Mario R. Sabbatini

Germination parameterization and development of an after-ripening thermal-time model for primary dormancy release of Lithospermum arvense seeds.

BACKGROUND AND AIMS Models based on thermal-time approaches have been a useful tool for characterizing and predicting seed germination and dormancy release in relation to time and temperature. The aims of the present work were to evaluate the relative accuracy of different thermal-time approaches for the description of germination in Lithospermum arvense and to develop an after-ripening thermal-time model for predicting seed dormancy release. METHODS Seeds were dry-stored at constant temperatures of 5, 15 or 24 degrees C for up to 210 d. After different storage periods, batches of 50 seeds were incubated at eight constant temperature regimes of 5, 8, 10, 13, 15, 17, 20 or 25 degrees C. Experimentally obtained cumulative-germination curves were analysed using a non-linear regression procedure to obtain optimal population thermal parameters for L. arvense. Changes in these parameters were described as a function of after-ripening thermal-time and storage temperature. KEY RESULTS The most accurate approach for simulating the thermal-germination response of L. arvense was achieved by assuming a normal distribution of both base and maximum germination temperatures. The results contradict the widely accepted assumption of a single T(b) value for the entire seed population. The after-ripening process was characterized by a progressive increase in the mean maximum germination temperature and a reduction in the thermal-time requirements for germination at sub-optimal temperatures. CONCLUSIONS The after-ripening thermal-time model developed here gave an acceptable description of the observed field emergence patterns, thus indicating its usefulness as a predictive tool to enhance weed management tactics.

Spurred Anoda (Anoda cristata) Competition in Narrow- and Wide-Row Soybean (Glycine max) 1

The effect of spurred anoda competition in narrow- (35 cm) and wide-row (70 cm) soybean was studied in field experiments for 2 yr. Vigorous early soybean growth in narrow- compared with wide-row soybean resulted in lower radiation transmitted through the canopy, which can partially account for greater competitiveness of narrow-row than wide-row soybean. Soybean plant height was not significantly influenced by the row spacing. Relative yield total (RYT), which is the relationship between yield in mixtures and in monocultures of the crop or the weed and indicates resource complementarity, was equal to 1 with 12 spurred anoda/m2 in the year with less precipitation. Regardless of the row spacing, spurred anoda gave resource use complementarity with the crop (RYT > 1) in all other treatments; therefore, partial avoidance of competition in mixed species was evident. Soybean aggressivity, which takes into account the effect of competition on both the crop and the weed and indicates competitive ability, decreased with weed density in both row spacings. Soybean yield loss at harvest was linearly related to relative dry weight 40 d after planting. Weed-free narrow- and wide-row soybean produced similar yields. In the presence of the spurred anoda, soybean yield was greater in narrow-row compared with wide-row soybean only in the most humid year. A management system that uses quick canopy closure with narrow-row soybean can provide excellent soybean yield and suppression of low spurred anoda densities. Nomenclature: Soybean, Glycine max (L.) Merr.; spurred anoda, Anoda cristata (L.) Schlecht. #3 ANVCR. Additional index words: Aggressivity, competition, crop and weed biomass, row spacing, weed density. Abbreviations: DAP, days after planting; PPF, photosynthetic photon flux; RDW, relative dry weights; RYT, relative yield total.

The impact of agricultural land use on stream chemistry and inputs to an inland reservoir: case of the Sauce Grande River, Argentina

Abstract Water quality in the Paso Piedras Reservoir has deteriorated, primarily due to periodic blue-green algal blooms, making it temporarily unfit for human consumption. This phenomenon results from the eutrophication of waters of the watershed, and nutrients are considered to be the primary contributors to degradation of freshwater quality. In order to reduce the levels of nitrogen and, most of all, phosphorus, entering the Paso Piedras Reservoir, it is necessary to understand the incidence of diffuse transfers of nutrients from agricultural soils. The nitrogen composition, soluble reactive phosphorus (SRP) and other physico-chemical parameters of the water were analysed in various different parts of the watershed in order to characterize water input to the reservoir from areas with different anthropogenic activity. The upper Sauce Grande basin was divided into three areas with different edaphic characteristics. The physico-chemical parameters measured in sampling surveys included: SRP, ammonium and nitrate, pH, conductivity, turbidity, and total solids. The results showed that the water of the Sauce Grande (areas 1 and 2) was not strongly contaminated, but in El Divisorio (Area 3, which is a sub-watershed whose stream flows directly into the Paso Piedras Reservoir), the levels of conductivity, total suspended sediments and SRP were so high that its contribution to the reservoir accounts for nearly 50% of the total SRP input. The main characteristics of El Divisorio watershed that make it a diffuse source of SRP were not only that its soils, under agricultural-livestock production, have significant slopes, so that water and wind erosion are faster, but also that human intervention had caused the destruction of aggregates. The results showed the influence of the soil characteristics, production systems and the micro-environment developed in the reservoir on the characteristics of waters flowing out of the reservoir. The two villages located in Area 2 had no significant influence on the concentration of nitrogen compounds and SRP found downstream.