Impact of branch crop load on pistachio nut growth and ripening

Despite the physiological impact that variable crop loads have on the vegetative growth and reproductive processes of pistachio, very few studies have examined the effect of fruit-sink strength on pistachio nut growth and quality. In this study, considering the branch as a semi-autonomous unit, with regard to carbohydrate availability, we manipulated fruit load with the following two treatments: high crop load (3 nut clusters) and low crop load (1 nut cluster), and evaluated the impact on carbon assimilation, nut growth and quality parameters. Results showed that during early nut development, where nut growth relies strongly on resources from the previous year, high crop load treatments experienced greater competition for available resources, resulting in smaller nuts. Crop load significantly affected nut and kernel dry weight, but not other quality parameters. Although the photosynthetic rate in July was higher in the high loaded branches, at the beginning of August, photosynthesis declined, resulting in a significantly lower photosynthetic rate by September. This mid-season reduction in the photosynthetic rate observed in highly loaded branches may have compensated for the different sink strength of the branches. Furthermore, differences observed in kernel dry weight by the end of the season demonstrated competition during a later stage of nut development, which was probably amplified by the late season reduction in assimilation associated with early senescence. As competition for resources can have a strong and complex impact on pistachio nut growth, managing the crop load should be integrated into orchard management practices to improve nut quality.

Emily Santos won the ISHS Young Minds Award for the best poster presentation at the VIII International Symposium on Almonds and Pistachios in USA in May 2023.

Emily Santos, Department of Plant Sciences, University of California Davis, Davis, CA, USA, e-mail: eadsantos@ucdavis.edu

The article is available in Chronica Horticulturae