Annals of Botany 85: 809-814 (2000)
Developmental Responses to Temperature and Photoperiod in Ecotypes of Medicago polymorpha L. Collected Along an Environmental Gradient in Central Chile
Alejandro Del Pozo, Carlos Ovalle, James Aronson, Julia Avendaņo

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°58'S, 72°l7'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 < 005) by both temperature and photoperiod. In the remaining accessions, no significant responses to temperatures were detected; 1/f was influenced significantly by photoperiod only.

Flowering time appears to be a common adaptive trait in annual legumes along aridity gradients, provided that they have sufficient seed dormancy (e.g. Ehrman and Cocks, 1996; Piano et al., 1996). Here it has been shown that, along the 1000-km-long aridity gradient of the Mediterraneanclimate region of central Chile, clear ecotypic differentiation in the reproductive phenology of burr medic has occurred, allowing adaptation to the various bioclimatic zones present. This has occurred despite the fact that very little genetic diversity has been detected in populations from Chile (Paredes et al., pers. comm.) or from Sardinia (Bullita et al., 1994). Work has now begun on the intra- and interpopulation variability in the reproductive phenology of this species in order to elucidate the mechanisms of ecotypic differentiation. Meanwhile, the identified ecotypic differentiation in the timing of phenophases and other agronomically-relevant traits has allowed the selection and registration of two spineless commercial varieties of burr medic, including a precocious one (derived from accession MPO-7-88) for the arid and semi-arid zones, and a lateflowering one (derived from the accession MPO-43-88) for the sub-humid zone.