Dissecting the influence of the orchard location and the maturity at harvest on apple quality, physiology and susceptibility to major postharvest pathogens

Highlights

• Altitude affected the flesh composition and ethylene production rate in ‘Golden’ apples.

• Differences in ethylene production rate among orchards was associated to differences in ACC.

• Enhanced ethylene production in fruit from late harvests was linked to increased ACO activity.

• Flesh composition had little influence on the susceptibility to postharvest fungal pathogens.

Abstract

‘Golden Reinders’ apple quality parameters, fruit physiology, biochemical composition and susceptibility to Penicillium expansum and Rhizopus stolonifer were analysed in fruit harvested from four different locations (two valley and two mountain orchards) and from the same valley orchard at six different maturity stages. Growing location strongly influenced the taste- and health-related fruit composition whereas the fruit maturity at harvest mainly affected the ethylene biosynthetic pathway and ethylene-dependant quality traits such as the fruit firmness and starch index. The fruit maturity at harvest, but not the growing location, also affected the severity of the infection caused by P. expansum and R. stolonifer, with mature fruit showing higher susceptibility to pathogen infection. Besides, by employing a Partial Least Square (PLS) regression model, our data showed that the severity of the lesions caused by R. stolonifer were intimately related to the fruit ethylene production. Overall, the results from this study demonstrate that differences in environmental conditions between orchards (mountain vs valley) strongly influenced the composition of ‘Golden Reinders’ apples without affecting the susceptibility of the fruit to two major postharvest pathogens.

Introduction

Although apple production in the European Union has remained stable over the past two decades, apple production for the same period in Spain has decreased by 20% (FAOSTAT, 2019). Such decline is due, in part, to the fact that more than half of the Spanish apple production is located in the Ebro Valley, an area characterized by dry and warm weather conditions, that detrimentally affect certain apple quality attributes, such as colour and firmness (Iglesias et al., 2008) as well as the fruit storability (Emongor et al., 1994). It is already well known that climatic differences between cultivation areas play an important role in the fruit physiology and, therefore, in its final quality (Corelli-Grappadelli and Lakso, 2004; Karagiannis et al., 2020) as well as its nutritional value (Crespo et al., 2010). Indeed, apples obtained in mountain areas are expected to have a better organoleptic quality (Faust, 2000) and enhanced content of antioxidants (Karagiannis et al., 2020). The main environmental differences between orchards located at the same latitude but at different altitude are generally temperature and solar radiation incidence (Körner, 2007). Temperatures reached on the field weeks before harvest strongly influence the taste-related composition of the fruit (Woolf and Ferguson, 2000), as well as its aroma (Dixon and Hewett, 2000). In particular, apples grown under warm temperatures tend to accumulate higher soluble sugars (Seo et al., 2003) and lower malic acid (Yamada et al., 1988). However, an excess of high temperature may inhibit starch metabolism in apples (Smith et al., 1979) as well as ripening and ethylene production in some plants (Biggs and Handa, 1988). Enhanced solar radiation, on the other hand, is also known to enhance the synthesis of antioxidants in fruit (Wang, 2006; Karagiannis et al., 2020).

In addition to the influence of environmental conditions, the fruit maturity stage at harvest clearly affects the final fruit quality and its market value in a wide range of pome fruit (Ingle et al., 2000; Lindo-García et al., 2020). Fruit picked immature may have a suboptimal organoleptic quality (Echeverrı́a et al., 2004) whereas over-mature fruit will exhibit a limited storability (Guerra and Casquero, 2010), poor firmness (Harker et al., 2010) and, to some extent, lower nutritional value (Huang et al., 2007). In this sense, it is feasible to speculate that differences in the fruit composition and physiology associated to orchard location and/or harvest date may lead to differences in the fruit susceptibility to postharvest diseases (Baró-Montel et al., 2019; Sun et al., 2017; Torregrosa et al., 2020; Vilanova et al., 2012). Penicillium expansum and Rhizopus stolonifer are considered two of the main apple postharvest pathogens due its wide incidence during storage (López et al., 2015). This said, few studies are currently available comparing the susceptibility to postharvest pathogens in apples grown in mountain and valley areas. Accordingly, our study was focused on comparing the fruit quality, physiology and susceptibility to major postharvest pathogens in ‘Golden Reinders’ apples grown in the Ebro valley and the Pyrenees Mountains, two areas very close to each other but with very different climatic conditions. Besides, on-tree ripening was monitored to further assess if differences observed between locations were caused by environmental conditions or by maturity differences.