Does alteration of ‘Koroneiki’ olive tree architecture by uniconazole affect productivity?

Abstract

High-density olive orchards (1000–3000treesha−1) allow use of the continuous straddle harvester that rides over the tree canopy, with very low harvesting and labor costs. However, tree size must be controlled, so that the harvesting machine can pass over the hedgerow and light can penetrate the tree canopy. To achieve this, high-density planted ‘Koroneiki’ olive trees, aged 5–8 years, were soil-treated with the gibberellin-biosynthesis inhibitor uniconazole at 0.1 or 0.2g per tree for four consecutive years (2007–2010) to reduce their growth rate. Uniconazole reduced 2007–2010 trunk cross-sectional area increment, 2009–2010 accumulated pruned branch weight, tree height and overall tree size. However, the treatment failed to affect the number of new leaves developed on a shoot, and shoot elongation was not consistently inhibited. Nevertheless, a higher leaf density was found for trees treated with 0.2g uniconazole per tree starting from the second year of the experiment and newly growing shoots on treated trees exhibited weeping-type growth. As a result, the architecture of the treated trees was changed, with the development of very thick and dense foliage. Cumulative fruit and oil yields for 2007–2010 were reduced in the uniconazole-treated trees, significantly so for the 0.2g per tree treatment. It was postulated that productivity reduction is a consequence of the altered ‘Koroneiki’ olive tree architecture, which decreases light interception in the tree canopy. Thus, although the results demonstrate that soil application of uniconazole can be used to control olive tree size in high-density orchards, it should be used carefully to ensure adequate tree illumination, so as to avoid crop loss.