EFFECT OF EC, AND CL AND NH4 CONCENTRATION OF NUTRIENT SOLUTIONS ON NITRATE ACCUMULATION IN LETTUCE
Partial replacement of NO3 by NH4 considerably lowered the nitrate content of winter-grown lettuce. A high level of Cl further decreased the nitrate content of the plants, but only when the nutrient solution contained NH4. The data suggest that, in the presence of NH4, Cl may replace NO3 as a vacuolar osmoticum. However, when NH4 was absent from the nutrient solution, NO3 apparently was preferred to Cl as an osmoticum.
It was assumed that the high EC that is often maintained in commercial practice to prevent glassiness may lead to nitrate accumulation in the crop. To test this assumption, the EC of the nutrient solution was increased without changing the mutual ratios between the nutrient elements. In solutions which exclusively contained nitrate as the nitrogen source, an increase in EC resulted in an increased nitrate content of lettuce in spring and in summer. In the presence of NH4, however, this increase was not observed.
In winter, elevated nutrient solution and air temperatures both stimulated growth rate; in spring, however, only an increase in nutrient solution temperature had this effect. Although an increase in temperature generally also increased the nitrate content of lettuce, the effect on growth rate was more pronounced.