Effects of Light and Nutritional Conditions on the Ascorbic Acid Content of Lettuce

Abstract

Effects of light and nutritional conditions on the ascorbic acid content in hydro-ponically grown lettuce of butterhead type (Luctuca sativa L. cv. Edogawa or Okayama) were investigated. With increasing levels of shading, the fresh weight and number of leaves decreased, the shape of leaves became narrower and ascorbic acid and sugars content decreased markedly, while nitrate nitrogen content inclined to increase. Plants grown in 1/4 strength standard nutrient solution were inferior in fresh weight and number of leaves to those grown in 1/2 strength solution. Though ascorbic acid and sugars content was higher for plants grown in 1/4 strength solution at higher light intensities, the effect of nutrient concentration gradually disappeared with increasing shading level. Diurnal changes in leaf constituents were studies on sunny and cloudy days. The contents of ascorbic acid, sugars and chlorophyll were higher in the daytime and lower during the night, while nitrate nitrogen content increased in the night time. These tendencies were more clear on the sunny day than on the cloudy day. The relations between the contents of ascorbic acid, sugars, nitrate nitrrogen etc. were discussed and possible techniques of practical use for improving the quality of hydroponically grown leaf vegetables were also discussed.