Location: Physiology and Pathology of Tree Fruits Research
2020 Annual Report
Objectives
The long-term goal of this project is to develop various tools to assist in quality management of deciduous tree fruits. Specifically, during the next five years we will focus on the following objectives.
Objective 1: Integrate pre- and postharvest environment and commercial horticultural management practices with genomic and metabolomic regulation of apple and pear fruit quality.[C1; PS 1.A]
Sub-objective 1A: Determine how fruit position within the tree impacts pear metabolic profile, superficial scald, postharvest quality, and ripening.
Sub-objective 1B: Determine if inconsistent post-storage ripening of 1-
methylcyclopropene (1-MCP) treated d’Anjou pears is relatable to differential
absorbance (DA) value at harvest.
Sub-objective 1C: Determine if pre-storage light exposure impacts apple peel
metabolic responses to postharvest chilling.
Objective 2: Enable new apple, pear and sweet cherry fruit biomarker-based quality management strategies. [C1; PS 1.A]
Sub-objective 2A: Determine if apple aroma volatile production changes when fruit are stored in environments conducive to development of physiological disorders.
Sub-objective 2B: Develop biomarker-based risk monitoring protocols using existing validated gene expression and metabolic biomarkers for early detection of apple and pear peel and cortex storage disorders.
Sub-objective 2C: Determine if sweet cherry fruit pitting, cracking and browning is relatable to fruit epidermis and wax composition.
The two objectives both rely on metabolomic and genomic techniques to investigate field and postharvest factors that impact fruit quality. The link from sub-objective 2B to 1A reflects biomarkers identified in the previous project period to be validated for apple (2B) as well as applied initially for pear (1A, 2B).
Objective 1 is focused on enhancing knowledge of how field horticulture impacts postharvest fruit quality with emphasis on fruit physiological disorders and ripening. The sub-objectives (1A, 1B) are designed to generate new information regarding the impact of pear field horticulture on fruit quality and ripening metabolism, particularly disorder-related metabolomics and genomics.
Sub-objective 1C also is focused on generating disorder-related metabolomic information for apple fruit sun damage originating prior to harvest.
Application of metabolomic and genomic techniques to disorders arising in the postharvest environment is the basis for Objective 2. Can assessment of apple fruit volatiles accumulating during storage in environments known to cause disorders provide a means to avoid disorder development. (2A) Biomarkers identified for apple disorders in the previous project plan will be validated with multiple fruit lots and cultivars (2B). Disorder metabolism of sweet cherries (2C) will be explored using a metabolomic approach.
Approach
Fruit from commercial orchards will be harvested then stored at ARS-Wenatchee and in commercial CA rooms. Fruit quality, metabolites, and mRNA will be characterized at harvest and after storage using standard methods.
Hypothesis 1A: Fruit position on the tree directly impacts maturation, superficial scald susceptibility, ripening and storability, and associated metabolism. Pears from two extreme light environments within the tree canopy will be grouped based on differential absorbance (DA). Fruit quality and disorders will be assessed at harvest and after storage. Metabolites and mRNA in peel collected from each canopy location/DA group will be analyzed. Results will be mined for metabolites and mRNA associated with physiological disorders.
Hypothesis 1B: Inconsistent post-storage ripening of 1-MCP treated d’Anjou pears is relatable to differential absorbance (DA) value at harvest. Pears will be exposed at harvest to 1-MCP for 16 hours, then stored at 1 degree C. After storage fruit will be evaluated for disorders and fruit quality characterized.
Hypothesis 1C: Apple peel metabolism following cold storage imposition is altered by pre-harvest light exposure. ‘Granny Smith’ apples exposed to direct sunlight will be harvested and sorted by sun damage. Apples will be stored at 1 degree C and after storage, untargeted metabolic profiling of ~800 metabolites will be performed on peel and cortex tissue collected at each sampling date. Multivariate and univariate statistical approaches will be employed to link changes in specific areas of metabolism with sunscald and superficial scald development.
Hypothesis 2A: Apple aroma volatile production changes when fruit are stored in environments conducive to development of physiological disorders. ‘Honeycrisp’ apples stored in controlled atmosphere chambers will be subjected to atmospheres known to cause physiological disorders. Volatile compound samples will be collected after various intervals and after 180 days, fruit will be removed from storage and evaluated for incidence and severity of external disorders.
