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Recording of Low-Oxygen Stress Response Using Chlorophyll Fluorescence Kinetics in Apple Fruit
(2023)
Long-term storage of apples (Malus x domestica, Borkh.) is increasingly taking place under Dynamic Controlled Atmosphere (DCA). The oxygen level is lowered to ≤ 1 kPa O2 and the apples are stored just above the Lower Oxygen Limit (LOL). Low oxygen stress during controlled atmosphere storage can lead to fermentation in apples if oxygen levels are too low. Chlorophyll fluorescence can be used to detect low-oxygen stress at an early stage during storage. The currently available non-imaging fluorescence systems often use the minimal fluorescence (Fo) parameter. In contrast, the use of chlorophyll fluorescence kinetics is insufficiently described. Therefore, this study aimed to gain more knowledge about the response of chlorophyll fluorescence kinetics to low oxygen stress in apples using a fluorescence imaging system. The results show that the kinetic fluorescence curves differ under aerobic and fermentation conditions. The fermentative conditions initiated a decrease in fluorescence intensity upon application of the saturation pulses during exposure to actinic light. This result was made at 18 °C and 2 °C ambient temperatures. Interestingly, the kinetic curve changed at 2 °C before fermentation products accumulated in the apples. Non-photochemical quenching (NPQ) decreased under fermentation conditions in the dark phase after relaxation. Upon entering the dark relaxation phase after Kautsky induction, ɸPSII began to increase. Under atmospheric oxygen conditions, ɸPSII reached values of 0.81 to 0.76, while under fermentation, ɸPSII values ranged from 0.57 to 0.44.
Currently, only non-imaging chlorophyll fluorescence measurements are used to identify the Lower Oxygen Limit (LOL) in Dynamic Controlled Atmosphere - Chlorophyll Fluorescence (DCA-CF) storage. The disadvantage of non-imaging fluorescence is that no statement can be made about the spatial heterogeneity of the sample. In contrast, chlorophyll fluorescence imaging can detect spatial heterogeneity of photosynthetic activity and has been established in research for some decades because the information benefit is higher. In this study, the chlorophyll fluorescence (Fo, Fm, Fv, Fv/Fm) of apples (Malus x domestica, BORKH.) was measured with a fluorescence imaging system in situ during storage. Intact apples of ‘Braeburn’ and ‘Golden Delicious’ were stored under low-oxygen stress conditions (< 1 kPa). The metabolic shift from aerobic to fermentative metabolism was made visible with the chlorophyll fluorescence imaging and was spatially localized on the sample. Furthermore, a method was developed to identify the LOL based on the chlorophyll fluorescence imaging combined with the histogram division method. This method considers the heterogeneity of the fluorescence and bundles the measured Fo data as histograms. Our results showed that the fluorescence imaging combined with the histogram division method can be a powerful tool for identifying the LOL.