Genetics of nitrate content in lettuce

Summary Adverse effects on human health makes the high nitrate content frequently found in lettuce (Lactuca sativa L.) grown under low light conditions an undesirable trait. Efforts have been made to breed cultivars with a reduced capacity for nitrate accumulation. In this study components of variance for nitrate content were estimated in F₂ and F₃ generations of ten lettuce crosses. Additive genotypic variances (A) were estimated from F₃ variance components and from the covariance between F₂ plants and corresponding F₃ lines. Estimates of wide sense heritability of the F₂ from crosses between a high nitrate genotype and four low nitrate genotypes ranged from 0.44 to 0.74 and the estimates for A ranged from 0.25 to 0.40 g·l^-1. Estimated wide sense heritabilities of F₂'s from six crosses involving two low nitrate parents ranged from 0.15 to 0.52. The parents of four of the low nitrate crosses showed relatively large effects of genotype x environment (GE) interaction in successive experiments: the nitrate content of the parents reacted differently to environmental changes between experiments. Estimates of A for crosses between low nitrate genotypes without large effects of GE interaction ranged from 0 to 0.19 g·l^-1. The estimated probability of selecting transgressive low nitrate lines in the progeny of a cross between a high and a low nitrate genotype was low (P=0.002–0.039), indicating that large populations should be evaluated to combine the positive traits of modern high nitrate cultivars with low nitrate content from genotypes not adapted to modern cropping practices. In the progenies from crosses between two low nitrate genotypes without important GE effects, only low estimates of the probability of obtaining transgressive low nitrate lines were obtained (P=0.04–0.06). With the growth conditions used in this study, the probability of selecting lines with a nitrate content compatible under all winter conditions with the proposed future maximum permissible level of 2.5 g nitrate per kg fresh matter is low. Therefore the solution of this problem should be found in a combination of low nitrate cultivars and cultural measures that reduce the nitrate content of the crop.