The morphology of a Walnut (Juglans regia L.) shoot is affected by its position in the canopy and correlated to the number and size of its fruits

In mature walnut trees, mono-cyclic shoots are the main bearing structure and are thus fundamental for tree productivity. In this line, a study was carried out in three commercial “Chandler” walnut orchards, located in the central valley of Chile. In each orchard, eight mature trees were selected and eighteen mono-cyclic shoots were marked according to three fruit load categories (shoots bearing one fruit (F1), shoots bearing two fruits (F2), shoots bearing three fruits (F3)). These shoots were distributed in three zones of the canopy (upper, lower-exposed, lower-shaded). This resulted in 432 marked shoots in the total of the experiment, in which the effect of position and fruit load on length, diameter, leaf area and specific leaf weight were studied. In addition, multilinear regression models for fruit diameter as a function of the above-mentioned characteristics were performed. The results indicate that the production of two or more fruits per shoot is associated with an average shoot length of 12.7 cm, an average shoot diameter of 8.1 mm and an average leaf area of 1034.7 cm². Linear models indicate that length and specific leaf weight significantly explain fruit size for all shoot categories. In addition, the leaf area, and leaf area plus basal diameter of the shoot, are significantly related to fruit size (equatorial diameter) for shoots with two and three fruits, respectively. According to the results of this study, the tree's upper and lower-exposed zones offer shoots with greater potential to produce more and larger fruits per shoot than the lower-shaded canopy zone. Thus, the morphological characteristics of the shoot (diameter, length and leaf area, among others) are directly related to a greater number and size of fruits, and therefore higher productivity.