their branch age and then the number of nodes, branching and length were measured to determine branch architecture and age. All the buds were excised and dissected, and their apices were examined under stereomicroscope. The fate (vegetative bud, female floral bud, male inflorescence, lateral shoot, blind node) of each axillary structure was evaluated according to the position along the axis. The bud content was noted recording the number of the appendages (cataphylls and developing embryonic leaves). Terminal buds were investigated also in 3 micropropagated plants and 4 apical shoots of young trees (2 years old). The female flower buds were more frequent in lateral bud of the medium terminal portion of the shoot. The flower in secondary buds were only male.