Ecotypic differentiation in Medicago polymorpha L. along an environmental gradient in central Chile. I. Phenology, biomass production and reproductive patterns

Burr Medic (Medicago polymorpha) is a widespread, polymorphic annual legume of Mediterranean origin, and as a self-reseeding, highly effective nitrogen fixer it has important value in various dryland farming systems. In central Chile, it is naturalized along a 1000 km long environmental gradient, from an arid zone (29 degrees-31 degrees S) to a perhumid one (37 degrees-38 degrees 24' S). Comparative and experimental studies of its ecotypic differentiation patterns along this gradient should be useful in elucidating the evolutionary forces at work along environmental gradients, and also for efforts to select annual legumes for pastures and fallow field improvement in mediterranean climate areas. In this first paper of a two-part series, we present results of two common-environment experiments conducted at a subhumid site in central Chile to compare vegetative and reproductive traits in 69 populations of Burr Medic collected in 1988 and 1994. Multivariate analyses based on 11 phenological and agronomic characters, including phenology and biomass accumulation, winter vigour and degree of hard-seededness, showed that three principal components (PCs) explained nearly 70% of total variability, in both collections. For the larger collection (1988), the diagram of dispersion for the first two PCs--phenology, winter vigour and above-ground biomass--provided good discrimination between accessions from arid-semiarid and humid-perhumid mediterranean zones. Onset of flowering was positively correlated with both latitude and longitude, as well as mean annual precipitation/potential evapotranspiration (PP/ETP) of collecting site; days from first flower to pod ripening was negatively correlated with PP/ETP. In contrast to reports from Syria and Sardinia, no clear trends in Burr Medic seed Abstract: or pod-related traits was found along the Chilean gradient, apart from the concentration of spiny pod accessions in the southern, more mesic end of the gradient. Winter vigour (according to a semi-quantitative analysis of growth) was greater in accessions from the warmer, arid and semiarid zones than in those from cooler, subhumid to humid ones. Both winter vigour and harvest index were negatively correlated with days to first flower. Applied and evolutionary aspects of the results are briefly discussed.