Optimising citrus fruit size by regulating flower numbers and crop load (CT03031)

This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

Fruit size was a major issue for the Australian citrus industry. The tendency for citrus to exhibit a biennial bearing pattern of flower, and hence fruit, numbers had a significant impact on fruit size at harvest. In “on” years in which a large numbers of flowers were produced, an excessive number of fruit were set, often in clusters of several fruit on a single branch. Competition between the developing fruit for assimilates reduced the size of all fruit, unless chemical or hand-thinning was used to reduce fruit numbers. Hand-thinning was labour-intensive and expensive while post-set chemical thinning could damage trees.

Previous research, both in Australia and overseas, demonstrated that one of the most effective ways of regulating crop load, and hence fruit size, was to regulate flower numbers with gibberellic acid (GA) sprays in winter. At present, a major difficulty with this approach was the inability to reliably predict flowering intensity in an orchard each spring, and hence the dosage of GA required in winter to achieve optimal flower and fruit numbers.

The overall aim of this project was to develop and test practical methods of predicting flowering intensity (a citrus floral index) in the coming spring by analysing dormant buds collected in winter for the activity of key flowering genes. This type of information allowed growers to use treatments such as winter gibberellic acid (GA) sprays more effectively to regulate flower production and hence fruit numbers and size.

Despite success in isolating citrus flowering genes and measuring their expression in dormant winter buds, a gene-based floral index test was not feasible at the time. However, this project had successfully generated knowledge and molecular tools that could be used to develop DNA fingerprinting for identification of citrus varieties or to improve seedling selection in conventional citrus breeding.