Composition of Hydroponic Lettuce: Effect of Plant Size, Season and Time of Day

The diurnal variation of nitrate and sugars may vary with plant size or the ability of plants to buffer the uptake, synthesis, and use of metabolites. Bibb lettuce was grown in hydroponics in a heated greenhouse. Plantings differing in size and age were sampled at 3-hour intervals throughout one day in August 2007 and November 2008. Samples were weighed, frozen and freeze dried, and dried tissue was extracted and analyzed for nitrate, soluble sugars, and organic acids, using LC chromatography. The increase in dry/fresh weight ratio during the daylight period was greater for small compared to large plants. On a fresh weight basis, tissue nitrate of small plants was only half that for larger plants. The variation in concentration with time was much less for nitrate than for soluble sugars. Soluble sugars were similar for all plant sizes early in the day, but they increased far more for small compared to large plants in the long days of summer.