Alternate Bearing in Olive (Olea europaea L.)
In alternate bearing olive trees, high yield on-crop years alternate with low yield off-crop years. Research was undertaken in support of the California olive industry to determine how fruit number one year impacts floral intensity and yield the following year. Removing the on-crop of fruit before endocarp hardening (mid-July) restored summer vegetative shoot growth and inflorescence number the following spring. Delaying fruit removal to 75 days after endocarp hardening (September) significantly reduced spring bud break and floral intensity. Quantification of the fate of buds on nonbearing and bearing shoots of off- and on-crop control trees provided striking results. The majority of buds on nonbearing shoots of off-crop trees produced inflorescences, whereas most buds on nonbearing shoots of on-crop trees remained inactive through spring bloom. In sharp contrast, 74% of buds on bearing shoots of on-crop trees had abscised prior to spring bloom. A putative O. europaea FLOWERING LOCUS T (FT) gene, known to upregulate flowering in woody perennials, was expressed in buds on nonbearing shoots of off-crop trees from June through March, but was repressed in buds of bearing and nonbearing shoots of on-crop trees starting in September and October, respectively. Whereas the lack of OeFT expression in buds on bearing shoots of on-crop trees may be due to the abscission of 51% of the floral buds by September, this result taken together with the repression of OeFT in buds on nonbearing shoots of on-crop trees in October, for which floral bud abscission was not a factor, suggests the possibility that the on crop may inhibit determination of the floral meristem between September and October. Buds on nonbearing shoots of off-crop trees subsequently expressed two floral organ identify genes in March; these genes were not expressed in buds of nonbearing and bearing shoots of on-crop trees. This study is the first to provide evidence that several mechanisms in addition to inhibition of summer vegetative shoot growth, including inhibition of spring bud break, inhibition of floral development at the level of gene expression, and abscission of potential floral buds, contribute to the perpetuation of alternate bearing in olive.