Concentrations of starch and sugars were measured in the cap leaf (leaf no. 1) and in every fifth leaf (5, 10, 15, 20) in heads of crisphead lettuce (Lactuca sativa L.) harvested at 0700 hr (AM) or 1400 hr (PM) PDT. Starch content increased in the cap leaf from the AM to the PM harvest, but remained unchanged in other leaves. Sucrose concentration was <5 mg·g−1 dry weight in AM-harvested lettuce, but the cap leaf, leaf 20, and stem tissue contained 43, 24, and 61 mg·g−1 dry weight, respectively, in lettuce from the PM harvest. AM harvested lettuce contained 70% to 260% more glucose and 20% to 120% more fructose than PM-harvested lettuce. Glucose and fructose concentrations were greatest in leaf 10 (110 and 120 mg·g−1 dry weight, respectively) and decreased 20% to 50% in inner and outer leaves. Exposure of lettuce to 7.5% or 10% CO2 for 12 days at 2.5°C followed by air for 3 days at 10° caused more severe injury on AM- than on PM-harvested lettuce. Injury occurred primarily on leaves 7 through 17, with those between leaves 10 and 15 being most severely affected. High reducing sugar content at harvest did not appear to decrease the sensitivity of lettuce to high CO2 during storage.
Received for publication 23 Feb. 1987. We thank Bruce Church, Inc. and Bud Antle, Inc., both of Salinas, Calif. for their cooperation in providing the lettuce used in these tests. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.