The association between metabolic status and susceptibility to Botrytis cinerea in strawberry fruit

Summary

Strawberry is a popular soft fruit worldwide due to its attractive appearance, aroma and nutritional value. However, strawberry fruit are highly susceptible to postharvest fungal infection, primarily caused by Botrytis cinerea. Breeding resistant strawberry genotypes is extremely challenging due to 1) the polygenic nature of resistance; 2) the octoploid genome structure of cultivated strawberry; 3) the functional redundancy of pathogenicity factors released by B. cinerea. Nevertheless, susceptibility is a quantitative trait which allows breeding for less susceptible genotypes resulting from the combination of certain physical and chemical characteristics of strawberry.

In this thesis, I aimed to explore the roles of physical and chemical characteristics of strawberry fruit (e.g., color, firmness, volatile compounds and soluble sugars) in susceptibility to B. cinerea and identify characteristics that could potentially serve as markers to assist in breeding for strawberry genotypes with reduced susceptibility.

To achieve these goals, I studied the subject from two angles. The first angle was at the genetic level, where I identified the association between the natural spoilage of strawberry genotypes and their metabolite profiles (Chapter 2). The second angle was to evaluate how fruit susceptibility was affected by pre- (Chapter 3) or postharvest (Chapters 4&5) practices and then to investigate the underlying physiological and metabolic characteristics that are associated with the trait.

In chapter 2, we aimed to characterize the association between shelf life and physical and chemical characteristics in strawberry fruit. Seventeen genotypes were selected based on contrasting flavor, color, firmness, and shelf life. Physical characteristics (e.g., weight, color, firmness) and certain primary (e.g., soluble sugars, organic acids) and secondary metabolites (e.g., proanthocyanins, anthocyanins, volatiles) of red-ripe strawberries at harvest were determined and the spoilage during the storage was assessed. The spoilage rate differed between genotypes, and this was consistent over two successive years. Among seventeen genotypes, a more intense red coloration of the fruit skin was associated with a lower spoilage rate. Higher levels of two volatile compounds, ethyl butanoate and 1-hexanol, were correlated to more rapid spoilage.

In chapter 3, we aimed to investigate the effects of light spectra on strawberry fruit susceptibility to B. cinerea. If any effects were observed, we aimed to explore the underlying physiological and metabolic mechanisms. Strawberry plants (Fragaria × ananassa, ‘Furore’) were grown in a climate chamber under different ratios of red to blue light and with or without far red light. Low ratios of red to blue light slowed down fruit ripening reflected in lower levels of sucrose and esters but the susceptibility of red-ripe fruit to B. cinerea infection (lesion expansion) was not affected by ratios of red to blue light. In contrast, FR light lowered levels of ascorbic acid and anthocyanins but stimulated the accumulation of soluble sugars and esters in strawberries, presumably indicating an accelerated ripening process. The susceptibility of red-ripe fruit to B. cinerea infection was not affected by FR light. Collectively, the effect of either ratios of red to blue light or FR light on changes of sugar and secondary metabolite levels were too minor to affect the susceptibility of strawberries to B. cinerea.

Controlled atmosphere techniques are commonly used in commercial practice to extend shelf life of fresh products. Strawberry is generally tolerant to the environment with high concentrations of CO2 (15 to 20 kPa), in which the spoilage by B. cinerea can be notably reduced. In chapter 4, we aimed to investigate whether the stepwise increment, up to 30 kPa CO2, is beneficial to reduce B. cinerea spoilage without inducing CO2 injury symptoms in strawberries. ‘Arabella’ strawberries were harvested at the red-ripe stage and subsequently stored for 11 d at 5 °C under CA conditions (10 kPa O2 with either 0, 20 or 30 kPa CO2). The CO2 levels were either realized within eight hours, or in a stepwise manner within the first four days. 21 kPa O2 and 0 kPa CO2 served as control. The application of high CO2 (20 and 30 kPa) in combined with 10 kPa O2 greatly suppressed fruit spoilage during storage and subsequent shelf-life. The stepwise increments of CO2 did not result in beneficial effects compared to the static application of high CO2.

To prolong the shelf life, strawberry fruit are generally harvested before they are fully ripe. In chapter 5, we aimed to investigate the effects of maturity at harvest and subsequent cold storage on consumer liking, which is largely dependent on sweetness, aroma attributes, and volatile compositions. Strawberries were harvested either at the ¾ red stage or fully red stage and stored at 4 °C for few days and subjected to sensory profiling, GC-MS and PTR-ToF-MS measurements. The sensory profile of strawberries harvested at ¾ red stage showed lower sweetness and aroma than full red harvested strawberries. VOC analysis of these strawberries showed lower presence of volatile fatty acids, furanones and most esters compared to full red strawberries. Ripe harvested fruit stored for nine days showed the lowest values for aroma attributes, perhaps related to the production of volatiles with off-flavors (acetaldehyde, ethyl acetate). PLS modelling showed that VOCs exist that are characteristic for both sweet and aromatic sensory attributes of ‘Lusa’ strawberries, based either on GC-MS (mainly volatile fatty acids) or PTR-ToF-MS analysis (mainly alcohol/ester fragments).