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Poor germination and slow establishment of Paspalum dilatatum Poir. seeds hinder the inclusion of this species in cultivated pastures. Laboratory and field experiments were conducted to test the effect of different pre-sowing treatments, aimed at improving germination and field emergence. Although initial and total germination at 25°C improved when seeds were mechanically scarified, the expected increase in water uptake was not observed. Alternating temperatures (20-35°C) increased total germination of both control and pre-treated seeds. Osmopriming with polyethylene glycol (PEG, 20 or 30% solution) improved initial and total germination of seeds, even at 20-35°C (the best germination temperature). Further experiments performed to evaluate different pre-imbibition and germination conditions on several seed sources determined it was better to osmoprime dried seeds at 20-35°C. The highest seedling establishment and tillering rates in the field were achieved by pre-germinated and osmoprimed seeds. Other possible physiological effects of osmopriming Paspalum dilatatum seeds are also discussed.

Dallisgrass (Paspalum dilatatum Poir.) is a warm‐season grass, native to South America. Its adoption as a pasture crop has been hindered by low seed germination and slow establishment. However, variability in germination behaviour for this species has never been systematically analysed. For Paspalum spp., dry‐storage and moist cold or warm pre‐conditioning treatments have been reported as effective for the relief or breaking of seed dormancy. In the present work, seed germination responses at 32°C were assayed for sixteen P. dilatatum genotypes, representing its known natural genetic variability, to two moist pre‐conditioning treatments (at 5 and 20°C) and without pre‐conditioning, and three seed storage periods (0, 3 and 6 months). Pre‐conditioning at 20°C showed high germination percentages (>80%), with ratios equal to or higher than pre‐conditioning at 5°C, suggesting that cold is not required to break dormancy. Longer storage times resulted in increases in germination responses for most of the genotypes, while other biotypes showed no effect. Biotypes Virasoro and Chirú showed a remarkably different behaviour with higher germination percentages of untreated freshly harvested seeds. Our results show that diversity for traits involved in seed germination exists among naturally occurring P. dilatatum genotypes and the characterization of these traits should be addressed during ecotype characterization and evaluation of potential domesticates of this species.

Seed germination and seedling emergence are key processes for population recruitment. Flooding and grazing are disturbances forming gaps that may strongly influence recruitment patterns in space and time, but their combined effects and action mechanisms have rarely been addressed. In this study we analysed the effects of microhabitat conditions associated with winter flooding and spring-summer defoliation on seed germination and seedling establishment of Paspalum dilatatum, a dominant perennial C4 grass in native grasslands of the Flooding Pampa, Argentina. The dynamics of seedling emergence from natural seed banks and buried seeds was studied in a factorial experiment with flooding and defoliation treatments applied to soil monoliths (mesocosms) collected from natural grassland. Additional laboratory experiments were applied to investigate seed germination under different combinations of temperature, light quality and simulated flooding. Seed germination and seedling emergence of P. dilatatum were promoted by flooding and high intensity defoliation. Gaps generated by flooding were maintained by high intensity defoliation exercising a synergistic effect on survival seedlings. Flooding resulted in the breaking of seed dormancy and higher germination rates associated with alternating temperature and the activation of the phytochrome system. Our results indicate that microhabitat conditions associated with the disturbances forming gaps, such as flooding and heavy grazing, synergistically promote the recruitment process of this dominant grass species.

Paspalum dilatatum Poir., is a perennial C 4 grass widely distributed in the Argentinean Pampas. The response to water availability for materials developed with forageproduction purposes is unknown. We hypothesized that genetic differences between commercial varieties are reflected in their regrowth capacity under water stress. The effect of five levels of constant water supply on three plant varieties (two derived from apomictic materials: 'Relincho' and 'Alonso' and one from sexually-derived material: 'Primo') were examined in the greenhouse. Leaf-and plant-response traits were followed during 38 d after a single defoliation event. Seven response variables were measured: three of them were morphogenetic (leaf elongation rate, leaf appearance rate and leaf elongation duration) and four were structural (number of live leaves, lamina length, tiller biomass and tiller production). The sexual material showed higher values for growth variables than the apomictic varieties (leaf elongation rate, leaf length and tiller biomass) across the environmental range. Apomictic varieties showed a proportionally similar drought response to the sexual material for the seven variables. No intra-specific trade-off (statistical interaction) was found between growth under high water availability conditions and drought tolerance.

