Fruit thinning using NAA shows potential for reducing biennial bearing of 'Barnea' and 'Picual' oil olive trees

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Abstract
Biennial bearing is a major horticultural and economic drawback of olive (Olea europaea L.) cultivation, which particularly affects the olive oil industry under intensive production systems. The number of fruits per tree in an on-year is a primary determinant of the biennial cycle. While fruit thinning using NAA shortly after full bloom is commonly practiced to increase fruit size in table olives, the extent of its influence on biennial bearing is unknown. In the present study, the ability of that common naphthaleneacetic acid (NAA) treatment (100 mg/L, 10 days after full bloom) to alleviate biennial bearing in two oil olive cultivars, Picual and Barnea, was poor, although significant influence on the number of fruit was evident solely in Barnea. Picual seemed less susceptible than Barnea to biennial bearing. Consequently, the effect of a broad range of NAA concentrations (0–320 mg/L, 10 days after full bloom) on various yield parameters was investigated during a biennial cycle of Barnea trees. There was a gradual proportional decline in the on-year number of fruits from ~50 000 to 10 000/tree in response to increasing NAA concentrations. The number of return fruits in the off-year was reciprocal to the on-year fruit load, but remained relatively small, below 15 000/tree. The dynamic relationship between fruit load and fruit size in both on- and off-years was a significant compensation factor in fruit and oil yields. In both cultivars, an on-year fruit load smaller than 20 000/tree is likely to provide consistent yearly oil yields ranging from 10 to 12 kg/tree. The results demonstrate the possibility of using NAA post-bloom spraying to balance biennial bearing in oil olives.