In this second article of a series analysing ecotypic differentiation ofthe Burr Medic (Medicago polymorpha L.),occurring along a 1000 km natural aridity gradient in centralChile, we report variations in growth potential at low temperature in relationto phenology and latitude of collecting site. In two experiments conducted inoutdoor 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 ofBurrMedic, was compared. In Experiment 1, the two early-flowering accessionscollected in the arid and semiarid zones (northern Chile) showedsignificantly greater winter growth potential than the twolate-floweringaccessions from humid, south-central Chile, at five different sowingdensities. In Experiment 2, winter growth was compared, at a single sowingdensity, among 18 accessions collected in various sites along the ariditygradient. Here again, growth patterns were closely correlated to latitude ofcollecting site, with early-flowering ecotypes from the north showingsignificantly greater winter growth potential than those from the south. In athird experiment, relative growth rate, net assimilation rate and leaf arearatio and phytomass production were compared among eight accessions grown undertemperature regimes simulating natural growth conditions at Cauquenes in June(winter) and October (spring), i.e.,12°/7 °Cand 18°/13 °C day/night, respectively. Asbefore, northern accessions showed higher growth potential in the lowertemperature regime than those from humid and perhumid areas at more southernlatitudes. These results suggest that in arid to semiarid environments, whererainfall is erratic and unpredictable, ecotypes of Burr Medic are naturallyselected which show early-flowering and high growth capacity duringwinter, the period of the year when rainfall is most likely to occur inmediterranean-climate regions. Conversely, in humid areas, with higherrainfall predictability, and higher frequency of killing frost, ecotypes havebeen selected with longer periods of growth potential but lower winter growth.Applied and ecological aspects of the results are discussed in the context of along-term program of annual pasture improvement programme.