Effect of NO3 and NH4 concentrations in nutrient solution on yield and nitrate concentration in seasonally grown leaf lettuce

The effect of suboptimal supply of nitrogen (N) and of replacing nitrate in the nutrient solution with ammonia on growth, yield, and nitrate concentration in green and red leaf lettuce was evaluated over two seasons (autumn and spring) using multiple regression analysis. The plants were grown in a greenhouse on a Nutrient Film Technique (NFT) system. Nitrogen concentrations in the nutrient solution were either 3 mM or 12 mM, and the form of N was varied as follows: 100% NO3, 50% NO3 + 50% NH4, and 100% NH4. In both seasons, the biomass (fresh weight) of lettuce heads increased with increasing NO3 concentrations and in autumn, NO3 even at 1.5 mM was sufficient for high yield. However, head dry weight was affected neither by the season nor by changes in the composition of the nutrient solution. The concentration of NO3 had no effect on root dry weight, but it decreased at higher concentrations of NH4. The number of leaves increased as the ratio of NO3 to NH4 in the nutrient solution increased and was higher in autumn because of the longer growth period. Increasing the concentration of NO3 in nutrient solution increased both total N and nitrate concentration in lettuce heads (dry weight) but decreased the concentration of total C. Also, leaf nitrate concentration was lower in spring than in autumn and decreased with increasing NH4 concentration. Nitrogen utilization efficiency was maximum when NH4 levels in the nutrient solution were either 0% or 50% irrespective of the season. Our results thus show that suboptimal N supply in autumn will not affect lettuce yield, and that nitrate concentration in leaves is lower when NH4 concentrations in nutrient solution are higher and also much lower in red lettuce than in green lettuce.