Increasing vitamin C content in strawberries(Fragaria × ananassa) through preharvest treatments.

MSc-thesis abstract (submitted 14 March 2016):
Vitamin C (ascorbic acid, AsA) is a vital micronutrient for humans. Strawberries are not only one of the fruits with the highest concentration of AsA, but also one of the model plants for investigating the different pathways of AsA biosynthesis. Increasing the AsA content of strawberries would therefore: Benefit consumers for health related issues, become a strong marketing tool for growers and provide new insights on AsA biosynthesis and

Aim: The experimental aim of this work is to investigate if the combination of deficit irrigation, additional LED lighting and a lower crop load can increase the AsA content of strawberry. At the same time, we will quantify the contribution of different metabolic pathways of AsA biosynthesis due to preharvest treatments, using gene expression analysis.

Approach: Experiments using strawberry cultivar ‘Elsanta’ was carried out in the greenhouses of Wageningen UR Greenhouse Horticulture in Bleiswijk. The preharvest treatments consisted of flower removal (0 and 50%), water deficit irrigation(100, 66% water supply) in combination with LED lighting (Top lighting, Top+Leaf lighting, Top+Fruit lighting). Strawberry fruits were harvested three times on November 2nd, 9th and 16th. Total AsA concentration and expression of relevant genes was determined.

Results: When compared to the control, the most effective combination of lighting, water supply (WS) and flower removal (FR) was extra fruit lighting with 100% water supply and 50% flower removal (Top+Fruit-100WS-50FR), which showed an increase of 16.5 ± 2.0%. ANOVA results of the preharvest experiment only showed a significant AsA increase for the effect of lighting on total AsA concentration. No interaction effect between lighting, water deficit and flower removal on AsA concentration was found. Within the three different lighting conditions, fruits with extra fruit lighting had significantly higher total AsA concentrations, which showed an increase of 9.4%. On the other hand, extra leaf lighting showed an increase of 3.3% compared with control top lighting. Lighting conditions and flower removal affected yield. Fifty percent flower removal decreased yield with approximately 20%. Treatment with extra fruit lighting had the highest fruit yield, which showed an increase of approximately 20% in comparison to top lighting.
The difference in the mRNA level for the four target genes was not statistically significant. Only a trend can be observed. According to the trend, GalUR, DHAR and MDHAR seemed to have a higher expression in fruit with extra fruit lighting, which might indicate that both the galacturonate and the recycling pathway are active in this experiment.

Conclusions and recommendations: The results suggest that extra fruit lighting can be an effective tool to increase the AsA content of strawberries, which also increases the yield. However, in commercial production, the energy cost for supplementary lighting should also be taken into account. The combination of crop load and regulated water deficit didn’t show any effect in this experiment. The amount of irrigation water can be therefore adjusted to save water. Flower removal is not recommended as it doesn’t increase the AsA content but decreases the yield according to this experiment.

To better understand the biosynthesis of AsA in strawberry, another aspect for further experiment is to include strawberries from different developmental stages. The prevalence of one pathway over the others depending upon the developmental stage has been reported in the fruits such as grape and tomato. In strawberry, the Smirnoff-Wheeler pathway seems to be more important during preharvest while the galacturonate pathway seems to play a more important role during ripening. However, in order to see the effect on the gene expression level, a significant difference in the AsA level is needed. More extreme treatments, such as higher light intensity or longer lighting period, more sever water stress conditions, are therefore suggested.