SUMMARY
Genotypic variation in nitrate concentrations of different lettuce genotypes belonging to four types (Butterhead, Cos/Romaine, Crisphead, and Stem lettuce) and its relationship to nitrate reductase genes (Nia) for low nitrate concentrations of leaves was investigated. The results showed that the Romaine genotype ' Lobjoit's Green' exhibited the lowest leaf nitrate concentration (4877.7 mg/kg dry weight). The highest nitrate levels were recorded in the Butterhead genotypes 'Augusta and Kennedy'. Nitrate concentrations in the roots were significantly higher in the Romaine genotype ' Little Gem' and the Butterhead genotypes 'Augusta and Merveille Des Quatre Saisons' than the other lettuce genotypes. Total N concentrations were higher in the Butterhead and Crisphead genotypes than in the Romaine and Stem lettuce genotypes. A significant positive association (r = 0.80, p > 0.01) was observed between leaf NO