Abstract

In this second article of a series analysing ecotypic differentiation of the Burr Medic (Medicago polymorpha L.), occurring along a 1000 km natural aridity gradient in central Chile, we report variations in growth potential at low temperature in relation to phenology and latitude of collecting site. In two experiments conducted in outdoor nursey trials at Cauquenes, in the 'intermediate' (i.e., subhumid) mediterranean zone (35°58? S, 72°17? W), biomass accumulation among 4 and 18 accessions of Burr Medic, was compared. In Experiment 1, the two early-flowering accessions collected in the arid and semiarid zones (northern Chile) showed significantly greater winter growth potential than the two late-flowering accessions from humid, south-central Chile, at five different sowing densities. In Experiment 2, winter growth was compared, at a single sowing density, among 18 accessions collected in various sites along the aridity gradient. Here again, growth patterns were closely correlated to latitude of collecting site, with early-flowering ecotypes from the north showing significantly greater winter growth potential than those from the south. In a third experiment, relative growth rate, net assimilation rate and leaf area ratio and phytomass production were compared among eight accessions grown under temperature regimes simulating natural growth conditions at Cauquenes in June (winter) and October (spring), i.e., 12°/7°C and 18°/13°C day/night, respectively. As before, northern accessions showed higher growth potential in the lower temperature regime than those from humid and perhumid areas at more southern latitudes. These results suggest that in arid to semiarid environments, where rainfall is erratic and unpredictable, ecotypes of Burr Medic are naturally selected which show early-flowering and high growth capacity during winter, the period of the year when rainfall is most likely to occur in mediterranean-climate regions. Conversely, in humid areas, with higher rainfall predictability, and higher frequency of killing frost, ecotypes have been selected with longer periods of growth potential but lower winter growth. Applied and ecological aspects of the results are discussed in the context of a long-term program of annual pasture improvement programme.