Maximising yield and reducing seasonal variation (AV16005)

In this reporting period, the project team quantified the active forms of candidate hormones to validate changes in the homeostasis of these signals associated with abscission. Based on these results, quantification of these hormones during early fruit abscission is critical for developing a strategic plan to mitigate irregular bearing using PGRs. Field trials were planned for identifying candidate hormones that act early in the abscission process. In addition, a preliminary PGR-based trial aimed at limiting summer immature fruit abscission will be performed.

Due to the irregular bearing behavior of avocado, which is primarily driven by low levels of fruit set and/or high rates of fruit abscission, average annual production levels across Australia are well below the theoretical yield potential. Methods to sufficiently improve avocado fruit set and reduce fruit abscission are currently lacking due to a limited understanding of the physiological and developmental basis of fruit set and abscission. Therefore, to effectively mitigate the primary drivers of irregular bearing, an in-depth understanding of fruit abscission and fruit set is required. New knowledge on fruit set and abscission can be leveraged to develop new methods to reduce irregular bearing in order to maximize yield and reduce seasonal variation.

This project is focused on understanding the physiological drivers of fruit abscission, which negatively impacts production in coastal regions of south western, WA, central QLD and Tristate area along the Murray River. To address this problem, the research team first characterised the seasonal patterns of fruit abscission to identify a specific fruit drop event for management intervention. The developmental basis of fruit abscission was addressed to identify what process should be managed. The hypothesis that trees adjust their crop load via fruit abscission in response to their carbohydrate status was tested, as this would align with carbohydrate management strategies proposed by Whiley and Wolstenholme, 1990. The project addressed the physiological drivers of fruit abscission, focusing on whether fruit drop is due to a: (1) decrease in water potential, (2) deficiency in a mineral element(s), (3) alteration in carbohydrate supply and metabolism and/or (4) change in hormone signaling during fruit development. Finally, research in AV16005 further addressed the role of tree carbohydrate status on flowering and fruit set.

Avocado fruit abscission has been studied for over 40 years, yet little progress has been made in understanding this developmental and physiological process. Research from AV16005 uncovered key knowledge gaps on fruit abscission and identified management intervention points for future research. First, management of summer fruit abscission has the potential to increase tree productivity by 24 per cent or more. Second, tree carbohydrate status, as well as dominance relations, impacts flowering, fruit set and fruit abscission. Third, fruit growth arrest is the first step in the abscission process and must be managed to limit fruit drop. Fourth, a decrease in water potential or deficiency in an essential mineral element(s) is not associated fruit growth arrest and abscission; therefore, irrigation and fertilization strategies will have little impact on reducing summer fruit drop. Fifth, fruit growth arrest is associated with an alteration in carbohydrate supply and metabolism, as well as global changes in gene expression profiles associated with the function of auxin, gibberellin, cytokinin, brassinosteroid, ethylene, abscisic acid and jasmonic acid. Sixth, preliminary studies suggest that the physiological basis of the initial fruit drop event is distinct from the factors that drive summer fruit drop.

Therefore, the research team has proposed that summer fruit abscission involves alternations in sugar and hormone signaling that impacts carbohydrate supply and metabolism necessary for fruit growth and development. Further research aimed at determining the hierarchy of hormone signaling, will provide the basis to develop a plant growth regulator application(s) aimed at limiting summer fruit abscission for managing irregular bearing in avocado.

One of the significant challenges facing the Australian avocado industry is irregular bearing, which is primarily driven by low fruit set and/or high fruit abscission, as well as biennial bearing. This investment is focused on understanding the physiological basis of summer fruit abscission, which is a major limitation for production in central QLD, southwest, WA and the Riverland, SA.

In this project, research conducted so far supports the hypothesis that avocado trees adjust their crop load via immature fruit abscission in response to fluctuations in tree carbohydrate status and dominance interactions between fruits and shoots, as well as among fruits. Further, in a number of experimental trials, the project team has demonstrated that fruits undergo growth arrest prior to abscission. Therefore, management intervention should be focused on understanding the physiological drivers and factors (hormones, sugars and developmental genes) that mediate the early step(s) of fruit growth arrest, in order to develop new innovative tools to limit summer fruit abscission.

In the last six months, the project team performed research activities to further identify key candidate regulators of fruit growth arrest. First, using a Gene Set Enrichment Analysis on the transcriptome data derived from arresting fruits, strong evidence showed that a conserved sugar starvation pathway is activated when fruits switch from a normal to low growth rate prior to abscission. Further, this activation is highly expressed in the seed coat under conditions in which fruits abscise naturally or response to defoliation. This result is consistent with previous data showing that sugar metabolites are depleted in the seed coat during fruit growth arrest.

To further identify candidate hormones involved in fruit growth arrest, the project team mined the transcriptome data. Results further suggest that the activity of growth-promoting hormones, including auxin, cytokinin, gibberellin and brassinosteroid, are reduced when fruit undergo growth arrest. At the same time, the transcriptome data supports the hypothesis that the activities of ethylene, abscisic acid and jasmonic acid, which inhibit growth, are increased in fruits undergoing growth arrest. The project team is in a position to quantify the key hormones to further validate the hypothesis.

