The microbiological safety of fresh produce is monitored almost exclusively by culture-based detection methods. However, bacterial food-borne pathogens are known to enter a viable-but-nonculturable (VBNC) state in response to environmental stresses such as chlorine, which is commonly used for fresh produce decontamination. Here, complete VBNC induction of green fluorescent protein-tagged Listeria monocytogenes and Salmonella enterica serovar Thompson was achieved by exposure to 12 and 3 ppm chlorine, respectively. The pathogens were subjected to chlorine washing following incubation on spinach leaves. Culture data revealed that total viable L. monocytogenes and Salmonella Thompson populations became VBNC by 50 and 100 ppm chlorine, respectively, while enumeration by direct viable counting found that chlorine caused a <1-log reduction in viability. The pathogenicity of chlorine-induced VBNC L. monocytogenes and Salmonella Thompson was assessed by using Caenorhabditis elegans. Ingestion of VBNC pathogens by C. elegans resulted in a significant life span reduction (P = 0.0064 and P < 0.0001), and no significant difference between the life span reductions caused by the VBNC and culturable L. monocytogenes treatments was observed. L. monocytogenes was visualized beyond the nematode intestinal lumen, indicating resuscitation and cell invasion. These data emphasize the risk that VBNC food-borne pathogens could pose to public health should they continue to go undetected.

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 inbed lines (RILs) derived from a cross between cultivated lettuce (Lactuca sativa cv. Salinas) and wild lettuce (L. serriola acc. UC96US23). Quantitative trait loci (QTLs) 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 and 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 post-harvest properties. By studying the population in two contrasting environments in northern and southern Europe, the genotype by environment interaction effects of the QTLs relevant to leaf development and shelf life were assessed. In total, 107 QTLs, distributed on all nine linkage groups, were detected from the 29 traits. Only five QTLs were common in both environments. Several areas where many QTLs co-located (hotspots) on the genome were identified, with relatively little overlap between developmental hotspots and those relating to shelf life. However, QTLs 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 QTLs 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.

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Baby salad leaves of salad roquette, arugula in the U.S. (Eruca vesicaria ssp. sativa) had an increased postharvest shelf life of 2 to 6 days, while lollo rosso lettuce (Lactuca sativa L. `Ravita') and red chard [Beta vulgaris L. var. flavescens (Lam.) Lam] baby salad leaves had increased shelf life of 1 to 2 days when harvested at the end of the day compared with leaves harvested at the start of the day. We have shown that improved shelf life of salad roquette and lollo rosso following end-of-day harvest was correlated with altered biophysical characteristics of the cell walls, with increased cell wall extensibility (percent plasticity and elasticity) measured at end of day. Leaf turgor pressure (P, MPa) was also highest in salad roquette and red chard at the end of day. Improved shelf life following `end of day' harvest was also associated with the accumulation of leaf sucrose in salad roquette but not lollo rosso and red chard following daily photosynthesis. Diurnal alterations of leaf starch concentration were detected in lollo rosso and red chard but not in salad roquette. The degree of leaf shelf life extension in salad roquette and red chard was further associated with the peak rates of leaf photosynthetic activity. These data suggest that, depending on species, significant improvements to postharvest shelf life could be achieved through the rescheduling of time of day for harvest and also provide relevant information on the selection of traits for future genetic improvement.

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