Lettuce (Lactuca sativa L.) Growth and Quality Response to Applied Nitrogen under Hydroponic Conditions

High nitrogen (N) fertilisation is a major cause of nitrate (NO3) accumulation in lettuce. With the objective of reducing N usage without compromising quality, growth responses in lettuce to low N levels were investigated to better understand plant plasticity reactions, expressed by partitioning and mineral content, A greenhouse experiment was conducted with red oakleaf lettuce (Lactuca sativa L. ‘Shiraz’) grown under hydroponic conditions for 4 weeks with six N application levels (40, 75, 150, 400, 1200, 2400 mg L -1 N). Commercial lettuce hydroponic fertiliser, in which all traces of N were removed, was applied equally as base fertiliser to all pots. Both base fertiliser and N levels (applied as Calnitrate at 19% Ca; 15.5% N) were applied manually on alternate days throughout the experiment. At the end of the trial leaf nitrates, yield, total N, carbon (C) and dry matter were measured in roots and leaves. Nitrates were high (5700 and 10000 mg kg -1 DW) in leaves grown under 1200 and 2400 mg L -1 N, respectively. Leaf nitrate content was lower and considered acceptable (26 to 200 mg kg -1 DW) in plants grown under N <1200 mg L -1 . Growth curves followed the expected N plasticity responses for shoots, C:N and root:shoot (R/S) ratios showed an exponential decay with maximum or minimum at 400 mg L -1 N. No significant differences were found between the 400, 1200 and 2400 mg L -1 N treatments for these variables, indicating that 400 mg L -1 N was the ideal application rate. Calcium (Ca) leaf content showed an exponential growth curve of absorption. The results show that high quality lettuce can be grown hydroponically with low nitrate levels and desired mineral accumulation by manipulating plant responses without decreasing yield substantially.