Hypothesis 2B: Metabolic and gene expression superficial scald based risk assessment can be used to indicate when scald risk in a storage room is elevated.‘Granny Smith’ and ‘Delicious’ apples, and Anjou pears will be stored in CA at 1 degree C. Superficial scald will be evaluated following various lengths of storage. Storage atmospheres will be evaluated for volatile compounds determined to be useful for scald risk assessment.
Hypothesis 2C: Pitting and cracking incidence of sweet cherries is associated with altered epidermal metabolic profile compared with undamaged fruit. ‘Rainier’ sweet cherries will be subjected to uniform bruising using a steel ball dropped onto the fruit. Micro-cracking and stomata/lenticel number will be estimated by staining fresh whole fruit with acridine orange and counting micro-cracks at five random positions on each fruit using florescence microscopy. Waxes extracted from fruit will be analyzed using HPLC-QTOF-MS. Metabolic differences linked with cracking and epidermal defects will be identified using untargeted metabolic profiling methods developed for apple.
Progress Report
This is a final progress report for 2094-43000-007-00D, "Developmental Genomics and Metabolomics Influencing Temperate Tree Fruit Quality," which has been replaced by new project, 2094-43000-008-00D,"Enhancement of Apple, Pear, and Sweet Cherry Quality." For additional information, please refence the new project report.
Research was conducted on Objective 1 to determine if inconsistent post-storage ripening of 1-methylcyclopropene (1-MCP) treated d’Anjou pears is relatable to differential absorbance (DA) value, a measure of peel components at harvest. Experiments with ‘d’Anjou’ and ‘Bosc’ pears did not support the hypothesis that DA value at harvest could be useful as a predictor of post-storage ripening of 1-MCP treated pears. Pears were sorted at harvest using a DA meter into four classes representing less to more peel chlorophyll content. Half the fruit was treated with the ripening inhibitor 1-MCP, then all fruit was stored at 31 degrees F for six or nine months. While a differential in pear firmness, as an indicator of ripeness, was observed after cold storage due to 1-MCP treatment at harvest, no relationship was observed due to DA class at harvest. With both pear varieties, DA value at harvest did not consistently provide a reliable predictor of ripening for 1-MCP treated fruit after cold storage. Research was also performed under Sub-objective 1C to determine if pre-harvest light exposure impacts how apple peel metabolism responds to postharvest chilling. Differences in peel composition relatable to pre-harvest light exposure and chilling were detected and patterns were similar for four different varieties (‘Fuji’, ‘Gala’, ‘Granny Smith’, ‘Honeycrisp’). Specific compounds including peel compounds and expressed genes were identified after chilling in asymptomatic fruit on which sunscald ultimately developed. These compounds and expressed genes have potential as biomarkers to indicate sunscald development risk. Fruit treatment with surfactants before sunscald symptoms developed reduced symptom severity indicating the potential of a postharvest treatment to reduce or prevent delayed sunscald.
Research was conducted on Objective 2 to develop biomarker-based risk monitoring protocols using existing validated gene expression and metabolic biomarkers for early detection of apple and pear peel and flesh storage disorders. Previously identified superficial scald (scald) risk assessment biomarkers proved effective in validation studies. Using fruit with scald risk identified using biomarkers, controlled atmosphere storage conditions were identified that may allow for post-storage scald prevention treatment using temperature or chemical treatments. These treatments, exposure to hot water, or the ethylene action inhibitor 1-MCP, were effective when applied after months of storage in a cold storage room with less than 1% oxygen. These new findings suggest cold chain management strategies for reducing or eliminating superficial scald from both crop protectant restricted and conventional apple cold chains. We also identified gene activity that may be useful to estimate sufficient warming time that allows the fruit to recover from the chilling injury and avoid superficial scald. Continued activity to determine if sweet cherry postharvest disorders are relatable to fruit epidermis and wax composition included treatments to “repair” peel microcracks that previously were shown to be related to tissue browning developing during cold storage. Although the treatments, cooling temperature protocols and/or fruit coating application, resulted in microcrack healing, no treatment reduced tissue browning.
Accomplishments
1. Improving pear quality by identifying metabolites associated with ripening consistency. Pear fruit quality inconsistency can contribute to significant annual losses due to spoilage. This variability can be detected in the natural pear chemistry affecting flavor, appearance, and texture. Changes in chemistry revealed sun exposure may be a prominent source of inconsistency based on fruit on-tree position. Differences in natural fruit chemistry and final product following storage indicates this variability would likely impact every cold chain management decision. Findings indicate that sorting of pears before they enter the cold chain potentially on the basis of levels of specific natural peel chemicals will enhance fruit quality consistency.
