Metabolic fingerprint of ‘WA 38’ green spot symptoms reveals increased production of epicuticular metabolites by parenchyma

Authors: SHEICK, RYAN - Washington State University; SERRA, SARA - Washington State University; MUSACCHI, STEFANO - Washington State University; Rudell, David

Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2023
Publication Date: 6/19/2023
Citation: Sheick, R., Serra, S., Musacchi, S., Rudell Jr, D.R. 2023. Metabolic fingerprint of ‘WA 38’ green spot symptoms reveals increased production of epicuticular metabolites by parenchyma. Scientia Horticulturae. 321. Article 112257. https://doi.org/10.1016/j.scienta.2023.112257.
DOI: https://doi.org/10.1016/j.scienta.2023.112257

Interpretive Summary: ‘WA 38’ is a recently released apple cultivar that is noted for its lack of susceptibility to developing most common browning disorders of the peel and cortex. However, it can develop high incidences of a disorder specific to the cultivar called “green spot” during the growing season. Symptoms of this disorder begin as superficial discoloration of the peel and can worsen to corking of the flesh immediately under the discolored peel surface. The symptoms can lead to a downgrade of the fruit or outright loss if the symptoms reach the flesh. The causes have yet to be determined and mitigation strategies are under investigation. We investigated the natural chemical profile of peel and flesh symptoms to determine the chemical fingerprint and then, followed levels of these chemicals during fruit development as influenced by reducing or nearly eliminating sunlight, a practice that showed promise for mitigating the disorder. We determined that symptomatic peel and cortex was chemically different compared with healthy tissues. These chemicals are associated with stress response but also may point to a process where the flesh tissue develops barriers, similar to those found in peel, as symptoms become more severe.

Technical Abstract: Green spot is a pre-harvest physiological disorder of ‘WA 38’ apple fruit that can culminate in corking immediately underneath the peel surface. Unlike bitter pit and many other corking disorders, superficial symptoms emerge in the middle of the growing season as areas of peel discoloration, with the most severe symptoms appearing as parenchyma necrosis or corking that worsens until harvest. Our previous work demonstrated the mitigative effect of early season fruit bagging on green spot appearance, but the mechanism of its onset and the biochemical signatures associated with it remained unknown. We used metabolic profiling to first, identify metabolites associated with green spot symptoms in the peel and cortex of ‘WA 38’ apples and then, track changes before and after disorder onset until harvest from apples that were bagged, under a shade net, or uncovered (control). We linked metabolites from multiple metabolic pathways, including carotenoids, phenolics, triglycerides, and triterpenes, with peel and cortex green spot symptoms. Many pigment and triterpene compounds associated with cortex symptoms are typically far less abundant in cortex tissue than in peel. Light-altering treatments also impacted levels of several of these compounds during the growing season; however, the inability to predict which pre-symptomatic tissue would later develop symptoms compromised the establishment of disorder genesis and latent development using this technique. Despite these challenges, we linked novel metabolites and pathways with the disorder, including those that may indicate strengthening and isolation of symptomatic tissue, possibly to provide protections similar to the fruit surface.