Effect of fruit on flowering in relation to biennial bearing in sweet orange (Citrus sinensis)

Flowering and fruit-setting in the sweet orange cv. ‘Late Valencia’ were observed for three consecutive seasons that comprised two “off-years” and one “on-year”. Twice as many flowers per unit area of tree canopy were observed in the “on-year” as in the two “off-years”. There was a significant negative correlation between the yield of the previous crop and the number of flowers per square yard of tree canopy.

There were no significant differences between an “off-year” and an “on-year” in the proportions of inflorescences classified according to the number of flowers and number of leaves per inflorescence. Furthermore, the previous crop had no direct effect on fruit-set, the percentage fruit-set being similar in all three seasons.

Observations made during blossoming, when the trees were bearing an “on-year” crop, showed that practically no flowers were produced on fruit-bearing wood. The few inflorescences that were recorded on fruit-bearing wood tended to be of the leafless types.

It is suggested that the cause of biennial bearing in sweet orange is the diffusion of a flower inhibitor, possibly a gibberellin-like substance, from the fruit into the wood on which it is borne.