QTL for shelf life in lettuce co-locate with those for leaf biophysical properties but not for leaf developmental traits

Developmental and biophysical leaf characteristics that influence post-harvest shelf life in lettuce, an important leafy crop, have been examined. The traits were studied using 60 informative F9 recombinant inbreed lines (RILs) derived from a cross between cultivated lettuce (Lactuca sativa cv. Salinas) and wild lettuce (L. serriola acc. UC96US23). Quantitative Trait Loci (QTL) for shelf life co-located most closely with those for leaf biophysical properties such as plasticity, elasticity and breakstrength suggesting that these are appropriate targets for molecular breeding for improved shelf life. Significant correlations were found between shelf life with leaf size, leaf weight, leaf chlorophyll content, leaf stomatal index and epidermal cell number per leaf, indicating that these pre-harvest leaf development traits confer postharvest properties. By studying the population in two contrasting environments in northern and southern Europe, the genotype by environment interaction effects of the QTL relevant to leaf development and shelf life were assessed. In total 107 QTL, distributed on all nine linkage groups, were detected from the 29 traits. Only five QTL were common in both environments. Several areas where many QTL co-located (hotspots) on the genome were identified, with relatively little overlap between developmental hotspots and those relating to shelf life. However, QTL for leaf biophysical properties (breakstrength, plasticity and elasticity) and cell area correlated well with shelf life, confirming that the ideal ideotype lettuce should have small cells with strong cell walls. The identification of QTL for leaf development, strength and longevity will lead to a better understanding of processability at a genetic and cellular level, and allow the improvement of salad leaf quality through marker assisted breeding