Rib Discoloration in F2 Populations of Crisphead Lettuce in Relation to Head Maturity

Abstract

Rib discoloration is a physiological disorder associated with heat stress in crisphead lettuce (Lactuca sativa L.). Rib discoloration resistance was studied in a 2-year field experiment using parental and F2 plant populations from a cross of ‘Emperor’, a resistant cultivar, and ‘Eldorado’, a susceptible cultivar. Rib discoloration was evaluated in terms of incidence (percentage of plants with symptoms) and severity (on a 1–5 scale) and was correlated with maturity traits. The rib discoloration severity ratings for the two reciprocal F2 populations were intermediate between the two parents and were not significantly different, indicating the lack of cytoplasmic inheritance for rib discoloration in ‘Emperor’ and ‘Eldorado’. In both parents and F2 progenies, rib discoloration severity was strongly correlated with stem length, head height, head diameter, and head weight, but not with head density. In the resistant parent, more severe rib discoloration was associated with denser heads, whereas in the susceptible parent, the expression of the disorder was independent of head density. The chi-square tests rejected the hypothesis for monogenic segregation in some plantings but not in others. Resistance to rib discoloration is likely to be controlled by more than one gene with a high heritability (h2 = 0.57, P < 0.0001).