Influence of Carbon Monoxide, Carbon Dioxide, and Oxygen Levels on Brown Stain, Respiration Rate, and Visual Quality of Lettuce1,2

Brown stain on crisphead lettuce ( Lactuca sativa L. cv. Calmar) was increased by exposure to carbon monoxide (CO) combined with elevated CO 2 (1, 2 or 5%) regardless of the O 2 level (2, 5, 10, or 21%). Carbon monoxide at 1, 3, or 5% added to air without added CO 2 did not induce brown stain. Holding under 5 or 10% O 2 in combination with 1% CO 2 and with or without 1% CO for 20 days followed by a 4-day period in air at 10°C resulted in better overall visual quality compared with air or 2% O 2 controls. Carbon monoxide in combination with air or 2% O 2 reduced the respiration rate of lettuce during a 10-day period at 2.5°C. Lettuce previously subjected to CO had higher respiration rates, relative to controls, during the subsequent 4-day period in air at 10°C.