Evidence of an antilisterial factor induced by wounding of iceberg lettuce tissues.

AIMS: To examine the influence of wound-associated reactions in cut iceberg lettuce (Lactuca sativa L.) tissues on the fate of Listeria monocytogenes. METHODS AND RESULTS: Aqueous extracts prepared from shredded iceberg lettuce before and after storage in high oxygen permeability film were inoculated with L. monocytogenes. Listeria monocytogenes grew in extracts prepared from fresh lettuce. In contrast, inhibition ranging from arrested growth to a decline in cell viability was observed in extracts prepared from samples stored for 1-3 days. Similar behaviour was evident in lettuce shreds inoculated with 10(5) CFU g(-1)L. monocytogenes immediately after processing or after 3 days in storage. Heat treatment of the cut tissues at 47 degrees C for 3 min before storage diminished the inhibitory effect. CONCLUSIONS: The results provided evidence that an antilisterial factor or factors are released by wounded iceberg lettuce tissues. Antilisterial activity was mitigated by heat treatment of the lettuce. SIGNIFICANCE AND IMPACT OF STUDY: This study indicates that intrinsic factors associated with plant metabolism could play a significant role in the ecology of human pathogens in packaged horticultural products.

Effect of freezing on the activity of catalase in apple flesh tissue.

Catalase (CAT, EC 1.11.1.6) activity was measured in flesh tissue of six apple cultivars (Malus domestica Borkh. cvs. Braeburn, Gala, Jonagold, McIntosh, Red Delicious, and Spartan). Activity of CAT was determined for fresh and frozen tissue of the same fruit. Freezing resulted in reductions of 50 to 90% in CAT activity compared with the activity measured in crude extracts from fresh tissues. The rate of freezing had an impact on the level of reduction of CAT activity, with slower freezing procedures leading to greater losses in activity. Six additives to the extraction buffer were tested to evaluate their potential to reduce the inactivation of CAT from frozen tissue, but only EDTA and Tween 20 showed any benefit. However, EDTA and Tween 20 provided only partial recovery in CAT activity. In contrast, crude extracts prepared from fresh tissue showed no appreciable loss in CAT activity after frozen storage for two weeks at -80 degrees C. Gel electrophoresis and immunological analysis indicated that the loss in CAT activity from tissue freezing could be attributed to loss of both the tetrameric CAT enzyme structure and total CAT protein. The implications of using freezing to preserve apple tissue samples prior to catalase activity analysis is discussed.

Regeneration of plantlets from in vitro cultured cotyledons of white spruce (Picea glauca (Moench) Voss).

Adventitious shoot production from seedling cotyledons was investigated for white spruce [Picea glauca (Moench) Voss]. The age of the seedling was found to be important for shoot induction response, the optimum seedling age being 7 to 8 days old. Prior to that age, although response was seen, the multiplication rate was lower. After 7 to 8 days, the capacity to produce shoots declined considerably. The optimum cytokinin (BA) concentration for bud induction was 2 µM BA. The response to specific BA concentrations was independent of seedling age. The shoot regeneration presented here was highly reproducible and provided primary multiplication rates of approximately 100 to 150 shoots per seedling which had an average of 6 cotyledons. Approximately 30% of 40 regenerated shoots were induced to differentiate roots and all the rooted plantlets were successfully transplanted to soil.