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Completed project

Physiological, metabolic and molecular basis of biennial bearing in apple (AP15002) and Physiological, metabolic and molecular basis of biennial bearing in apple – Australian component of AP15002 (AP15013)

Key research provider: University of Hohenheim, Germany (AP15002) and The Victorian Department of Economic Development, Jobs, Transport and Resources (DEDJTR) (AP15013)
Publication date: Wednesday, May 19, 2021

What were they all about?

Biennial bearing is a major constraint to apple flowering and production, and it’s estimated that around 30 per cent of commercial cultivars are susceptible. While this cropping irregularity is usually managed by chemical, mechanical or manual thinning methods, the underlying physiological, metabolic and molecular plant processes are largely unknown.

Running from 2016 to 2020, these two related projects increased understanding of the mechanisms involved in biennial bearing and in turn why apple crops fail to develop sufficient flower numbers in the year after a high crop load.

Project AP15002 (International component)

Project AP15002 was an international body of research with two field trials involved in identifying factors that either suppress or promote flower induction in apples. They examined the roles of plant hormones (signals from developing fruit), gene expression and carbohydrates.

The findings from this study will result, in the medium to long-term, in specific recommendations for the apple industry in Australia and worldwide. It is particularly expected that information on genes that trigger biennial bearing may lead to breeding of non-biennial bearing apple cultivars and that the identification of compounds that affect floral integrator gene activity and thus the transition to flowering in apple trees may lead to the commercial applications for regulating flowering.

Project AP15013 (Australian component)

Project AP15013 involved field trials in a commercial orchard setting in Victoria’s Yarra Valley. The study focused on the effects of crop load on the biennial cultivar ‘Nicoter’ (marketed as Kanzi) and the cultivar ‘Rosy Glow’ (marketed as Pink Lady), which is less susceptible to biennial bearing. A number of aspects of tree performance were studied including return bloom, fruit yield and quality, tree growth and carbohydrate reserves.

Results showed return bloom to be inversely proportional to the applied crop loads, and that high crop loads resulted in smaller, later maturing fruit with reduced quality (lower SSC content, firmness, and intensity of colour). Tree shoot growth was also found to be inversely proportional to crop load, and lower crop load trees showed lower leaf conductance. The project team recommend that higher crop loads should not be used as a means of producing smaller fruit to meet market demands as this could instigate biennial bearing and will produce less attractive fruit with lower quality.


Watch two webinars produced by APAL on the program:

Read the Effects of Crop Load on Apple Fruit Maturity industry report, on page 36 of the Australian FruitGrower magazine, Autumn 2019.

Hear the two project section leaders interviewed at the Hort Connection conference in Brisbane in June 2018 Understanding Biennial Bearing in Apples – 2018 Update

Read Do Australian growers need a tool for secondary thinning? on the APAL website.

Related levy funds


Funding statement:
These projects were funded through the Hort Innovation Apple and pear Fund using the apple and pear R&D levy and contributions from the Australian Government.

Copyright © Horticulture Innovation Australia Limited 2021. The Final Research Report (in part or as a whole) cannot be reproduced, published, communicated or adapted without the prior written consent of Hort Innovation, except as may be permitted under the Copyright Act 1968 (Cth).