1a. Objectives (from AD-416)
The long-term objective of this project is to develop tools to assist in marker-assisted selection and quality management of deciduous tree fruits. Over the next 5 years this program will focus on the genetic, metabolic and physiological mechanisms underlying fruit texture, aroma, and physiological disorders (Appendix 1) based on the following objectives: Objective 1: Identify genetic factors regulating apple cultivar-specific fruit texture and aroma production. Objective 2: Characterize the physiological basis for external necrotic disorders and identify genetic and/or metabolic markers associated with disorder development. •Sub-objective 2.a. Discover biomarkers that predict, diagnose, and/or distinguish apple superficial scald and soft-scald. •Sub-objective 2.b. Validate prospective biomarkers that predict, diagnose, and/or distinguish apple superficial scald and soft scald. Objective 3: Determine how environmentally-induced changes in apple fruit physiology affect onset of physiological disorders.
1b. Approach (from AD-416)
Research to elucidate genetic regulation of apple fruit texture and aroma will utilize established molecular analytical techniques. All analyses will use resources in place in the research unit. The apple germplasm to be utilized is available via an ongoing collaboration with the Washington State University Apple Breeding Program. This includes reference cultivars as well as a large cross population specifically established for studies related to fruit texture. Metabolomic studies will utilize fruit obtained from commercial and research orchards with chemical analyses conducted using location resources. Chemical analyses (GC- and LC-MS) will be performed using established methods where available with new methods developed as needed. Fruit will be stored using the location’s cold storage and controlled atmosphere facility which consists of 140-2 bushel chambers in which N2, CO2, and air are manipulated to achieve desired proportions in each chamber. Individual chamber temperature control is available for 20 chambers. Field experiments will be primarily conducted in commercial orchards to allow microclimate effects to be evaluated under a range of geographic and horticultural management conditions. Fruit microenvironment prior to harvest will be manipulated using polyethylene bags. Evaluation of fruit epidermis by SEM will utilize equipment in place in the research unit.
3. Progress Report
Apple fruit texture is a critical determinant of commercial success for any variety. Understanding the genetic control of textural properties during fruit ripening will facilitate the development of tools breeders can use to develop new apple cultivars with superior texture attributes. During 2011, ARS scientists at the Tree Fruit Research Laboratory conducted genetic and physical analyses of fruit produced by trees bred from crossing two cultivars, ‘Honeycrisp’ and ‘Pink Lady’ which have distinct ripening behavior and texture attributes including crispness and firmness. Genetic analysis indicated that a number of genes that function in metabolism of plant hormones express differentially in relation to ripening time, firmness, and fruit size. Results of detailed genetic analyses may provide a means to increase efficiency of the apple breeding process. Many apple cultivars can develop internal browning if fruit are exposed to high levels of carbon dioxide after harvest. Symptoms can take months to develop, but tissue injury occurs soon after harvest. The processes by which injury and browning occur are not well understood, therefore, in 2011 ARS scientists conducted experiments that showed internal browning provoked by high carbon dioxide during storage is accompanied by increased amounts of several unique volatile compounds that could serve as indicators of CO2-induced injury. This finding raises the possibility of monitoring volatile compounds in apple storages to assess risk for CO2-induced internal browning and to predict disorder occurrence prior to development of symptoms. Moisture loss during apple fruit storage is a significant factor limiting fruit consumer acceptance and industry profitability. During 2011, ARS scientists conducted a series of experiments in an orchard with sub-soil irrigation (very low ambient water vapor) in which humidity surrounding individual fruit was increased to 100% for 1, 2, or 3 weeks each month beginning 5 months before harvest. Assessment of fruit quality after cold storage indicated 1) high humidity during the last 2-3 weeks of fruit development prior to harvest caused the greatest weight loss (shrivel) in storage, and 2) 3 or more days at high humidity 1 week before harvest confers the greatest vulnerability to fruit quality loss.
1. Apple fruit superficial scald is a postharvest disorder that can result in significant losses of fruit after storage. Strategies to avoid or predict this would contribute to orderly marketing and industry profitability. From 2009 through 2011, ARS scientists identified key apple fruit metabolites associated with storage conditions leading to superficial scald development. The research revealed a link between several unique compounds and chilling stress that leads to scald development. Measurement of these compounds may be valuable indicators of chilling stress in cold stored apples and could serve as predictors of disorders in advance of symptoms.
Felicetti, E., Mattheis, J.P., Zhu, Y., Fellman, J.K. 2011. Dynamics of ascorbic acid in ‘Braeburn’ and ‘Gala’ apples during on-tree development and storage in atmospheres conducive to internal browning development. Postharvest Biology and Technology. 61:95-102.