Beschreibung:
<jats:p> Cultivars of Lolium perenne L. when removed from their area of adaptation, especially in the latitudinal sense, are usually less persistent than adapted varieties and disappear after a few years. This phenomenon is particularly remarkable for North European varieties when cultivated in Mediterranean regions which are characterized by a period of drought stress (from the 2nd week of June to 2nd week of August). Two varieties of Lolium perenne L., Vejo (adapted to Central Italy) and Lenta (North European cultivar), were sown in September 1979 and compared to evaluate the effects of water stress on persistence from June 1980 to January 1982 at Perugia, central Italy. Measurements recorded included ground cover, plant survival, dry matter yield (DMY), number of tillers per plant, and water soluble carbohydrate (WSC) content. Trials were performed, according to a split-plot design with three replicates, in dense stands (seeding rate of 2000 viable seeds/m<jats:sup>2</jats:sup>) and in spaced plants (40 × 60 cm apart) which were either irrigated or nonirrigated. The results show that Vejo, starting from the 2nd year from sowing, had higher values than Lenta for ground cover, plant survival and number of tillers per plant. The same behavior was observed for DMY with the exception of the August 1981 sampling in which the situation was reversed because of the summer dormancy of the local variety. The pattern of WSC content also differed between the two cultivars with the adapted type characterized by higher values than the unadapted one except in winter. Irrigation during the dry season increased DMY by preventing dormancy of the local cultivar and by stimulating vegetative activity of the unadapted one, but neither influenced the mortality of the foreign cultivar or the WSC content in either variety. These results suggest that the low DMY produced in summer by the unadapted cultivar is obtained at the expense of carbohydrate reserves. The reduction of WSC (mg∙plant<jats:sup>−1</jats:sup>) in Lenta from May to August was 43% in dry and 47% in irrigated conditions versus 22% and 32%, respectively, in Vejo. Severe depletion of reserves seems to be the main cause of the thinning of the unadapted variety (the ground cover evaluated in October 1981 was 18% and 22% in Lenta and 82% and 93% in Vejo in nonirrigated and irrigated conditions, respectively). For these reasons the lack of persistence of the unadapted cultivars of perennial ryegrass in Mediterranean regions cannot be attributed to the extent of summer water shortage characterizing these areas.Key words: Lolium perenne L., environmental adaptation, persistence, carbohydrate reserves, stress in Mediterranean areas </jats:p>