Vol.:(0123456789) 1 3 Trees https://doi.org/10.1007/s00468-018-1708-x ORIGINAL ARTICLE The position in the canopy and the bearing status of 1-year-old shoots affect the bearing potential and morphology of current-year shoots in walnuts (Juglans regia L.) cv. Chandler Daniela Valdebenito 1  · Daniela Farías 1  · Bruce Lampinen 2  · Aude Tixier 2  · Maciej Zwieniecki 2  · Sebastian Saa 1 Received: 6 February 2018 / Accepted: 9 May 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Key message From 1 year to the next the number of shoots increases, but their individual size declines. Shoots located in more lightened canopy zones and bearing two fruits maintain productive sustainability. Abstract 432 shoots in 24 mature trees from three commercial orchards located in the central valley of Chile were inspected for 2 years. Tree canopy was divided into three zones according to light exposure (upper, lower-exposed, lower-shaded). In the first year of the study, six shoots were randomly selected in each canopy zone and divided into three bearing statuses [shoots bearing one fruit (F1), shoots bearing two fruits (F2), and shoots bearing three fruits (F3)]. In the second year, all new shoots (1068) born on the 432 shoots were studied. Specific leaf weight and current-year shoots (CY) characteristics (length, diameter, number of leaves, number of leaflets, leaf area) were evaluated as a function of the previous season fruit load and canopy position of the 1-year-old (1Y) shoots. The results indicated that 1Y shoots located in more light-exposed canopy zones bore on average 0.5 more CY shoots than 1Y shoots in the lower-shaded canopy zone. Specifically, the 1Y shoot probability of bearing CY shoots that had two or three fruits was 72.8% in the upper canopy zone, 44.6% in the lower-exposed zone, and 34.9% in the lower-shaded zone of the canopy, respectively. 1Y shoots in the lower-shaded zone had on average a 7.7% higher probability of dying than 1Y shoots in the upper and exposed positions. In addition, CY shoots born on 1Y shoots exposed to light were on average 2.7 cm longer, more than 1 mm thicker, and had 53.7% greater leaf area than CY shoots born on shaded 1Y shoots. Finally, the results of this study showed that 1Y shoots that had two fruits in the previous season produced CY shoots with the greatest bearing potential in the current and future seasons. Keywords Bearing structure · Light · Leaf area · Source · Sink Introduction At the tree level, studying the behaviour of bearing struc- tures provides greater understanding about the factors that influence fruit production and generates important infor- mation about how to manage trees to reach potential yield (kilograms of fruit produced per hectare). In walnuts, fruit is produced on current-year shoots. These shoots can be mono- cyclic (formed by one growth unit) or bi-cyclic (formed by two successive growth units) (Sabatier et al. 1998, 2003). In mature trees, mono-cyclic shoots provide the greatest amount of fruit, while bi-cyclic shoots are less frequent. Depending on variety, current-year fruiting shoots develop at the terminal or lateral positions of 1-year-old shoots (Kelc et al. 2010; Solar and Stampar 2003). Thus, the number of current-year mono-cyclic shoots born on annual shoots and the number of fruits per mono-cyclic shoot are variables that define productivity in walnut trees. Light availability is a significant factor affecting poten- tial yield in fruit trees. Light plays a central role as the major factor involved in photosynthesis and affects cur- rent season growth and development of shoots (Henriksson Communicated by T. Fourcaud. * Sebastian Saa sebastian.saa@pucv.cl 1 Escuela de Agronomía, Pontificia Universidad Católica de Valparaíso, Casilla 4D, Quillota, Chile 2 Department of Plant Sciences, University of California, One Shields Avenue, Davis, CA 95616, USA