Dry Weight Production and Partitioning inMedicago minimaandErodium cicutariumUnder Water Stress

Medicago minima and Erodium cicutarium are two naturalized annual species in extensive semi-arid, temperate rangelands of central Argentina. A field study was conducted during 1989 and 1990 to evaluate the effects of different levels of soil water availability on above- and below-ground dry weight production and partitioning in these species. Dry weight production by M. minima was more sensitive to water stress than that by E. cicutarium . Although the response was more marked in M. minima , both species allocated a larger proportion of total plant dry weight to fruits under water stress than under irrigated conditions during early spring. Percentage allocation of total plant dry weight into reproductive organs in both species, and stems and peduncles in E. cicutarium , was correlated with total dry weight of these organs; this might indicate a correlation between sink size and strength. Persistence of both species in the local flora appears to be associated with their capacity to colonize open, degraded areas and grow in association with native perennial grasses, as well as to tolerate severe drought periods.