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Ongoing 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)

What are 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.

Beginning in 2016, these two related projects aim to increase 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 is an international body of research with two field trails involved in identifying factors that either suppress or promote flower induction in apples. They are looking specifically at the roles of plant hormones (signals from developing fruit), gene expression and carbohydrates, and include…

  • One trial involving the Spencer Seedless cultivar, at the Horticultural Research Centre of the University of Hohenheim in Germany
  • One trial comparing a biennial cultivar (Fuji) to a non-biennial cultivar (Royal Gala), at the Centre of Competence for Fruit Cultivation near Lake Constance in the Alps.

Project AP15013 is the Australian component, which involves field trials in a commercial orchard setting in Victoria’s Yarra Valley. Here there is study of biennial cultivar Nicoter (Kanzi) and non-biennial cultivar Cripps Pink (Pink Lady), similarly looking at the effect of gene expression and metabolic signals on flowering, in response to plant resources, plant development, cultural practices and environmental cues.

Australian researchers in local project AP15013 harvested the 2018/19 fruit of Nicoter and Rosy Glow cultivars and analysed for quality standards including size, weight, soluble solids concentration (SSC), firmness and maturity. A significant relationship was found between crop load (number of fruit/cm2 of trunk area) and most of the quality factors measured, with quality increasing as crop load decreased.

Winter pruning wood was also collected, weighed and dried for further analysis. Return bloom was assessed by counting flower clusters on trees, with the team finding this also improves with decreased crop load.

Climate data from the orchard is being collected through a weather station and shared with other researchers at NSW Department of Primary Industries, who are adapting an apple carbon balance model originally developed for the US (called MaluSim) to Australian conditions. The model helps growers make chemical thinning decisions. Model simulations were performed to compare Australian conditions with other important apple growing sites within Australia and around the world as part of the adaptation process.

Meanwhile, after extensive evaluation, the overseas team in project AP15002 determined the best method to successfully extract plant hormones as part of their investigation into flower bud initiation, with seven phytohormones successfully detected in apple spur buds.

RNA sequencing from additional Gala samples covering the onset of flower initiation is completed. Carbohydrate analysis is now underway and will be completed in January 2020.


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


Work overseas (AP15002) continues with ‘Cripps Pink’ and ‘Nicoter’ apple spur buds collected in Australia (during the growing seasons of 2015/16, 2016/17, and 2017/18) now being analysed to reveal flower initiation time points for each cultivar and to evaluate to what extent crop load influences flower initiation time.

Proteins and other metabolic compounds found in ‘Gala’ samples have now been found in ‘Fuji’ as well, but the abundance of each molecule varied with cultivar and treatment. Researchers are now relating each compound with genes in the sample trees.

Around 6,000 genes were found to differ between ‘on’ and ‘off’ trees at floral initiation time and just prior, providing many potential factors influencing biennial bearing. Next steps are to group the genes according to the metabolic pathways in which they are involved.

Meanwhile, Australian researchers (AP15013) have continued taking various measurements of tree responses to crop load, including fruit growth, leaf conductance, light interception, fruit maturity and dry matter. Pruning wood was collected in winter 2018 to assess tree growth and starch and soluble sugar content.

Orchard climate data is being collected from weather stations and shared with researchers in NSW who are verifying new apple carbon balance models.  This allows Australian conditions to be incorporated into the modelling, which was originally developed in the US.  In the US, the apple carbon model MaluSim is used to assist growers in making chemical thinning decisions, based on four-day carbon balance averages. The work aims to make the technique available to Australian growers.

In a modification to the original research proposal, the team has adopted a new way of measuring the efficiency of photosynthesis by using a Bistatic LiDAR, which will allow them to identify any efficiency variations due to crop load.


Read about the responses of fruit maturity to crop load on page 36 of the Autumn issue of the Australian Fruitgrower magazine.

The overseas component of the study (AP15002) has been continuing to investigate flower bud initiation in the laboratory. Among the areas of investigation is an analysis of buds from ‘on’ and ‘off’ trees to find differences between them. In all, 95 proteins and 23 other metabolic compounds have been found to differ in amounts and will be investigated further to reveal the effect they have on budding.

In the Australian component of work (AP15013), fruit of ‘Nicoter’ and ‘Rosy Glow’ cultivars have been harvested and analysed for comparison along with other measures. Buds collected last season were sent to Germany for analysis.

In October 2018, the team published an article about secondary thinning: Do Australian growers need a tool for secondary thinning?


For international project AP15002, the researchers report that analysis of plant tissue samples and collected data from their trials is ongoing, with outcomes to be communicated to industry when available.

Initial lab tests have identified a range of metabolites (substances used in plant metabolism) in apple buds, including several known to influence hormonal signalling pathways, that could play a regulatory role in flower induction.

Another early finding has been that floral bud initiation is related to crop load, with ‘on’ trees having fewer floral buds than ‘off’ trees. Depending on variety and treatment in the trials, the transition time from vegetative to floral meristems in apple buds differed significantly.

As part of the Australian component, project AP15013, biennial cultivar Nicoter (Kanzi) and non-biennial cultivar Cripps Pink (Pink Lady) are being looked at. The researchers report that during the 2016/17 season, fruit from both cultivars was harvested and analysed standard quality attributes including size, weight, soluble solids concentration, firmness, colour and maturity. There was a significant inverse relationship between crop load (number of fruit/cm2 of trunk area) and the majority of variables measured. Winter pruning wood was also weighed, collected and dried for further analysis.

Crop loads for the 2017/18 growing season were applied within three weeks of full bloom with fruitlets smaller than 15mm in diameter. Bud collection for the new season molecular and histological analyses (to be sent to University of Hohenheim) commenced four weeks after full bloom and was set to continue for two months.

The postgraduate program at the University of Queensland has also been involved in regular tree imaging to identify and analyse aspects of flowering, fruit set, growth and canopy size during the growing season, while climate data collection from the orchard is also ongoing.