The role of post-harvest supplementary light exposure for ripening and quality development of tomato fruit

Abstract

The major aim of the present work was to evaluate the optimal exposure time and effects of red light and short periods of daily ultraviolet (UV) on the post-harvest quality of green tomato fruit during ripening. The concomitant objective was to work out fundamental knowledge about the mechanisms of secondary metabolite compounds in lab-scale of post-harvest tomato under optimal light condition. For this purpose, experiments were conducted in a climate chamber equipped with red light-emitting diodes (LED) and UV tubes. The studies on green stage-1 tomatoes include investigations on the impact of light exposure on level of external fruit color Normalised Difference Vegetation Index (NDVI), Normal-ised Anthocyanin Index (NAI), Simple Chlorophyll Fluorescence ratio (SFR_R), firmness and total soluble solid content after post-harvest. To prove suitability of the light to track change in secondary metabolite compounds, lycopene and β-carotene concentration, determined by (means of) HPLC analyses. Total phenolic, and total flavonoid concentrations and antioxidant activity were analyzed by spectrophotometric detection. Here again, the fluorescence-based indices were compared to the well-established reflection-based ripening index a*/b*. The results ascertained in the single chapters can be summarized as follows: <br /> 1. Green tomatoes were exposed for 30 min to UV radiation, continuous red light or a combination of both for up to 20 d. Non-treated (control) fruits ripened within 15 d while fruits exposed to red light and a combination of red light with UV radiation required five days less to reach the same maturity level. Moreover, the exposure to red light alone or in combination with UV raised concentrations of lycopene, ß-carotene, total flavonoids and phenolics. This possibility to steer the concentrations of health promoting antioxidants through light treatments is a reliable method to increase fruit quality according to customer wishes and demands. <br /> 2. The impact of post-harvest red light irradiation via LED modules on the content of health promoting compounds was evaluated on green tomatoes stage 1 exposed to intermittent or continuous irradiation. The first experiment studied the overall effects of different duration periods of red light radiation (darkness, continuous red light for 10 d, continuous red light for 15 d and continuous red light for 20 d) while the second experiment focused on effects of intermittent red light (darkness, red light for 30 min per day, red light for 6 h per day, red light for 12 h per day and continuous red light). In both experiments, tomatoes exposed to darkness served as control. Continuous red light irradiation accelerated the ripening process of green tomatoes. In addition, continuous red light also significantly increased lycopene, β-carotene, total phenolic content, total flavonoid concentration and antioxidant activity compared to all other treatments, suggesting that continuous red light exposure positively influences metabolic processes and contributes to a higher content of health promoting compounds in tomatoes. <br /> 3. In the last section, green stage-1 tomatoes were harvested and treated daily with red light for 12 hours per day, for 15 days (followed by storage in darkness for additional 6 days) or continuously radiated with red light for 21 days, as previously proved to be effective. Control (untreated) tomatoes were kept in the dark for the same period. Color parameters of the outer part were strongly influenced by the application of continuous red light. In different parts (outer and inener) of fruit, significant differences between treatments were analyzed for major compounds such as lycopene, β-carotene, total phenolic and total flavonoid concentration. Fruit treated with continuous red light showed the highest concentration in all parameters. <br /> In summary, application of continuous red light has proved effective to enhance color and firmness of tomato fruits by stimulating accumulation of carotenoids and antioxidative com-pounds during post-harvest storage and accelerating full tomato ripening.