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Turning off biennial bearing

Research & Extension

Improving our understanding of the metabolic processes associated with biennial bearing and identifying chemicals that could be applied to ensure more consistent flowering could lead to easier management of biennial bearing.

Biennial, or alternate, bearing in apples occurs where high fruit numbers in an ‘on’ season result in reduced flower and fruit numbers the following ‘off’ season. Careful management of crop load by thinning flowers and fruitlets to sustainable levels is generally necessary but is a costly process. Even with best practices, biennial bearing can still reduce yields by more than 20 per cent, leading to loss of potential income for growers. Therefore, the ability to even out biennial bearing would be extremely valuable for growers. Research currently being undertaken by Agriculture Victoria as a component of the PIPS3 Program’s Advancing sustainable and technology driven apple orchard production systems project (AP19003) aims to identify potential chemical signalling compounds within the trees, which could lead to a better understanding of biennial bearing and the development of future management strategies.

Biennial bearing is controlled within trees by a combination of genetics and environment (carbohydrate availability), and most likely mediated by metabolites, which act as chemical signals to either stimulate or inhibit the activation of the genes that control the formation of new floral buds (floral induction). Studies have shown that the floral induction period occurs in the early part of the growing season. It is believed that phytohormones or signalling compounds produced in the young fruitlets may be responsible for the suppression of floral induction, with excessive fruit numbers leading to higher levels of signalling compounds within the tree and lower return bloom in the following growing season.

In a recent study as part of the project, the chemical signals associated with this irregular cropping behaviour were determined in buds collected in the floral induction stage of ‘on’ and ‘off’ trees of the biennial-bearing cultivar ‘Nicoter’ (marketed as Kanzi®), and the less susceptible cultivar ‘Rosy Glow’ (sold domestically as Pink Lady). This study was the first to show that chemicals similar to salicylic acid were associated with floral induction in apple trees. Furthermore, an additional mechanism associated with cytokinin was also observed in the less susceptible cultivar ‘Rosy Glow’.

Identification of chemical structures associated with floral induction in ‘Nicoter’ and ‘Rosy Glow’.

The outcomes from this research are consistent with gene expression studies and similar to some chemical thinning agents, thus are likely new candidates for future crop load management strategies for increasing return bloom.

Chemical signalling was further substantiated by the formation of zones within trees that showed biennial behaviour independent to the rest of the tree. These zones were generally branches or areas, such as treetops, that started with high flower numbers but became biennial while the rest of the tree showed relatively constant flower numbers. The fact that flower induction in these zones was different from the rest of the tree indicates a localised signal. Furthermore, the fruit in these zones showed the quality pattern of the rest of the tree, suggesting that these zones are not autonomous with respect to carbohydrate utilisation. For example, small fruit with low sugar content in trees with a high crop load were found in zones where the number of fruit was very low. This was contrary to expectation due to the high local carbohydrate availability.

In our current experiment, we are advancing our understanding of the systemic processes within the tree by analysing buds, leaves and fruitlet seeds from ‘Ruby Pink’ apple trees trained to a two-leader system, with different combinations of crop loads applied to the leaders on each tree. This will help identify important signalling compounds and give us a better understanding of how localised the chemical signals are within the tree.

Scales were removed from the buds using a scalpel, and the inner meristem isolated for analysis.

By identifying chemicals within the trees which are differentially affected in the ‘on’ or ‘off’ year of biennial cultivars, we are improving our understanding of the metabolic processes associated with biennial bearing and helping to identify potential chemicals that could be applied to trees to ensure more consistent flowering year to year and help to even out the variability of biennial bearing. If successful, this could lead to easier management of biennial bearing in the future.

This article was first published in the Spring 2022 edition of AFG. It was written by Priyanka Reddy, Tim Plozza, Simone Rochfort and Ian Goodwin from Agriculture Victoria.

For more information, watch this video on Crop load management: Signalling compounds that induce flowering.


The PIPS3 Program’s Advancing sustainable and technology driven apple orchard production systems project (AP19003) has been funded by Hort Innovation, using the apple and pear research and development levy, contributions from the Australian Government and co-investment from Agriculture Victoria. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.

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