This article appears in the Winter 2019 edition of Talking Avocados (Volume 30 No 2).

By Amnon Haberman and Harley M. Smith, CSIRO Agriculture and Food

Avocado is a low yielding tree crop with average annual production levels equivalent to approximately 10t/ha, which is considerably lower than the theoretical value of 32.5t/ha, as estimated by Wolstenholme 1987. Low yields are attributed to the semi-domesticated nature of avocado¹, due to unfavourable traits including vigorous shoot growth, excessive flowering, low fruit set and high fruit abscission^2,3. In addition, avocado has a high propensity for alternate (biennial) bearing⁴. The predicted rise in global temperatures will likely enhance these traits and further reduce annual yields⁵. Together, the additive effects of these yield-associated traits limit production and present a major challenge for Australian orchard management for maximising yields and reducing seasonal variation.

Challenges in Australian avocado production

Yield associated traits are controlled by the interaction between the genetics, climate and management inputs⁶, as well as the age of the tree. To increase Hass yields, new management tools must be developed to reduce the impact of excessive vigour, low fruit set, high fruit abscission and biennial bearing. To achieve this, it is necessary to have a basic understanding of the physiology that drives these yield associated traits. Fruit abscission is a central component controlling fruit production and this process is poorly understood in avocado, as well as other fruit tree crops. Therefore, this article is focused on fruit drop, which is the major aim of the Hort Innovation funded project, AV16005.

Early fruit abscission

During the early fruit abscission event, a majority of fruitlets abscise within the first five weeks after fruit set^7,8. The initial phase of the early fruit drop event is due to the abscission of unfertilised fruitlets⁷. The later phase of this fruit abscission event involves the abscission of fertilised fruitlets, typically between 6-10mm in size. Growers estimate that approximately 30-50% of the fertilised fruitlets drop during the early fruit abscission event.

Interaction between the spring flush and developing fruitlets

Due to the coincidence of vegetative and reproductive growth in the spring, it has been proposed that the early fruit abscission event is mediated in part by the growing spring flush, which competes with the developing fruitlet for photosynthates and nutrients (reviewed by Salazar-García et al. 2013). In support of this hypothesis, Salazar-Garcia and Lovatt (1998) reported that ‘functionally determinate’ inflorescences are three times more productive than indeterminate inflorescence shoots. Paclobutrazol and uniconazole are growth retardants that reduce stem elongation and leaf expansion via inhibition of gibberellin biosynthesis. As elongating stems have a high growth potential, an increase in yield by applications of paclobutrazol at flowering was associated with an augmentation in the number of fruits^11,12. Applications of paclobutrazol also increased fruit size, which also contributes to higher yields¹³. However, other reports showed that applications of paclobutrazol, as well as uniconazole, at flowering did not increase yield^14-16. In addition, studies showed that removal of the spring flush or applications of paclobutrazol increased fruit set; however, yield was not increased due to a heavy fruit drop during the summer in the treated trees^17,18. The discrepancy of the effect of paclobutrazol and/or uniconazole on fruit drop and yield demonstrates the underlying complexity of the early fruit abscission event and mechanism(s) the mediates resource (carbohydrates) distribution to actively growing tissues in the tree.

Challenges for studying the early fruit abscission event

One of the major challenges for studying the early fruit abscission event is the low rate of fruit set followed by a high rate of fruitlet drop (reviewed by Salazar-García et al. 2013). The combined effect of these two traits severely reduces the ability to directly compare developmental profiles between retained and abscising fruitlets. This is extremely important, as studies in model plant systems, show that early fruit development is marked by massive changes in fruit physiology, including hormone signalling and gene expression^19,20. Therefore, if retained and abscising fruits are not collected at the same developmental age, then it becomes extremely difficult to compare the physiological differences in order to identify the key factors that mediate abscission. Moreover, this hindrance also obstructs the ability to effectively study the interaction between the vegetative flushes and developing fruits. However, a basic understanding of the physiological basis of the summer fruit drop event will likely apply to the early fruit abscission event.

