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Variations in phytochrome status in grasses were provoked by end of day red and far-red treatments and above canopy irradiations. Lolum perenne L. and Lodium mutjiJ&orum Lam. plants developed more tillers when illuminated with higher red/far-red ratios. These results show that branching of grasses is controlled by pbytochrome activity in a way similar to that in dicotyledon plants.The effect of light quality upon branching of dicots has been reported for several species. It was noted that Xanthium strumarium L. (1) The results obtained with dicotyledoneon plants and some other information led us to hypothesize that, in grasses, tillering may be affected in a similar way. Some authors (8,14, 15) consider it necessary to remove some of the aboveground biomass periodically to maintain satisfactory light conditions at the bottom of pasture canopies. They often base this on experiments (2,10) showing that an increase in light at the base of plants favored tiller development.Although light intensity effects cannot be ignored, changes in light quality in the lower levels of the canopies might be the cause of the reduction in tillering rates. Such changes are caused by preferential absorption by leaves of R compared to FR wavelengths (4, 11). We considered that in this environment axillary buds would not grow unless green biomass were removed by clipping or grazing. In a previous work (3)

Studies of aerial net primary productivity (ANPP) were made on a grassland that had been excluded from livestock grazing for four years. ANPP was calculated by summation of individual species and corrections based on fluctuations of standing dead litter. The grassland produced a minimum of 4 kg of dry material *ha-'*d-l in the fall and a maximum of 30 kg of dry material-ha-'d -' during the spring. Salado River Basin occupies an area of 5,800,OOO ha in Buenos Aires Province, Argentina, 80% of which is covered by native grassland utilized for cattle production. The proximity of the area to the city of Buenos Aires makes it important as a source of beef to this large area of population. The general aspect of the basin is that of an extensive plain with little or no slope. This results in a great number of permanent ponds and areas subjected to frequent flooding. However, vegetation of the area also suffers from severe summer droughts because of shallow soils. Climate is temperate and humid. Annual precipitation is 900 mm of rainfall that is evenly distributed throughout the year. No snow deposition occurs. This mild weather permits the grasses to maintain productivity during the entire year. The objectives of this study were to describe the (I) dynamics of the aerial biomass and (2) the dynamics of the aboveground net primary productivity (ANPP) of a native grassland of the Salado River Basin and of its major species throughout an entire year. The study was conducted in one of the most conspicuous communities of the basin. This community was described by Leon (1975) using Braun-Blanquet (1950) techniques and named Piptochaetium montevidense, Ambrosia tenuifolia, Echpta bellidioides, and Metha pulegium. Methods Estimates of ANPP were made using a method of successive harvests throughout the entire calendar year. Clippings were made in an ungrazed area that had been excluded from livestock grazing for 4 years. Six harvests were made during the year utilizing rectangular sample plots 0.2 X 3 m. This type of sample plot was selected because it best covered the heterogeneity of the grassland (Fonseca et al. 1976). The number of plots clipped during each sampling period was such that the required level of accuracy for biomass of the main species was fixed at 30% of the mean at 5% level (Milner and Hughes 1970). This resulted in 30 to 40 samples being clipped for each sampling period. Quadrats were randomly located and never occurred in previously clipped places. Litter was collected by hand from each harvested plot. Standing crop samples were clipped to ground level and kept in a freezer Authors are research scientists,