Ranking Mediterranean-Type Shrubs and Trees by their Allelopathic Activity is Not Independent of How Extract Concentration is Expressed

Water extracts from 19 Mediterranean-type shrubs and trees were screened for phytoactivity on germination of lettuce. The existing model for the effects of pH and osmotic pressure on germination requires refitting. Extract concentrations were expressed as plant fresh weight, plant dry weight and extract dry weight, and final ranking of the six phytoactive species was found to strongly depend on the way the concentrations are expressed. This methodological issue requires consideration when designing allelopathic bioassays. Extract dry weight is conceptually the most adequate way to express concentration and should be used, despite the increase in time and labour it requires.