Summer fruit drop

The integration of genetic determinants, climatic events and management practices has impact on tree physiology and resource (carbohydrates) availability. As tree carbohydrate levels are essential for growth, the adjustment of crop load in response to resource availability is hypothesized to be a major factor that regulates the summer fruit drop event ^3,21. Therefore, understanding how tree crop load is adjusted in response to resource availability and the physiological mechanism(s) that mediate fruit abscission may provide the knowledge required to develop new management tools to reduce fruit drop and increase production. Moreover, new management tools aimed at reducing the summer fruit drop will likely be effective for reducing the early fruit drop event.

Role of seed coat in fruit abscission

Experimental studies demonstrate that seed development is required for fruit retention and development^7,8. The seed coat is the maternal component of the seed, which functions to provide the embryo with photosynthates and nutrients required for growth²². Moreover, the seed coat also synthesises plant growth regulators/hormones critical for regulating embryo development (reviewed by Bower and Cutting 1988; Robert et al. 2018). Interestingly, seed coat senescence is an observable characteristic associated with abscising fruits^8,25 (Figure 1). Therefore, the seed coat function appears to be a critical tissue that determines the fate of a fruit, retained versus abscised.

Model of fruit abscission

We have developed a model to explain fruit abscission in avocado. In this model, a subset of fruit in a tree undergoes abscission in response to a resource availability signal(s). As pointed out above, the physiology of the tree is speculated to be a key determinant of fruit drop (illustrated in Figure 2). In addition, competition for resources between fruits and with shoots also drive fruit drop. At this time, the nature of this resource availability signal(s) is unknown. The fruit abscission event is viewed as a multistep process in which the resource availability signal(s) mediate fruit growth cessation.

Given that the seed coat plays a major role in fruit development and senescence of this tissue is associated with abscission, it is highly likely that seed coat mediates the cessation of fruit growth. After growth cessation, the seed coat undergoes senescence and the abscission zone is activated in the pedicel, which leads to the physical separation of the fruit from the tree. Therefore, the primary event of fruit abscission is fruit growth cessation, while the secondary event involves the process that mediates fruit drop. Based on this model, fruit abscission can only be reversed during fruit growth cessation. Once seed coat senescence is initiated, the cessation of fruit growth cannot be reversed. Therefore, in order to develop new tools to limit fruit abscission, an understanding of the physiological basis of fruit growth cessation is required.

The AV16005 Hort Innovation funded project

The primary aim of the AV16005 Hort Innovation funded project is to study the physiology of fruit growth cessation, as well as seed coat senescence and fruit abscission. However, we are lacking the capability to distinguish fruits fate to develop to maturity from fruits targeted for abscission, during early stages of fruit growth cessation. To overcome this problem, trials were performed to identify treatments that would induce a massive fruit drop event by limiting carbohydrate availability. Results from these trials showed that extensive removal of new vegetative growth promotes a massive fruit drop event. Using this approach, different fruit tissues, as well as pedicels and stems, were collected at regular time intervals from treated and control trees.

We are currently analysing the tissues using analytical and molecular methods to identify candidate hormones, metabolites, carbohydrates and genes that correlate with fruit growth cessation. This information will be integrated and used to construct the physiological and developmental pathways that mediate fruit growth cessation. Finally, these pathways will be incorporated into the model above, which will serve as a knowledge base for developing new management tools to limit fruit abscission.

We acknowledge and thank the contribution of Jasper Farms (WA), Delroy Orchards (WA), Chinoola Orchards (SA) and Thiel Orchards (SA) to the project and technical assistance from Jacinta Foley (Jasper Farms) and Declan McCauley (WA DPIRD).

 Acknowledgement

The Maximising yield and reducing seasonal variation (AV16005) project has been funded by Hort Innovation, using the Avocado research and development levy and contributions from the Australian Government.

 

 

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This article was prepared for the Winter 2019 Talking Avocados magazine.