Lastly, the project team presented experimental trials to the Project Reference Committee to further characterise fruit growth arrest in order to understand the outlook of management intervention.

The team continued to progress research outcomes for the project:

• Seasonal trends in fruit abscission were evaluated and showed that a primary fruit drop event occurred during early stage fruit development and gradually declines throughout the season. A small but significant peak of abscission was observed in mid-summer in WA and SA.
• Using defoliation and shading trials the team showed that tree carbohydrate status correlates with fruit abscission, also finding that that fruits undergo growth cessation prior to abscission, naturally and under conditions that induce fruit abscission. Seed coat senescence is initiated when fruit growth arrest is completed and fruit carbohydrate status is key for the maintenance of fruit growth.
• Results from the transcriptome/gene expression study also provided useful research insights, with possible management interventions also investigated.
• The team found that irrigation is not a management solution for fruit abscission and that fertilisation is not a direct management tool to prevent growth arrest.

Over the past six months, the team reported progress with:

• Development of an integrated model of avocado fruit abscission – derived from gene expression and carbohydrate analysis. Hormones implicated in abscission were also obtained from gene expression data.
• A role for tree carbohydrate status for flowering and fruit set developed.

Understanding the physiological basis of fruit drop

A fruit growth cessation pathway implicated in immature fruit abscission was identified via transcriptome analysis (used to identify genes and molecular/signalling pathways involved in biological processes). The analysis indicated that:

• The embryo, seed coat and pericarp respond differentially to fruit growth arrest
• The switch from high to low growth rate involves changes in numerous metabolic pathways
• Defoliation is an appropriate method to dissect early physiological drivers of fruit abscission
• Decline in auxin, cytokinin and gibberellin in the seed coat may be linked to reduced sucrose and glucose levels in the seed
• A major axillary bud suppression pathway is activated in the seed coat to inhibit fruit growth
• Activation of EIN3 via reduced glucose levels may act to promote seed coat senescence.

Given that hormones and plant growth regulators are often developed to manage trees, the next phase of the project will validate candidate hormones identified in the transcriptome study. To achieve the outcome, the project is collaborating with project National tree genomics program (AS17000) to identify and quantify hormones involved in the abscission of immature avocado fruits.

Since the last project update, the team performed the third year of fruit abscission trials aimed at understanding the physiological basis of fruit drop, along with second year trials to evaluate the role of tree carbohydrate status on flowering and fruit set.

Understanding the physiological basis of fruit drop

Results indicated that carbohydrate metabolism and signalling are key physiological drivers of fruit abscission. They also showed that in well managed orchards nitrogen and essential elements do not play a role in fruit abscission, however a relationship between aluminium and fruit abscission was discovered. Further studies will be undertaken to better understand and expand on these findings.

Evaluating the role of tree carbohydrate status on flowering and fruit set

The team found that while tree carbohydrates influence the level of flowering and fruit set, increasing avocado yield will require new management tools to decrease fruit abscission and manipulate fruit set.

Field trials planned for the 2020-21 season

Field trials for 2020-2021 will address the physiology of fruit abscission and the potential for practical applications for managing fruit drop.

The trials include:

• Drought stress-fruit abscission trial with the aim of linking changes in tree carbohydrate status with fruit abscission
• Fruit growth trial utilising fruit dendrometers to characterise changes in fruit growth rates with abscission and determine if they can predict fruit abscission events
• Analysis to determine if fungal infections are involved in avocado fruit abscission and, if so, evaluate which pathogens are involved and whether fungicide treatments can reduce impact.

Since the last update in October 2019, the project team have completed trials aimed at better understanding the drivers of natural fruit abscission, as well as fruit drop induced by abiotic and carbon stress. Three sets of trials were established to impact tree carbohydrate status and induce fruit abscission: (1) defoliation, (2) drought stress and (3) shading. An additional trial captured natural fruit abscission. The findings corroborated previous results and demonstrated that fruit growth cessation is a universal phenotype that occurs early in the fruit abscission process.

This method was also used to sample fruits with high (retained) and low (abscising) growth rates to identify the early physiological pathways that control fruit abscission. Results indicate that fruit abscission is not induced by changes in the fruit water potential.  

The project team collaborated with Hort Innovation Avocado Fund project Avocado industry capacity building – Western Australia (AV17006) to evaluate management solutions in fruit abscission induced trees. Further evaluation is required before recommendations can be made to industry. Gene expression profiling and carbohydrate and hormone studies will be conducted to provide the new knowledge required to trial and develop management tools to limit fruit drop.

The research team has expanded its understanding of how avocado trees adjust crop load depending on the tree’s carbohydrate and nutrient levels. There is strong evidence to show that the carbohydrate status of the tree is critical to maintaining productivity.

The physiology of fruit drop was examined by measuring carbohydrate levels in stems and fruits. Results showed defoliation (leaf loss) led to a significant reduction in stored and soluble carbohydrates in the tree stem and seed coat. Low carbohydrate levels in the tree correlate with high abscission (fruit drop).

