Branching of annual shoots in common walnut (Juglans regia L.) as affected by bud production and indol-3-acetic acid (IAA) content

Relationship of bud production (axillary and terminally) of annual shoot (1Y) and/or the content of bud-derived indol-3-acetic acid (IAA) to branching of the 1Y was studied in common walnut (Juglans regia L.), cvs. Franquette and Lara. Cultivar-related branch architecture was determined. Lara tended to branch more densely than Franquette (53 vs. 42%). Significantly more fruiting off-spring shoots (FO) than vegetative ones (VO) grew-out per 1Y in both cultivars, whereas the ratio FO/VO of Lara exceeded that of Franquette by four times. An acrotonic branching pattern was more strongly expressed in Lara compared to Franquette. Bud-derived IAA was influenced by the cultivar (Franquette had 3.6 times more cumulative IAA along the 1Y than Lara), and by the relative position (terminal, subterminal, medial and basal) of the buds along 1Y. An opposite relationship between branching density and cumulative IAA content was established in both cultivars. At the 1Y relative position level, the opposite ratio between branching density and IAA content was clearly shown only on the basal position of the bud along 1Y in the Lara cultivar. Such an inconsistent linkage between bud production and the IAA spatial distribution along the 1Y illustrated that hormonal factors probably weakly affect the branching of Franquette and Lara. The length of the parent 1Y, the position of the buds along the 1Y-length, and the fate of the buds seemed to have a stronger influence on the bud out-growth and further development of the off-springs. In further analyses, seasonal fluctuations of the IAA, and the following activity of the buds should be investigated in order to improve the understanding of a complex branching phenomenon in walnut.