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Rangelands from central Argentina characterize by insufficient warm-season, palatable perennial grasses to domestic livestock. Two palatable and water-stress tolerant cultivars ("Magnar" and "Trailhead") of Leymus cinereus were introduced into Argentina with the purpose of increasing the warm-season forage offer. Some mechanisms involved in determining defoliation tolerance and competitive ability, and subsequent dry matter production, were investigated in the study genotypes. Studies were conducted within an exclosure to domestic livestock. Two severe defoliation treatments were applied in 2006/07 and 2007/08. Only Pappophorum vaginatum, the native genotype, produced and dispersed seeds. Natural water stress during the second study year advanced phenology of all three genotypes. This advancement, together with the high production of total, reproductive and daughter tillers in P. vaginatum, even under defoliation, might contribute to explain its greater abundance than the other native, warm-season perennial grasses in rangelands of central, arid Argentina. Although plant survival and forage production were lower (p0.05), but not lower, in the introduced than in the native genotype. Future research is needed to substantially improve plant establishment of L. cinereus, genotype which can be seeded by conventional drilling. Fil: Torres, Yanina Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Universidad Nacional del Sur. Departamento de Agronomía. Laboratorio de Ecología; Argentina Fil: Busso, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Universidad Nacional del Sur. Departamento de Agronomía. Laboratorio de Ecología; Argentina Fil: Montenegro, Oscar Alberto. Provincia de Buenos Aires. Ministerio de Asuntos Agrarios. Chacra Experimental de Patagones; Argentina Fil: Giorgetti, Hugo Dosindo. Provincia de Buenos Aires. Ministerio de Asuntos Agrarios. Chacra Experimental de Patagones; Argentina Fil: Rodriguez, Gustavo Dionisio. Provincia de Buenos Aires. Ministerio de Asuntos Agrarios. Chacra Experimental de Patagones; Argentina Fil: Ithurrart, Leticia Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina. Universidad Nacional del Sur. Departamento de Agronomía; Argentina

Species historically heavily grazed may have been selected for grazing-resistance traits associated with improved nutrient use efficiency (Chapin and Slack, 1979). Higher root length density (RLD) and vesicular-arbuscular mycorrhizae (VAM) colonization have been associated with greater nutrient acquisition in perennial grasses, thus contributing to competitive ability (Jackson and Caldwell, 1996). This is especially true in unproductive environments where competition for belowground resources is strong (Jackson and Caldwell, 1996). Mycorrhizal fungi can affect competitive interactions among plants by improving soil nutrient uptake (Marschner and Dell, 1994; Mohammad et al., 1998). However, lack of or even negative responses have been obtained under increased levels of mycorrhizal colonization (Fitter and Hay, 1983). Relationships among soil resource acquisition mechanisms and components of shoot tissue production were studied in the late-seral, grazing tolerant and competitive Stipa clarazii Ball and in the earlier-seral, less grazing tolerant and competitive S. tenuis Phil and S. ambigua Speg. History of grazing and/or fire determines abundance of these C3 perennial tussock grasses in temperate, semiarid rangelands of Argentina. We hypothesized lower values for root length density (RLD) and percentage vesicular-arbuscular mycorrhizal (%VAM) colonization on defoliated than on undefoliated plants, and also greater values for RLD and %VAM colonization on defoliated and undefoliated plants of S. clarazii than on those of the other two species, because of its greater competitive ability and defoliation tolerance. Field study was conducted within a 2-year exclosure to domestic herbivory. One group of plants was defoliated once to 5cm stubble height in early spring, while another group was defoliated twice, in early and mid-spring. A third group remained undefoliated (control). Measurements were conducted 6-10 days after each defoliation, and at the end of the growing season. RLD and %VAM colonization were similar on defoliated plants and on undefoliated controls in all three species. Greater regrowth production in S. clarazii was not associated with higher RLD and %VAM colonization in the more competitive species. The positive relationship between RLD and either dry weight production or tissue N and P concentration in these species suggests that their root activity must be maintained after defoliation to rapidly reestablish a photosynthetic canopy and keep resource supply at equilibrium. All perennial grasses showed consistently high %VAM colonization, suggesting they may function as obligate mycotrophs. The negative correlation between %VAM colonization and RLD or soil available P for S. clarazii could represent a mechanism to avoid shifts from mutualism to parasitism in the plant-mycorrhizae relationship. Estudaram-se as relações entre os mecanismos de aquisição de recursos do solo e os componentes de produção de tecido foliar na espécie de etapas serais tardias, tolerante ao pastoreio e competitiva Stipa clarazii Ball e nas espécies de etapas serais mais prematuras, menos tolerantes ao pastoreio e competitivas S. tenuis Phil. e S. ambigua Speg. A história de pastoreio e/ou fogo determina a abundância destas gramíneas perenes cespitosas C3 nos pastizais temperados semi-áridos da Argentina. A hipótese de trabalho foi de que as plantas desfolhadas teriam menor densidade de longitude de raízes (DLR) e porcentagem de colonização por micorrizas vesiculares-arbusculares (%MAV) que aquelas não desfolhadas, e também que as plantas desfolhadas e não desfolhadas de S. clarazii teríam maiores valores de DLR y %MAV que aquelas das outras espécies, por sua maior capacidade competitiva e tolerância à desfolhação. Se conduziu um estudo sob condições de campo em uma área excluída ao pastoreio por herbívoros domésticos durante 2 anos. Um grupo de plantas foi desfolhado uma vez a 5 cm do solo no início da primavera, enquanto que outro grupo foi desfolhado duas vezes, no início e na metade da primavera. Um terceiro grupo permaneceu não desfolhado. As medições se conduziram 6-10 dias depois de cada desfolhação, e no final da estação de crescimento. DLR y %MAV foram similares em plantas desfolhadas e não desfolhadas nas três espécies. A maior produção de rebrote em S. clarazii não esteve associado com maiores DLR y %MAV na espécie mais competitiva. A relação positiva entre DLR e produção de peso seco, ou a concentração de N e P no tecido nestas espécies, sugere que sua atividade radical deve se manter após a desfolhação para restabelecer uma superficie fotossintética e manter a distribuição de recursos em equilíbrio. Todas as espécies mostraram valores consistentemente altos de % MAV, sugerindo que podem ter uma associação mutualística obrigada com as micorrizas. A correlação negativa entre %MAV e DLR ou P disponível do solo para S. clarazii poderia representar um mecanismo para evitar mudanças de associação mutualística a parasítica na relação planta-micorriza. Fil: Saint Pierre, Carolina. Universidad Nacional del Sur; Argentina Fil: Busso, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina Fil: Montenegro, Oscar Alberto. Provincia de Buenos Aires. Ministerio de Asuntos Agrarios. Chacra Experimental de Patagones; Argentina. Universidad Nacional del Sur; Argentina Fil: Rodriguez, Gustavo Dionisio. Provincia de Buenos Aires. Ministerio de Asuntos Agrarios. Chacra Experimental de Patagones; Argentina Fil: Giorgetti, Hugo Dosindo. Provincia de Buenos Aires. Ministerio de Asuntos Agrarios. Chacra Experimental de Patagones; Argentina. Universidad Nacional del Sur; Argentina Fil: Montani, Tomás. Universidad Nacional del Sur; Argentina Fil: Bravo, Oscar Abel. Universidad Nacional del Sur; Argentina

