Interested Members of the University Community are invited to attend the Final Oral Examination for the Degree of Doctor of Philosophy of Karthika Sriskantharajah of the Department of Plant Agriculture

DATE: Monday, December 13
TIME: 10.30 am
LOCATION: Room 307, Crop Science Building for those who are students/faculty of the University of Guelph, or on Zoom for those who are unable to attend in person.

Join Zoom Meeting: https://zoom.us/j/99716303049?pwd=dkRUcE0wd1RLOUY5OWdkaVRrU3hZQT09#success
Meeting ID: 997 1630 3049
Passcode: 432278

Thesis Title: “Enhancement of Fruit Retention and Postharvest Quality in ‘Honeycrisp’ Apples (Malus domestica Borkh) using Hexanal”

Advisory Committee
Dr. Jayasankar Subramanian, Advisor
Dr. Gopinadhan Paliyath
Dr. Alan Sullivan
Dr. Evan Fraser

Examination Committee
Dr. John Cline, Chair
Dr. Sadanand Dhekney, External Examiner
Dr. Praveen Saxena
Dr. Gopinadhan Paliyath
Dr. Jayasankar Subramanian
                                     

ABSTRACT

‘Honeycrisp’ (Malus domestica Borkh), a premium apple cultivar, is prone to preharvest fruit drop (PFD), bitter pit (BP) and decline in quality during long-term storage. To enhance the fruit retention and storage quality in ‘Honeycrisp’, an aqueous composition containing hexanal was applied as a preharvest spray. Hexanal treated trees retained 18% more fruit compared to control trees. Fruit firmness was significantly improved by hexanal while fresh weight, and total soluble solids (TSS) did not vary in response to the treatment at the field. Abscisic acid (ABA) in the fruit abscission zone (FAZ) was substantially reduced by hexanal at commercial maturity. At this stage, a total of 726 differentially expressed genes (DEGs) were identified between treated and control FAZ. Functional classification of the DEGs showed that hexanal downregulated ethylene biosynthesis genes (SAM2, ACO3, ACO4, and ACO4-like), while it upregulated the receptor genes (ETR2 and ERS1). Genes related to ABA biosynthesis (FDPS and CLE25) and signaling were also downregulated. Further, hexanal downregulated the expression of genes related to cell wall degrading enzymes, including polygalacturonase, glucanase, and expansin. These findings revealed that hexanal reduced the sensitivity of FAZ cells to ethylene and ABA. Simultaneously, hexanal reduced the cell wall degradation of FAZ cells by regulating genes involved in cell wall modifications. Thus, delayed fruit abscission by hexanal in ‘Honeycrisp’ is most likely achieved by minimizing ABA through an ethylene-dependent mechanism. Further investigation of effects of hexanal on long-term storage quality in ‘Honeycrisp’ revealed that treated apples had higher TSS under both cold and cold followed by room temperature storages. In addition, treated apples had lower incidence, and progression of BP, lower phospholipase D (PLD) enzyme activity and decreased expression of MdPLDα1, MdPLDα4, MdCaM2, MdCaM4 and MdCML18 genes. Therefore, improved postharvest quality, including the lower incidence of BP in hexanal treated ‘Honeycrisp’ apples may be associated with lower membrane damage due to downregulation of PLD. Thus, hexanal application promises to be a great technology to control fruit drop, reduce bitter pit, enhance fruit qualities, and marketability in ‘Honeycrisp’ apples, given that this cultivar is categorized as susceptible to fruit drop and bitter pit.

PUBLICATIONS:

Sriskantharajah, K, El Kayal, W, Torkamaneh, D, Ayyanath, MM, Saxena, PK, Sullivan, AJ, Paliyath, G, Subramanian, J. (2021). Transcriptomics of Improved Fruit Retention by Hexanal in 'Honeycrisp' Reveals Hormonal Crosstalk and Reduced Cell Wall Degradation in the Fruit Abscission Zone. International Journal of Molecular Sciences, 22(16), 8830. https://doi.org/10.3390/ijms22168830  

Sriskantharajah, K, El Kayal, W, Ayyanath, MM, Saxena, PK, Sullivan, JA, Paliyath, G, Subramanian, J. (2021). Preharvest spray hexanal formulation enhances postharvest quality in 'Honeycrisp' apples by regulating phospholipase D and calcium sensor proteins genes. Plants, 10(11), 2332; https://doi.org/10.3390/plants10112332