Water supply affects seed germination and seedling establishment of shallow-rooted warm-season grasses. This may explain the difficulty of incorporating Dallisgrass (Paspalum dilatatum Poir.) into humid temperate grasslands through interseeding. We studied the effects of water availability on seed germination and seedling growth under controlled conditions to determine which step of the establishment process was most affected. In a laboratory experiment, seeds were germinated at 0,-0.25,-0.5,-0.75, and-1 MPa water availability generated with solutions of polyethylene glycol. Although both maximum rate and total germination (P , 0.05) significantly decreased with increased water stress, the speed of germination was even more sensitive. In a greenhouse experiment, variations in seedling emergence and morphological characteristics were measured in relation to water availability. Pregerminated and dry seeds were sown in pots that were irrigated at 1-, 2-, 4-, or 7-day intervals. This species showed high sensitivity to water stress during germination and early emergence. High emergence was obtained from the daily irrigation treatment. In all other treatments, where watering frequency was extended, emergence was decreased. Results suggest that rapid germination and early adventitious root growth can be obtained only with reliable availability of water. These conditions, combined with the high temperatures required for breaking seed dormancy, occur infrequently, explaining the difficulty of achieving successful establishment of Dallisgrass. Water availability during seed germination and seedling emergence is a critical factor for survival of this species. Resumen El suministro de agua afecta la germinació n y el establecimiento de las gramíneas megatérmicas de raíces superficiales. Esto puede explicar la dificultad para incorporar Pasto miel en pastizales templado hú medos por medio de la intersiembra. Se evaluaron los efectos de la disponibilidad hídrica sobre la germinació n y el crecimiento de plá ntulas bajo condiciones controladas para determinar cuá l es el paso del proceso de establecimiento que es más afectado. En un experimento de laboratorio se incubaron semillas variando la disponibilidad de agua mediante el uso de soluciones acuosas de Polietilenglicol (PEG 8000) (0, À0.25, À0.5, À0.75, y-1 MPa). La velocidad de germinació n fue más sensible que la germinació n total al aumento del estrés hídrico, aunque ambas disminuyeron significativamente (P , 0.05). En un experimento, realizado en invernáculo, se caracterizó morfoló gicamente el crecimiento aéreo y radical de plá ntulas con diferente disponibilidad hídrica. Se sembraron semillas pre-germinadas y secas en macetas sometidas a diferentes frecuencias de riego (cada 1, 2, 4, y 7 días). Esta especie mostró una sensibilidad elevada al estrés hídrico durante la germinació n y la emergencia temprana. La mayor emergencia se obtuvo bajo el tratamiento de riego diario. En los otros tratamientos, la emergencia de plá ntulas disminuyó con la menor frecuencia de riegos...

Forage production in silvopastoral systems of the Flooding Pampa is based on cool season grasses with a relatively asynchronous phenology regarding their accompanying deciduous trees. However, the productivity of cool season grasses in these systems is usually low. The hypothesis of this work is that the low productivity of cool season grasses is caused by tree litter constraining plant recruitment. Emergence and establishment (reproductive propagation), and tillering (vegetative propagation) patterns of tall fescue, a cool season grass in the region, were studied in two pairs of adjacent non-afforested and afforested poplar stands (tree age 26-28 years, tree density 453-797 plants ha -1 ). Observational and manipulative (i.e. addition of seeds, leaf litter removal) experiments indicated that the recruitment of tall fescue plants is strongly limited by the fall of poplar leaves over emerged seedlings, during autumn. Results suggest that any management practice capable of removing poplar litter, either through grazing or machinery, could neutralize this limitation enhancing the herbaceous primary production of the system.

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