BACKGROUND: Lettuce accumulates high levels of nitrate when grown in winter in heated greenhouses. Temporarily removing nitrate from the nutrient solution may lower tissue nitrate. The composition of hydroponic lettuce was examined over a 6- to 13-day interval of nitrogen depletion or re-supply.
RESULTS: Growth responses were delayed by 6 days or more after changing N supply, except root growth responded in 3–4 days. The total nitrogen increased or decreased for 6 days. When measured in the light, nitrate concentration changed 10-fold in roots within 2 days. The change in leaf and petiole tissue was slower. Amino acids responded to a change in N-supply within 1 day in leaf and root. All tissues of N-depleted plants had more sugars compared to N-sufficient plants within 2 days after the start of depletion. Nitrogen depletion had reversible effects on growth or tissue concentrations if applied for only 6 days.
CONCLUSIONS: Tissue nitrate changed more rapidly than total nitrogen, or growth responses. However, nitrate changed more slowly in petiole than in leaf blade or roots, and larger plants have more biomass in petioles. Thus, clearing nitrate from lettuce is slower for large compared to smaller lettuce plants. Copyright © 2012 Society of Chemical Industry