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The phenological development of nine Chilean accessions of Medicago polymorpha, collected along a north-south aridity gradient, and of two commercial cultivars of the same species, were compared in 12 sequential outdoor sowings at Cauquenes (35 degrees 58’s, 72 degrees 17’W, elev. 177 m), in the sub-humid Mediterranean climate zone of Chile. A glasshouse experiment was also conducted to evaluate the effect of photoperiod on phenophase timing. There was a clear gradient in precocity among the Chilean accessions in both experiments: accessions MPO-9-88 and MPO-7-88, from the arid zone, were the earliest-flowering accessions, whereas MPO-36-88 from the humid Mediterranean zone was the latest. Both experiments revealed significant variation among the Chilean accessions in the response of flowering time to variation in photoperiod regime. Differences in days to flowering between the least- (8 h) and the most- (16 h) inductive photoperiods were lower in precocious accessions from arid and semi-arid zones, than in late-flowering accessions from more humid zones. Rate of progress to flowering, defined as the inverse of time from emergence to first flower appearance (1/f), was related to mean diurnal temperature, or to both mean diurnal temperature and mean photoperiod. In two early-flowering accessions from the arid zone, and in the Australian cultivar ’Circle Valley’, 1/f was affected significantly (P < 0.05) by both temperature and photoperiod. In the remaining accessions, no significant responses to temperatures were detected; 1/f was influenced significantly by photoperiod only. (C) 2000 Annals of Botany Company.