Breeding for Improved Flavour of Fresh Market Tomatoes: Breeders' Sensory Test and Molecular Markers

Until recently, flavour was not a primary breeding target in tomatoes (<i>Solanum lycopersicum</i> L.). However, the poor flavour of tomatoes is a major cause of consumer complains. Flavour results from a complex interaction of sugars, acids (taste) and volatile compounds (aroma). Therefore, flavour assessment is difficult and expensive. Classical sensory methods are not suitable for evaluating early segregating generations, which are characterised by hundreds of small samples, and simple physicochemical measurements are not sufficient to predict perceived aroma. Selection in the first segregating generation could enhance breeding for flavour by preventing the loss of favourable alleles. To meet consumer demands for better flavour, plant breeders need alternative selection methods such as phenotypic methods adapted to early breeding generations or marker-assisted selection (MAS). So far, sensory attributes have often been neglected in genetic studies. Studies verifying molecular markers in multiple genetic backgrounds, as required for a broader application, are even rarer. Therefore, the so-called Breeders’ Sensory Test, designed to asses a large number of small samples by a small team, was evaluated using F₂ plants from 32 crosses and their parents (910 individuals) in two cultivation systems, organic low-input and hydroponic. In addition, physicochemical traits (total soluble solids, titratable acidity, dry matter), and volatile compounds were analysed. Corresponding physicochemical measurements and sensory attributes were significantly correlated. For most of the studied traits, selection between single plants in the F₂ generation was possible. Thus, the Breeders’ Sensory Test is a promising method for flavour improvement. Subsequently, the Breeders’ Sensory Test was applied to an F₂ mapping population (‘Resi’ × ‘Auriga’) derived from two parents with excellent flavour. Sensory attributes, physicochemical measurements, volatile compounds and fruit weight were assessed in the same contrasting cultivation systems. Of the total number of 100 significant QTL, 27 were co-localised between both cultivation systems and their mean values. Novel QTL for the sensory attributes sweetness, sourness, and tomato aroma were identified on the chromosomes 5 and 10. Nine distinct QTL clusters for flavour-related traits were identified including a large cluster on chromosome 6 comprising five sensory and nine volatile QTL. Finally, two unrelated crosses (‘Resi’ × ‘Auriga’, ‘Roterno F₁’ × ‘Black Cherry’) were chosen to evaluate the efficiency of phenotypic selection based on the Breeders’ Sensory Test (breeders’ sensory selection, BS) and MAS for sweetness, sourness and tomato aroma in the same cultivation systems as before. Five QTL for sensory attributes, partly within clusters, were chosen for verification in these crosses. Both selection methods were more effective in the cross ‘Roterno F₁’ × ‘Black Cherry’ derived from more diverse parents. A trend towards slightly higher efficiency of MAS in early segregating generations was observed, most evident for tomato aroma. To maximize the response to selection and capture all genetic loci including unidentified ones, a combination of BS and MAS is recommended. MAS is particularly interesting for preselection of seedlings and allows a higher selection intensity, as a very large number of plants can be analysed. A major advantage of BS is its universal applicability and it is particularly interesting for organic plant breeding, where phenotypic selection is considered most important. Furthermore, the results of all three studies emphasizing the challenge of breeding large-fruited tomatoes with good flavour. In conclusion, genetic improvement starting with selection in the F₂ generation is expected to be successful. Combing phenotypic and molecular selection methods is recommended. A genotype-by-year interaction probably decelerates the selection progress. Our results provide relevant information to improve the flavour of fresh market tomatoes, a trait demanded by consumers. Breeding is the first step in the value chain and improved genetics form the basis for flavourful tomatoes.

2023-12-08