Growth of Lettuce Under Controlled Humidity Levels1

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Abstract
The influence of atmospheric moisture levels of 85% and 50% relative humidity (RH) (3.5 and 11.6 mb saturation vapor deficit respectively) at 20°C for 16:8 long day (LD) cycle on butterhead lettuce, (Lactuca sativa L. cv. Meikoningen), during a growth period of 4 weeks from seeding, was studied in organic soil and liquid cultures. Significantly faster growth rates were evident on plants developing under 85% RH than under 50% in both soil and liquid culture with the largest increases occurring with plants grown in soil culture. The higher humidity level increased leaf number 15%, leaf size 30%, dry weights 62%, and leaf water contents from 93% to 94%. The differences in leaf number and dry weight equalled about 2 days growth difference at 4 weeks after seeding. The density of stomata was greater on plants grown under 50% RH but the total number of stomata per leaf under the 2 humidity levels was the same. The leaf resistance was significantly higher on plants grown under 50% than under 85% RH. The principle significance of high humidity level during growth of lettuce is the production of larger marketable heads with a higher water content in a slighly reduced period of time.

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