The 2012 ASHS Annual Conference

Lettuce tissues can accumulate 4000 ppm nitrate on a fresh weight basis when grown under low light intensity in heated greenhouses. Timely removal of nitrate from the nutrient solution in hydroponics may lower tissue nitrate. The time course of changes in composition of hydroponic lettuce tissues were examined over a 6 to 13 day interval of nitrogen depletion or resupply. Most changes in growth were delayed by 6 days or more after changing N supply, except root growth responded in 3 to 4 days. The total reduced nitrogen increased or decreased for 6 days after nitrogen was depleted or resupplied. Tissue nitrate changed more rapidly. Amino acids responded to a change in N-supply within one day in leaf and root. Within 2 days after the start of depletion, all tissues of N-depleted plants had more sugars compared to N-sufficient plants. Effects of N-depletion on tissue concentrations were reversible within 6 days of the start of depletion. Tissue nitrate changed more rapidly than total nitrogen, or growth responses. However, nitrate in petiole tissue changed more slowly than in leaf blade or roots, and larger plants have more biomass in petiole tissue. Thus, clearing nitrate from lettuce by depleting the nutrient solution occurred more slowly for large compared to small lettuce plants.