Quantitative trait loci associated with tipburn, heat stress-induced physiological disorders, and maturity traits in crisphead lettuce.

Crisphead lettuce (Lactuca sativa L.) crops exhibit several economically important, physiological disorders when grown in high temperature conditions. These include tipburn, rib discoloration, premature bolting, ribbiness, and internal rib cracking. We evaluated seven physiological disorders and three agronomic traits segregating in a recombinant inbred line (RIL) population consisting of 152 F7 RILs derived from an intra-specific cross between two crisphead cultivars, L. sativa cv. Emperor x L. sativa cv. El Dorado; evaluations were carried out at each of two parental maturities in one planting and at one intermediate maturity in a second planting in each of 2 years for a total of six evaluations. A genetic map was developed using 449 polymorphic SNP markers; it comprises 807 cM in 20 linkage groups that covered 51 % of the nine lettuce chromosomes. Composite interval mapping revealed a total of 36 significant QTLs for eight out of the ten traits evaluated. Significant QTLs were distributed in 11 linkage groups on seven of the chromosomes and accounted for up to 83 % of the phenotypic variation observed. The three largest QTLs for rib discoloration, which accounted individually for 7-21 % of the variation, were clustered with stem length, two with ribbiness and one with head firmness. Three major clusters of QTLs revealed pleiotropic effects or tight linkage between tipburn incidence and severity, head type, stem length, head firmness and ribbiness. One QTL, qTPB5.2, was detected in multiple trials and described 38-70 % of the variation in tipburn incidence. qTPB5.2 is, therefore, a useful candidate gene for breeding for tipburn resistance using marker-assisted selection.