Poa ligularis Nees is one of the dominant perennial grass species in Patagonian grasslands. This is the first field study which investigated the effects of defoliation frequency on its number of viable axillary buds, and subsequent dry matter production during one growing season. Experiments were arranged in a randomized complete block design. Treatments included one defoliation height (10 cm), from one to five times a year, and undefoliated controls. Bud respiratory activity was examined usingthe tetrazolium test and the vital stain Evan's blue. At the end of the growing season, plants that have been defoliated more than three times showed significatively (p

The importance of several factors in limiting recovery from defoliation was investigated in field-grown plants of Agropyron desertorum and Agropyron spicatum exposed to drought , natural or irrigated conditions. Leaf extension rate, components of leaf area production, number of metabolically active axillary buds and carbohydrate availability were examined on the same plants immediately after defoliation and/or in the following spring from 1984 until 1986. The diurnal course of leaf growth did not relate to turgor pressure in the expanded portion of leaf laminae. Rather growth was apparently associated with air temperature. Leaf extension rate was lower under drought than under better moisture levels during 1984 to 1986. For both species, reduced growth rates and shorter growth periods resulted in reduced tiller height, leaf number and leaf size under drought compared with natural or irrigated conditions in 1985 and 1986, but not in 1984. As a result, leaf area and/or yields were also lower under drought in 1985 and 1986, and lowest under drought plus defoliation in 1986. Production of daughter tillers immediately after defoliation was also lowest under drought. Regrowth capacity of both species was not limited by axillary bud number, size or viability immediately after defoliation under any water level in 1986. In early spring, however, tiller number and growth were lower on clipped than on unclipped plants of both species under drought and irrigated conditions in 1986, and under all water levels in 1987; this resulted in considerably reduced photosynthetic canopies on clipped plants. Crown and root total nonstructural carbohydrate (TNC) pools were higher under drought than under better moisture levels in A. desertorum and A. spicatum in early spring 1986. These high pools of TNC apparently enhanced the production of etiolated regrowth in both species when meristematic limitations did not exist in early spring. The productive potential of both Agropyron species will probably not be affected following a late and severe defoliation under drought. However, vegetative growth and/or productivity, and probably the persistence of these species in the community, will be reduced after two or more years of late and heavy defoliations under drought.