*4.1. Low Oxygen Storage Alleviated CI in Tomato Which Might Be Related to Lower Oxygen Uptake and Improved Lycopene Synthesis*

When low temperature was combined with reduced oxygen concentrations, lower decay and lower weight loss was observed during shelf-life for both MG and R tomatoes (Figures 1 and 2). Our results showed that O2 consumption decreased with lower oxygen levels while CO2 production rates were similar (Figure 8). Low oxygen storage is reported to suppress respiration and ethylene production [15,35]. Low oxygen uptake might reduce O2 availability for ROS production, such as singlet oxygen (1O2) and superoxide anions (O2 .−) [36]. O2 − is dismutated into H2O2 by the action of SOD [4,36,37]. Lower levels of O2 − are expected to yield lower levels of H2O2. However, we did not observe a lower level of H2O2 in the low oxygen stored fruit (Figure S3), perhaps indicating that low oxygen did not suppress oxidative stress initiated by the presence of O2 −. As tomato stored under low oxygen showed further red colouration close to or even higher than the non-chilled control (Figure 4B) and faster red colouration (Figure 7) after transfer to 20 ◦C, we hypothesise that lycopene acted directly to quench 1O2. Carotenoids are able to quench 1O2 due to its high number of conjugated double bonds, whereas lycopene and its precursors, are the most effective 1O2 quencher [38–41]. Quenching of 1O2 by lycopene or its precursors might have resulted in delayed lycopene synthesis or lycopene degradation [31,42]. Therefore, uninterrupted colour synthesis might indicate that low oxygen prevents lycopene degradation as well as preserving the lycopene biosynthetic machinery during cold storage allowing new lycopene synthesis during shelf life [43–45].

The lowest oxygen concentration to delay or prevent CI symptoms was 1 kPa (Figure 7). A lower oxygen level (0.5 kPa) resulted in necrosis and fungal infection (data not shown), probably because of excessive fermentation. It was reported that MG 'Bermuda' tomatoes stored at 22 ◦C under 0.5 kPa O2 developed identical symptoms after three days of storage [35].
