Characterization of fruit quality traits for organic acids content and profile in a large peach germplasm collection

Highlights

• Fruit acidity greatly varied in peach germplasm with a range of 1.18–12.31 g L⁻¹.

• Among the ten organic acids detected, malate and citrate were the most abundant.

• The large phenotypic diversity could be useful for the breeding of novel varieties.

Abstract

Organoleptic properties play a pivotal role in determining peach fruit quality, affecting both consumers’ and market acceptance. In spite of a narrow genetic diversity, peach [Prunus persica L. (Batsch.)] has a remarkable range of fruit taste, mainly driven by the relative amount of sugars and acids. Characterization and exploitation of such variability is a major objective of fruit-quality orientated breeding programmes. In this study, a peach collection of 201 accessions was dissected for important fruit quality traits, with a particular focus on acidity and organic acids content. Fruit acidity was titrated and ten organic acids (oxalate, cis-aconitate, citrate, tartrate, galacturonate, malate, quinate, succinate, shikimate and fumarate) were detected through HPLC and UHPLC-MS. Malate and citrate are the most abundant accounting for the 62 % and the 22.6 % of total organic acids, respectively, and suggesting their largest contribution to the overall peaches acidity. Results reveal also a genotype-dependent contribution of specific organic acids and a low seasonality-effect on peach acidity and organic acid patterns. Collectively, this work provides an overview of the phenotypic variation associated with organic-acid related traits useful for supporting the ongoing breeding works.

Introduction

Peach [Prunus persica L. (Batsch.)] is a perennial crop setting climacteric fleshy fruits widely cultivated in the temperate regions, mainly. Annual peach production in Europe has showed a decreasing trend in the last decade (from about 4 Mt in 2008 to 3.5 Mt in 2018), following a reduction of fresh consumption, likely due to the overall low quality of commercialized fruits (Etienne et al., 2002; Cirilli et al., 2016). Many efforts have been done in the last years in developing novel varieties with improved external fruit quality, related to size, texture and skin colour (Dirlewanger et al., 1999; Fideghelli et al., 1998; Moreno, 2005; Cantín et al., 2009, 2010). Nevertheless, a poor taste or lack of the characteristic “peach” aroma still remains the major consumers’ complaint (Bassi and Selli, 1990; Cirilli et al., 2016). Among basic tastes, sweetness and sourness are the prominent eating quality attributes in peach (Crisosto et al., 2006). Their perception mostly depends on the relative abundance of sugars (the most abundant water-soluble compound) and acidity levels (dependent to the organic acid content, particularly malic and citric) (Vizzotto et al., 2002; Colaric et al., 2005; Batista-Silva et al., 2018). Acidity is a major driver of fruit quality perception in peach directly affecting consumers’ liking degree. Peach varieties are usually classified as normal or low-acid, based on fruit pH respectively below 3.8 or higher than 4 (Yoshida, 1970; Dirlewanger et al., 1998). The low-acid (LA) type has been often classified as the so called ‘honey peach’ group, widely spread in the Far-East countries (China, Korea and Japan) (Reimer, 1906). In contrast, most of the European and USA varieties tend to have normal-acid (NA) fruits. The low-acid trait has been further elucidated, revealing a simple, dominant genetic inheritance (locus D, for Douceur) (Monet, 1979; Boudehri et al., 2009), making this phenotype a relatively easy breeding target (Byrne, 2002). Apart from the D locus, titratable acidity and pH still show a remarkable variability, as suggested by an early study of Yoshida (1970). The content and profile of organic acids (OAs) seem to be quantitatively and qualitatively variable among accessions. OAs are responsible of protons bulk in mesocarp cells that determines the titratable acidity of fruits (Famiani et al., 2020). Acidity correlates with taste and aroma perception: sweetness is influenced by citric and shikimic acids and sugars/organic acids ratio, while aroma correlates with total acidity and malic acid content (Colaric et al., 2005). Peach accumulates mainly malate (dicarboxylic acid) and citrate (tricarboxylic acid) in the flesh, while shikimate, fumarate, oxalate, succinate, quinate and ascorbate are reported at low contents (Moing and Svanella, 1998; Etienne et al., 2013; Nowicka et al., 2019; Zheng et al., 2021). Total OAs content range from about 400 to over 3000 mg L⁻¹ (Cao et al., 2016), being malic and citric acids the most abundant averaging from 300 to 600 and 100–300 mg kg⁻¹ of fresh weight, respectively. The OAs content and profile are markedly regulated by fruit development and ripening, as well as post-harvest storage conditions and treatments, although generally well-correlated across seasons (Cano-Salazar et al., 2013; Desnoues et al., 2014). In NA accessions, malate content remains relatively constant along fruit development and ripening, while in LA accessions it tends to decrease. Citrate tends to be accumulated from pre-climateric stage onwards in NA, and only in reduced amount in LA (Moing and Svanella, 1998; Lombardo et al., 2011). Differences among varieties for both titratable acidity and OAs content seem to be mainly dependent to their aptitude to store OAs in the vacuole of mesocarp cells rather than metabolic synthesis or degradation (Moing and Svanella, 1998), although other factors are also reported to play a role in citrate accumulation (Zheng et al., 2021). In LA fruits, some divergence in expression of vacuole proton pumps may explain the cells’ reduced capability to sequester OAs, thereby making them accessible to catabolic enzymes (Etienne et al., 2002). Higher levels of PEPC activity and reduced accumulation of sugars in the LA fruits during ripening may suggest that the acids are constantly recycled (Moing et al., 2000).

This study reports an analysis of fruit acidity and OAs content in a large and genetically diverse peach germplasm collection with the aim of providing valuable information on this important class of metabolites to support eating quality-oriented breeding programmes.