DEMOGRAPHY AND GROWTH OF MEDICAGO-MINIMA AND ERODIUM-CICUTARIUM - WATER-STRESS EFFECTS

The demography and growth of two herbaceous annual species, Medicago minima Grufb. and Erodium cicutarium L'Herit, were studied under water stress, natural or irrigated conditions at the field. These species constitute an important forage resource for cattle during spring in the Calden District or caldenal (approximate to 10 million ha), a semi-arid phytogeographic region in central Argentina where droughts are very common. After the first 2 months of the 4-month field study period, the total number of leaves, number of green leaves, height, total lengths and petioles, green leaves and secondary ramifications were lower (p less than or equal to 0.05) under water stress than under better moisture levels in M. minima. The number of flowers and fruits showed a similar response. However, these parameters were similar (p greater than or equal to 0.05) in the different water availability treatments in E. cicutarium during the study period. Plants of both species survived until the end of the study in the water stress treatment, where soil water potentials were lower than -2.0 MPa at this time. This response could explain, at least in part, the persistence of M. minima and E. cicutarium in the south of the caldenal, occupying degraded areas or associated to native grasses.