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 varieties differ in fruit quality attributes as well as in ripening patterns. While maturation and ripening are driven by the plant hormone ethylene, what sets ethylene in motion to promote apple fruit maturation and ripening is unknown. ARS scientists studying apple maturation and ripening examined gene expression known to promote activity of another plant hormone auxin as well as ethylene. The goal was determine if interactions between auxin and ethylene exist that contribute to apple fruit maturation and ripening patterns. Results indicate expression of auxin and ethylene related genes during apple fruit maturation and ripening is consistent with auxin stimulating ethylene production necessary for maturation and ripening. The timing of these changes is consistent with the onset of ripening in multiple apple varieties. This information may contribute to development of pre- and post-harvest technologies to manipulate fruit quality and storability such that fruit edibility is maintained over long storage periods. This information could also have utility for apple breeders designing new apple varieties. Many apple cultivars develop disorders during storage due to low storage temperature. Symptoms such as browning of the peel or internal tissues, can take months to develop, but tissue injury occurs soon after harvest. The processes by which injury and browning occur are not understood, therefore, in 2012 ARS scientists conducted experiments that showed peel and internal browning provoked by low storage temperature is accompanied by increased amounts of unique compounds that could serve as indicators of low temperature injury. This finding supports the possibility of monitoring these apple fruit compounds during storage to assess risk for peel and internal browning and to predict disorder occurrence prior to development of symptoms. The information could contribute to the development of predictive and diagnostic tests that would aid industry in development of marketing strategies for particular fruit lots, and also to identify fruits injured by exposure to low temperature. Moisture loss during storage is implicated in a number of storage disorders of apples and is a limiting availability of fresh apples throughout the year. ARS scientists have previously demonstrated high humidity during the last 2-3 weeks before harvest results in the greatest moisture loss and lenticel breakdown during storage. In 2012 it was observed that ‘Gala’ apples with more lenticels had fewer lenticel-related disorders. Analyses of peel wax components from apples grown in dry vs. humid growing environments suggest there are no differences in the ratios of individual wax components, only in the amounts thereof. Moreover, cuticle thickening occurs on the fruit surface in addition to the walls surrounding cells, several layers beneath the epidermal layer as desiccation pressure increases. This information may contribute to development of pre- and post-harvest practices that enhance fruit resistance to moisture loss during storage and subsequently reduce development of physiological disorders that render apples unmarketable.
Varanasi, V., Mattheis, J.P., Rudell Jr, D.R., Zhu, Y., Shin, S.B. 2011. Expression profiles of MdACS3 gene suggest function as an accelerator of apple (Malus x domestica) fruit ripening. Postharvest Biology and Technology. 62:141-148.