The seed coat also appears to play a fundamental role in regulating fruit growth and drop. Results showed that soluble carbohydrates are rapidly reduced in this tissue in response to defoliation.

These insights show managing fruitlet drop is key for increasing production. The team continue to focus research efforts on investigating fruit abscission, given its recent progress in this area, with the potential for crop management techniques to be developed in light of this research.

The project team has shed light on two factors that play a role in fruitlet abscission: carbon stores and water stress.

Carbon stores

To determine the role that carbon levels play in fruitlet drop, the researchers removed new vegetative growth from avocado trees and found that this induced a significant fruit drop event. As well as confirming the part that low tree carbon stores play, this ability to bring about fruit abscission will be useful in future trials.

Next steps are to learn more about the physiological changes in a tree that control fruit growth cessation, seed coat senescence and fruit abscission, so the project team is collecting fruit and shoot tissues at particular points in both treated and control trees for further analysis and comparison.

Once identified, these pathways will be incorporated into a model, which will serve as a knowledge base platform for developing new management tools to limit fruit abscission.

Water stress

The experiments also showed demonstrated the impact of water deficit on fruit abscission. The researchers found that in drought stress, fruit abscission doesn’t occur until trees are in severe water deficit and that fruit abscission is reversible in severe water stressed trees, which may prove useful in reducing fruitlet drop in the field.  

The field trial at Waikerie, SA, was completed, with the aim to develop a method to increase stored starch reserves in the trees during the winter months, but results will need to be repeated, as the trees in the trial were in an off-year with low crop load. The second year trial has begun, and the trees are carrying a significant crop so data on the effect of cytokinin applications on stored carbohydrates and yield should be forthcoming.

The team is continuing trials to establish the effect of heavy flowering on fruit set, as well as comparing rootstocks.

The project team report that experimental studies indicate that early fruit drop (fruitlet abscission) is caused by growth of the spring flush. Following this early fruit abscission event, immature fruit drop occurs during the summer, either in a wave or at a steady rate until the amount of fruit is stabilised on the tree.

Trials are ongoing to gain a better understanding of the dominance inhibition and/or competition between roots and shoots, to help deliver better management solutions to reduce the effect of shoot growth on fruitlet development. New trials for these studies have been established in South Australia since the project team’s last reporting to Hort Innovation.

The researchers are also continuing to look at the physiological mechanisms of early fruitlet abscission and summer fruit drop, with field trials in Western Australia.

Insights will be collected over the project period, before being collated and communicated to industry.

The project team have been continuing to bring together information on how reproduction events (including pollination, fertilisation, fruit set and fruit abscission) are regulated in avocado trees, and what the physiological mechanisms of resource competition and fruit growth cessation are. This information is what’s needed to allow key intervention points to be identified, to in turn develop effective management tools for limiting abscission and enhancing growth and development.

Preliminary field trials have taken place on a commercial orchard in Western Australia, looking at how pruning and application of fertilisation treatments (high nitrogen) affect the timing and degree of fruit abscission. Here, the team is looking at ways to induce substantial shoot growth, resulting in extensive fruit drop, to better understand the mechanisms at play.

The researchers are also looking at ways to manipulate stored carbohydrates in trees, to better understand the role of carbohydrates in resource competition. To this end, there is a field trial on a commercial orchard in South Australia, looking at how stored starch reserves can be increased. Starch accumulates over the winter months in the stems and woody tissues of avocado trees, and levels decline in spring as the reserves are mobilised for flowering, fruit set and growth, as well as the initiation of the spring flush. For this trial, treatments began in mid-April 2018 and continued through the winter months, with starch levels now being evaluated. The work will continue into future seasons.

In working towards tools and systems to address the physiological basis of high productivity, an established rootstock trial at Waikerie in South Australia is being used to investigate the role of rootstock/scion interactions in regulating tree yield, with information set to be published in an upcoming edition of the levy-funded Talking Avocados magazine.

As the project team report, fruit tree production is dependent upon mechanisms regulated by the nutrient status of the tree, as well as environmental cues. However, production is significantly limited by the fact that vegetative shoot growth coincides with fruit development, which results in resource competition for the carbohydrates, other nutrients and hormones necessary for maintaining growth.

This competition impacts on the early stages of fruit development, as shoot growth diverts resources, resulting in fruit growth cessation followed by abscission – a key event that limits avocado production.

The project has begun looking at relevant knowledge and knowledge gaps in this area, working with international experts and bringing together information from model plant systems and existing industry research to delve into how reproduction events are regulated (including pollination, fertilisation, fruit set and fruit abscission) and what the physiological mechanisms of resource competition and fruit growth cessation are.

This information is what is needed to allow key intervention points to be identified, to in turn develop effective management tools to limit abscission and enhance growth and development.

Field trials are upcoming, including those looking at factors affecting timing and degree of fruit abscission including pruning and fertilisation treatments.

In its course, the project will also be developing tools and systems to address the physiological basis of high productivity. It will begin using a rootstock trial to look at the role of rootstock-scion interactions in regulating yield, and will look to develop a method to manipulate stored carbohydrates to better understand the role of carbohydrates in resource competition.