2. Reducing apple cold chain losses caused by excess sun exposure. Exposure to excessive sunlight can contribute to 10-25% annual losses due to multiple disorders in many of the world’s most productive apple growing regions including the western United States. Our results demonstrate that heat events in the orchard contribute to sunscald development, a common source of postharvest losses of susceptible apple varieties. Results reveal physical changes to apple peel surface that may confer resistance to solar stress that leads to sun related postharvest disorders. We identified novel peel chemicals associated with risk of delayed sunscald as well as fruit quality and ripening. Monitoring amounts of these compounds may have potential to non-destructively sort apples according to sunscald risk at harvest. This information would allow producers to store fruit with low sunscald risk and to sell high-risk fruit before the disorder develops soon after harvest.
3. New management strategies for apple superficial scald control. Superficial scald (scald), a postharvest browning disorder of apple peel, continues to contribute to annual fruit quality loss, especially in markets where residues of scald control compounds are restricted. We have identified gene activity that, after the chilling injury that causes scald occurs, may be useful to define storage management practices to prevent scald. Our results reveal that chilling injury is cumulative and preventable with optimal controlled atmosphere (low oxygen, high carbon dioxide relative to air) storage conditions started within the first week following harvest. We have demonstrated that post-storage crop-protectant or hot water treatment effectively controls scald if the chilling injury that induces scald has been controlled. Monitoring levels of natural compounds in fruit associated with scald risk may indicate if this criterion is met and post-storage treatments will be effective. These findings indicate novel strategies for reducing or eliminating scald from both crop protectant restricted, and organic cold chains are possible using existing commercial technologies.
4. ‘Honeycrisp’ apple bitter pit is reduced by 1-MCP and/or short-term controlled atmosphere (CA) storage. The apple physiological disorder bitter pit is an unsightly cosmetic defect on the fruit surface which results from several factors existing in orchards prior to harvest. Bitter pit symptoms often do not arise until the fruit have been harvested and placed into cold storage. Bitter pit prevention typically relies on application of calcium sprays and crop load management prior to harvest. ARS scientists collaborating with Washington State University scientists in Wenatchee, Washington, conducted studies to evaluate postharvest technologies as an additional means to reduce bitter pit development. Apples were exposed to the ripening inhibitor 1-methylcyclopropene (1-MCP) and/or stored in a CA with low oxygen and high carbon dioxide content relative to air for 1 or more weeks beginning within 2 days of harvest. Both 1-MCP and CA of 1 week or more reduced bitter pit in most orchard lots with the combination of 1-MCP then CA providing the best bitter pit prevention. This postharvest prevention protocol can be readily adopted by commercial producers as all technology necessary is currently in place in most apple warehouses having CA storage rooms. The efficacy for bitter pit reduction of a week of CA immediately after harvest may allow producers to reduce disorder potential while allowing fruit to be marketed early in the harvest season.
5. Knowledge of gene expression differences among tree fruit varieties enhances efforts to discover and utilize genes as biomarkers. One way to enhance postharvest tree fruit quality is to apply existing technology with greater precision and accuracy using biomarkers – fruit compounds that predict future fruit quality. ARS scientists in Wenatchee, Washington, in collaboration with scientists at Washington State University, the Pennsylvania State University, and the California State University, have conducted studies to identify new fruit quality biomarkers which are in this case genes. The research identified genes that are unique among fruit varieties of the same type, for example genes that make ‘d’Anjou’ different from ‘Bartlett’ pears and ‘Granny Smith’ different from ‘Golden Delicious’ apples. Scientists can now also explore larger-scale genetic differences that can help explain differences in ripening among some apple and pear varieties, information that may be useful in designing modified storage protocols specific for the needs of each type of apple and pear.
Review Publications
Poirier, B.C., Mattheis, J.P., Rudell Jr, D.R. 2019. Extending ‘Granny Smith’ apple superficial scald control following long-term ultra-low oxygen controlled atmosphere storage. Postharvest Biology and Technology. 161. Article 111062. https://doi.org/10.1016/j.postharvbio.2019.111062.
McTavish, C.K., Poirier, B.C., Torres, C.A., Mattheis, J.P., Rudell Jr, D.R. 2020. A convergence of sunlight and cold chain: The influence of sun exposure on postharvest apple peel metabolism. Postharvest Biology and Technology. 164. Article 111164. https://doi.org/10.1016/j.postharvbio.2020.111164.
