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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.
Dynamic Controlled Atmosphere-Chlorophyll Fluorescence storage (DCA-CF) uses a fluorescence-based measurement method to detect fermentation in apples (Malus × domestica BORKH.) caused by low-oxygen levels at an early stage. In recent years, it has been observed that individual apples of the same variety and origin can exhibit different fermentation behavior when stored under completely identical conditions. The causes of the different fermentation behavior must be found in order to be able to use DCA storage optimally. This study aimed to find the causes of the different fermentation behaviors of individual apples. Our results show that fruit ripeness can affect the lower oxygen limit (LOL), especially immediately after harvest, when the starch degradation in the fruit is not yet complete. A significant increase in the LOL was observed in ‘Elstar’ (2020: 0.3 kPa, 0.6 kPa, 0.9 kPa; 2021: 0.3 kPa, 0.4 kPa, 0.6 kPa). ‘Braeburn’ also exhibited this behavior regarding the LOL at a lower level. The LOL could not be identified for some of the fruit (varying from 12.5% to 41.7% of the examined apples) previously stored in Ultra Low Oxygen (ULO) storage for 4 months. Also, the chlorophyll content in the apple skin influences the fluorescence measurement method. Within 2 weeks, the chlorophyll content in the apple skin was halved. If the chlorophyll content drops, the reliability of the fluorescence measurement also decreases. It turned out that apples with an Fv/Fm < 0.7 were unsuitable for valid LOL identification.
Derzeit wird nur die nicht-bildgebende Chlorophyllfluoreszenz in der DCA-Lagerung eingesetzt, um den LOL zu identifizieren. Das Ziel dieser Untersuchung war es, die Chlorophyllfluoreszenz von einzelnen Apfelfrüchten mit einem Fluoreszenz-Bildgebungssystems (FluorCAM 701 MF, Photon Systems Instruments, Brno, Tschechische Republik) unter aeroben und fermentativen Bedingung zu messen. Der Grad der Heterogenität der Fluoreszenz am Apfel wurde ermittelt und der Fluoreszenzanstieg visualisiert. Mit der Methode der Histogramm Division konnten alle Fluoreszenzinformationen gebündelt werden.
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.
Chlorophyllfluoreszenz als Werkzeug für die DCA-Lagerung von Äpfeln (Malus x domestica BORKH.)
(2024)
Das Ziel der Untersuchung war es, die Bedeutung von Eigenschaften der einzelnen Apfelfrucht für die DCA-CF-Lagerung (Dynamic Controlled Atmosphere – Chlorophyll Fluorescence) bei der Sorte ‘Elstar, PCP’ zu untersuchen. Die untersuchten Eigenschaften waren der Deckfarbenanteil, der Erntetermin, die Fruchtbehangstärke und die Fruchtgröße. In den letzten Jahren wurde seitens der obstbaulichen Praxis wiederholt beobachtet, dass sich die Früchte derselben Sorte bei gleichem Sauerstoffgehalt in der Lagerraumatmosphäre unterschiedlich hinsichtlich der Neigung zur Gärung verhalten. Während ein Teil der Äpfel noch keine Gärung aufweist, bildet ein anderer Teil der Äpfel bereits erhebliche Gärstoffmengen oder befindet sich im Gärungsprozess. Die Ursache für dieses unterschiedliche Verhalten ist nicht bekannt. Für die vorliegende Arbeit wurde das Gärungsverhalten einzelfruchtweise untersucht. Der Sauerstoffgrenzwert wurde mittels Messungen der Grundfluoreszenz (F0) sowie der Sauerstoffkonzentration in der Lagerraumatmosphäre bestimmt. Des Weiteren wurde das Gärungsverhalten durch die Analyse der fruchteigenen Ethanol-Konzentration beschrieben. Die Ergebnisse zeigen, dass sowohl der Erntetermin als auch die Fruchtbehangstärke das Gärungsverhalten bei ‘Elstar, PCP’ beeinflussen können.