Using chlorophyll fluorescence as an indicator of brown stain (CO₂ injury) in Romaine lettuce

Chlorophyll fluorescence is a tool developed to study photosynthesis. Chlorophyll fluorescence can be used to identify changes in membranes or membrane-bound parts of chloroplasts that lead to changes in their fluorescence emission characteristics. In postharvest studies, chlorophyll fluorescence could be used as a very sensitive measure of membrane changes in products containing chloroplasts. This would permit postharvest researchers to gain useful information on early responses. Romaine lettuce that had been harvested at typical commercial maturity was stored in air or in two CA treatment conditions: 2% O₂ plus 5% CO₂ (CA1) and 2% O₂ plus 2% CO₂ (CA2). The lettuce was stored for 20 days at 0.5°C to simulate a marine container transport period, followed by one day in air at 20°C. Chlorophyll fluorescence, color changes, weight loss, chlorophyll a and b, total chlorophyll, total sugars, total amino acids, and overall visual quality were evaluated after 10 and 20 days of storage at 0.5°C and after 20 days of storage at 0.5°C plus one day shelf life at 20°C. Symptoms of CO₂ injury (brown stain) developed upon transfer to air at 20°C after 20 days storage in CA1, but not in air- or CA2-stored lettuce. The Fv/m measurement declined much more during storage in the injurious CA1 than in air or the non-injurious CA2. The other measured parameters did not differ between the treatments. Our results suggested that chlorophyll fluorescence can be used as a sensitive indicator of CO₂ injury before visual symptoms occur.