Tissue Elemental Content and Growth Response of Lettuce to Hydroponic Solution Concentration Varied with Cultivar

The growth and tissue mineral elemental content of lettuce [Lactuca sativa L. ‘Paris Island Cos’ (PIC) and 'Black Seeded Simpson' (BSS)] in response to nutrient solution source and concentration were evaluated in a Nutrient Film Technique (NFT) hydroponic culture system. Thirteen-day old seedlings were grown; and harvested 27 days after transfer into NFT hydroponic culture. Fertilizer sources for the two hydroponic solutions comprised of either 100%– or 50% Steiner's nutrient solution formulation, or a soluble fertilizer (15N–2.2P‐12K) at 200 mg·L^-1 of N, plus micronutrients (CAMG). Mean nutrient solution pH was maintained between 6.0 and 6.7, while electrical conductivity (EC) ranged from 1.61 to 2.83. With the exception of copper (Cu), whole lettuce-head tissue analysis indicates no significant (P = 0.05) fertilizer × cultivar interaction for tissue elemental content, or growth parameters measured. Compared with 100% Steiner's and CAMG, average leaf count (LCNT) decreased at 50% Steiner's, in both BSS and PIC (P ≤ 0.002). In addition, head fresh- and dry weight (HFW, HDW) of PIC were reduced at 50% Steiner's (P ≤ 0.001), compared with the 100% level, despite increases in tissue content of nitrogen (N, 2.7–fold) and phosphorus (P, 13.4%). Similarly, 100% Steiner's significantly increased the HFW and HDW of PIC, compared with CAMG and 50% Steiner's (P ≤ 0.001). With BSS, there were no differences in HFW, HW, macro- and micronutrient content among the nutrient solution sources and concentration. Except for copper (Cu), zinc (Zn) and aluminum (Al), which increased at 100% Steiner's (P ≤ 0.0001) in PIC, tissue concentration of iron (Fe), manganese (Mn), boron (B), molybdenum (Mo), and sodium (Na) were highest at 50% Steiner's. Reductions in tissue elemental content at 100% Steiner's may have resulted from the 43% increase in EC (2.83 mS/cm) over the 50% level (1.61 mS/cm). The higher EC at 100% Steiner's caused root tip death and root stunting in seedlings, and delayed plant establishment by about two weeks compared to the latter. With both BSS and PIC, plants grown with CAMG had higher P and potassium (K) content than both levels of Steiner's, and higher N than 100% Steiner's nutrient solution. Similarly, with micronutrients, tissue Fe, Mn, B, and Mo content were higher with CAMG than at 100%, but not 50% Steiner's (P < 0.05).