Bud structure, position and fate in walnut cv. Chandler

In walnut, studies on tree growth show that the morphology and growth pattern of a shoot are related to its position in the plant architecture and to plant age. More precisely, the morphological features of a shoot may vary according to morphogenetical gradients linked with shoot architectural position and developmental stage of the tree. After bud break, determined monocyclic annual shoots are built up during a single spring flux whereas long bicyclic annual shoots are formed during two successive fluxes of growth (spring and summer) separated by a short resting phase where the second flux of growth is undetermined. In walnut, distribution and type of bud complex are important for pruning and for the application of dis-budding techniques aiming to shoot growth manipulation during the training of the orchards with different training systems and later for the production pruning. Our study on bud topology related to the propagation technique was carried on 1-yr-old cv Chandler walnut trees, micropropagated or grafted on seedlings (J. regia), in two orchards located in Emilia Romagna (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 before bud break in 2018. The results evidenced that on average, micropropagated and grafted trees had a similar bud distribution along the shoot 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 when the internodes were shorter. The bud topology was described in 8 years-old micropropagated Chandler trees. Samples of 4 different branching structures were selected in mid of October 2017: 4-years branch, 2-years branch, 1-year shoot, sylleptic shoot. Shoots were classified according to their supporting branch age and then the number of nodes, branching and length were measured. 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. The female flower buds were more frequent in lateral bud of the medium terminal portion of the shoot with second cycle growth, while in the short monocycle shoots the male flowers were more abundant and only the terminal one was female. The flowers in secondary buds were almost exclusively male, but it is possible to find some female flowers in secondary buds on vigorous shoots. © 2019 Italus Hortus. All rights reserved.