How bushfires affect apple productivity: preliminary cues from simulated injury

Published on May 17, 2023

Research & Extension

How do bushfire injuries to apple trees affect carbohydrate reserves, fruit development and return flowering, and does the site of the injury play a part in fruit productivity and tree recovery? 

The Australian apple industry was severely affected by bushfires in the 2019–20 growing season, with long-term production losses being an unfortunate consequence. Following the fires, the Developing management strategies to enhance the recovery of horticulture from bushfires (AS19002) project was established between the NSW Department of Primary Industries and the SA Department of Primary Industries and Regions with support from Hort Innovation.  

As part of this project, and with additional support from Charles Sturt University, a study was conducted to simulate the effects of bushfires on apple tree carbohydrate reserves, fruit development and return flowering. The aim of the study was to improve our understanding of how fire injury of the trunk and canopy may affect fruit productivity, and what the implications are for tree recovery. 

Bushfires generally cause immediate crop losses but may also reduce apple productivity in subsequent seasons following damage to the permanent structure of trees. Bushfires are commonly classified as crown or surface fires (Wiese et al. 2018). Crown fires spread through tree canopies, resembling a blowtorch, whereas surface fires involve the burning of vegetation near the ground, often imitating a slow-cooker. Fire damage can impair apple tree functioning in various ways, including damage to the phloem vascular system and loss in leaf area (Bär et al. 2019). As a result, trees may exhibit diminished photosynthesis in addition to restricted sugar transport. Such limitations in carbohydrate supply may reduce fruit quality in the current season and cause restricted reproductive and vegetative development the following spring (Breen et al. 2020).  

To better understand some of the effects of fire injury on apple production, defoliation treatments were imposed during fruit maturation to simulate fire-induced leaf loss (Figure 1). On 2 February 2022, approximately three months prior to harvest, treatments were implemented on mature Pink Lady apple trees, grown on the NSW Department of Primary Industries research station in Orange, NSW. 

Defoliation of the top third of the canopy, simulated blowtorch fire injury, while defoliation of the bottom third of the canopy, in conjunction with trunk girdling, simulated slow-cooker damage to phloem in addition to loss of basal leaf area. No alterations were made to the control trees.  

The treatments were used to assess repercussions for fruit development, including fruit numbers and composition, and to assess how carbohydrate reserves in woody tissues were affected. 

Fruit development

At harvest, the average weight per apple did not significantly differ between the control trees and the ones that received treatments, nor did the number of apples present per tree. However, the following spring, trees that were previously subjected to simulated blowtorch injury exhibited 53 and 38 per cent less flowers at the top third of the canopy compared to the middle and bottom sections, respectively (Figure 2). In contrast, on trees with simulated slow-cooker injury, a reduced number of flowers were present at the bottom and middle parts of the canopy compared to the top; by 58 and 68 per cent, respectively. A loss in leaf area may curtail the replenishment of carbohydrate reserves in the perennial structure, often in a localised fashion, meaning woody tissues near the zone of defoliation may enter the next season with low reserves (Sprugel et al. 1991). Consequently, flowering may be impeded since remobilised carbohydrates are needed for spring reproductive development including flower formation (Breen et al. 2020). 

Fruit starch concentrations reduced rapidly during the final seven weeks of apple maturation, concurrent with an increase in sugars (data not shown). At harvest, in the middle of the canopy, fruit from the control trees and trunk girdled trees exhibited greater starch levels than those subjected to any form of defoliation, suggesting defoliation exacerbates starch depletion during late ripening. Furthermore, defoliation treatments negatively affected fruit sugar levels by harvest, with proximity to the zone of leaf removal playing an apparent role (Figure 3). At the top and middle canopy sections, apples from the blowtorch treatment exhibited lower juice Brix levels than all other trees; 11 and 7 per cent lower than control fruit from each of those sections, respectively. At the bottom, fruit from the control and trunk girdled trees had greater juice Brix levels than apples from both the defoliation treatments. Here, blowtorch and slow-cooker injury resulted in a 7 and 8 per cent reduction in Brix, respectively, relative to the control. A localised detrimental effect of defoliation on fruit sugar accumulation therefore occurred. This may follow a reduction in the availability of sugars produced by the canopy, meaning less carbohydrates are available for allocation to fruit. Such localised effects suggest a degree of branch autonomy in terms of implications of carbohydrate depletion on fruit maturation.

