In this work, diagnosis of boron (B) nutritional status based on leaf B concentrations were compared for the most common leaf sampling times for olive trees, January and July. For this purpose field experiments were conducted over four years (2003-06) in two rainfed olive groves located in Mirandela and Bragança, in north-eastern Portugal. Leaf samples were collected in January and July and analysed for B by standard procedures. Fruit harvest occurred in December of each year.
The crops followed typical alternate fruiting cycles. During the four years of the study, mean olive yields in the Bragança orchard, for instance, fluctuated, yielding 3.6, 28.1, 5.5 and 22.7 kg tree-1. Yield variation per individual tree was also great. In the Bragança orchard and in the 2004 harvest, for instance, yields ranged from 1.2 to 52.7 kg tree-1. Leaf B concentrations also varied greatly between individual trees. In the Bragança orchard in the July sampling of 2004, values for individual trees varied from 12.2 and 23.7 mg B kg-1. From a total of 16 scatterplots generated from the relationship between leaf B concentrations and olive yields, 10 significant linear relationships were established; six of them were related to July sampling dates and four were related to January dates. The number of significant linear relationships established between leaf B concentration and olive yield was used as a criterion of the accuracy of the B nutritional diagnosis, since this represented the lowest experimental variability. By using this criterion, the July sampling date was proved as better B nutritional diagnosis, although the difference from January sampling date was not sufficient to disregard this. Leaf B concentrations were consistently higher in July than in January. Averaged across the four years of the study in both orchards, the difference was 4.3 mg B kg-1. This difference should be taken into account in the interpretation of leaf analysis results when B levels are close to the deficient critical concentration.
