Genotype × Environment Interaction for Nitrate Concentration in Lettuce

High nitrate concentration is a problem when lettuce (Lactuca sativa L.) is grown under low light intensities. Genotypes with low nitrate concentrations have been identified and are being used in breeding programmes. Previous results indicated the occurrence of genotype × environment (GE) interactions. Because of the important influence of light intensity on nitrate accumulation, two types of interactions may be expected: interactions related to daily variation, and those related to annual variation. In the present investigation both types were investigated using eight butterhead genotypes of lettuce which were repeatedly harvested. No daily variation in nitrate concentration and no corresponding GE-interactions were found, irrespective of the level of global radiation. In contrast, a large annual variation and important corresponding GE-interactions were found. Joint regression analysis on environmental means and on physical factors related to light intensity showed a differential response of genotypes to changing environmental conditions. Multiple joint regression on daylength and change in daylength accounted for two-thirds of the interaction variance. However, deviations from regression were still significant indicating non-linearity of the relationship, or, the existence of other environmental factors contributing to GE-interaction.