Visual colour scoring of apples at harvest indicate blowtorch injury had a profound impact on the red colouration of fruit located at the top third of the canopy (data not shown). In fact, all relevant fruit exhibited a colouration score of greater than 80 per cent, a significantly higher proportion than other trees. At the bottom third of the canopy, slow-cooker treatment induced a greater proportion of fruit with more than 80 per cent red colouration than all other trees. A positive localised effect on red colouration of skins therefore occurred following defoliation. Light exposure is a known stimulus for anthocyanin accumulation in apples (Takos et al. 2006). Defoliation treatments had an apparent positive effect on light availability leading to greater skin red pigmentation. 

Carbohydrate reserves

By harvest, blowtorch injured trees exhibited a reduced level of root starch relative to the control trees (Figure 4). Concurrently, blowtorch injury induced higher levels of root soluble sugars. Root sugar accumulation in conjunction with starch depletion may relate to the remobilisation of carbohydrate reserves to support fruit development when leaf area is insufficient (Rossouw et al. 2017). Furthermore, in terminal growth units of fruiting shoots collected from the middle of the canopy, control and trunk girdled trees had higher levels of starch, compared to blowtorch and slow-cooker treatment trees, by harvest. Defoliation therefore appeared to limit reserve accumulation in fruiting shoots by harvest, likely contributing to the reduced flowering observed the next spring. 

Take-home messages 

• Loss in leaf area during fruit maturation following bushfire injury is likely to impair apple fruit composition and return flowering. 
• Damage to trunk vascular tissues, similar to girdling, appears less damaging, at least if injury is moderate. 
• Blowtorch-type fire injury could be particularly detrimental by restricting the storage of carbohydrate reserves. 
• Negative defoliation effects on fruit sugar levels appear somewhat localised and may be related to reduced carbohydrate availability from shoots, suggesting a degree of branch autonomy. 
• Loss in leaf area may stimulate fruit skin colour development due to increased light exposure. 

Acknowledgements 

This work was supported by funding from the Faculty of Science and Health, Charles Sturt University, and associated with the Developing management strategies to enhance the recovery of horticulture from bushfires (AS19002) project, funded by the Hort Frontiers Advanced Production Systems Fund, part of the Hort Frontiers strategic partnership initiative developed by Hort Innovation, with co-investment from the NSW Department of Primary Industries and the South Australian Research and Development Institute (SARDI) and contributions from the Australian Government. 

References 

Bär A, Michaletz ST and Mayr S (2019) “Fire effects on tree physiology”, New Phytologist, 223:1728–1741.  

Breen K, Tustin S, Palmer J, Boldingh H and Close D (2020) “Revisiting the role of carbohydrate reserves in fruit set and early-season growth of apple”, Scientia Horticulturae, 261:109034.  

Rossouw GC, Orchard BA, Šuklje K, Smith JP, Barril C, Deloire A and Holzapfel BP (2017) “Vitis vinifera root and leaf metabolic composition during fruit maturation: implications of defoliation”, Physiologia plantarum, 161:434–450.  

Sprugel DG, Hinckley TM and Schaap W (1991) “The theory and practice of branch autonomy”, Annual Review of Ecology and Systematics, 309–334.  

Takos AM, Jaffé F W, Jacob SR, Bogs J, Robinson SP and Walker AR (2006) “Light-induced expression of a MYB gene regulates anthocyanin biosynthesis in red apples”, Plant physiology, 142:1216–1232.  

Weise DR, Cobian-Iñiguez J and Princevac M (2018) “Surface to crown transition”, Encyclopedia of wildfires and wildland-urban interface (WUI) fires, ed. SL Manzello, 1–5, Cham: Springer International Publishing, doi:10.1007/978-3-319-51727-8_24-1. 

This article was first published in the Autumn 2023 edition of AFG.

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