Biennial bearing is the term used to describe the annual cyclical changes in apple orchard yields characterised by ‘on’ and ‘off’ years that yield ‘heavy’ and ‘light’ fruit loads, respectively. The lack of regular cropping may occur in an entire fruit-growing region, triggered by adverse climatic conditions or crop management practices, although they are more commonly observed at the tree or branch level.

There is a large variation among apple varieties in their tendency for fruitfulness and regular cropping behaviour. Biennial bearing is a major constraint for production in key growing areas in Australia, Germany and many other countries with certain apple varieties such as Fuji and Kanzi^®. It is estimated that about 30 percent of commercial apple cultivars are susceptible to some level of biennial bearing, causing a significant annual financial loss to growers.

The exact plant mechanism, or the underlying trigger that leads to biennial bearing, is still poorly understood, but it is often linked to the lack of efficient plant control of the flower development cycle. There is good evidence that a high yield one year reduces the next year’s flower density and hence crop load via 1) transmissible signals formed by developing fruit (plant hormones) or 2) lack of certain nutrients (carbohydrates). Simultaneous and competitive growth processes might inhibit flower bud formation.

Consequently, crop load management through effective flower and fruitlet thinning strategies are well known to have positive effects on flower bud formation and return bloom. This can help growers to reduce biennial bearing and even out production across years. If growers can better understand what makes one cultivar biennial and another one not, they might also understand how they can reduce the amplitude of the biennial bearing and establish consistent cropping.

Underlying triggers of biennial bearing

In this project the researchers aim to advance the industry’s knowledge on processes inside the plant (at the physical, chemical and molecular level) that may be major constraints to flower bud development in apple trees that result in biennial bearing.

By establishing different trials, the researchers aim to take a close look at and test if and how changes in biochemical signals within the apple bud may modulate flowering gene activity and therefore bud development, or lack thereof.

In trees that have invested in many flowers and thus fruitlets, higher levels of negative signals (e.g. plant hormones) or lower levels of positive signals (e.g. hormones or carbohydrates) are sent from the seeds, fruitlets or leaves to the bud, which changes the activity of flowering regulator genes. Flowering regulator genes are fundamentally responsible for whether a plant produces floral buds and flowers, or not. As a result, downstream target genes required for flowering are activated at lower levels and fewer buds will switch to floral production.

Potential outcomes for growers

By identifying which genes are differentially affected in the ‘on’ or ‘off’ year of biennial and non-biennial cultivars and if there are different versions of these genes present in biennial and non-biennial cultivars, the researchers can get closer to determining the fundamental triggers of biennial bearing.

If there are differences at the gene level, they want to determine if they can easily identify these differences (through molecular markers) because this would help with the breeding of non-biennial bearing apple varieties.

New non-biennial bearing apple varieties would still be a long-term solution given breeding time frames. To help growers in the medium term, the researchers have also set ourselves the challenge to see if they can identify specific compounds that, upon application to a biennial cultivar in an ‘on’ or ‘off’ year prior to seasonal flower induction, would reduce or increase flowering gene activity, and thus keep the tree relatively balanced in its annual fruit load. This would indeed give an excellent horticultural tool to modulate crop levels.

Likewise, the researchers hope that through their better understanding of the processes involved in biennial bearing, they will be able to identify more effective crop management practices that reduce biennial bearing in different susceptible apple cultivars.

Experiments to date

Three experimental field trials have been established across Germany and Australia: 1) Spencer Seedless experiment, 2) Cultivar experiment, and 3) Crop load experiment.

Spencer Seedless experiment

The Spencer Seedless field trial has been established at the Horticultural Research Centre of the University of Hohenheim, Germany. The apple cultivar used in the experiment is the Spencer Seedless cultivar, which can be either seedless without pollination or seeded if pollinated and serves as a model cultivar to study biennial bearing in detail.

For this research, the trees were either pollinated by hand to produce seeded fruit or not pollinated to produce seedless fruit. Samples of fruit and buds were then taken from the differently pollinated trees at regular intervals throughout the growing season to determine which specific mobile signals (e.g. proteins, carbohydrates, hormones) from the fruit either repress or promote flowering regulator genes and hence inhibit or favour floral bud formation.

Cultivar experiment

In Germany from March to May 2016 (spring in the northern hemisphere), field trials were set up using a biennial (Fuji) and non-biennial (Royal Gala) apple cultivar at the Centre of Competence for Fruit Cultivation, Germany.

All flower clusters were manually removed on half of the trees to simulate an ‘off’ year, while the other half of the trees were not thinned at all to simulate ‘on’ trees. Bud samples were taken from each to determine the onset of floral bud formation which allows the selection of critical time points prior to this event for gene expression analysis as well as proteomic and metabolomic studies.

Crop load experiment

In spring 2015, on a commercial orchard in the Yarra Valley, Victoria, we established different crop loads on the biennial bearing Nicoter (Kanzi^®) and the non-biennial bearing Cripps Pink (Pink Lady^®).

Bud samples were taken from both types of trees to examine the tissues under microscope to identify any differences and to determine what genes were being expressed in the different samples.

Acknowledgement

This project (Physiological, metabolic and molecular basis of biennial bearing in apple) is funded by Horticulture Innovation Australia Ltd using the apple and pear industry levy paid by growers and matched funds from the Australian Government.

Contributing authors

Jens Wünsche, Julian Kofler and Anton Milyaev, University of Hohenheim (Germany); Henryk Flachowsky, Julius Kühn-Institute (Germany); and Dario Stefanelli, Department of Economic Development, Jobs, Transport & Resources (Victoria).

Source: APAL