The effects of seed mass and emergence time on growth and reproduction of the annual, C grass, Panicum dichotomiflorum Michx., were investigated in an experimental field study. Individual seeds of P. dichotomiflorum of known mass were sown into marked plots in four newly-abandoned fields that differed in soil nutrient availability, soil moisture and the time of the year in which they were abandoned. Seedling emergence was monitored daily for four weeks following sowing. In none of these fields was there any relationship between seed mass and emergence time. Analyses of covariance indicated that the effects of seed mass and emergence date on plant performance varied among fields. Seed mass affected performance in one field (SOUTH EARLY) where abiotic stress after emergence was high. In contrast, later emergence significantly reduced performance in all four fields. We used a path analysis to decompose the direct and indirect effects of seed mass and emergence time on plant performance in each field. The path analysis revealed that in all four fields, final weight had a significant positive effect on seed production. The effect of seed mass and emergence time had variable (and often opposite effects) on these traits. Seed mass only had a significant positive, direct effect on seed production in the most abiotically stressful field (SOUTH LATE). In the other fields, the effects of seed mass on fecundity were mediated by the effect on final weight. Thus, the effects of seed mass and emergence time on growth and reproduction in P. dichotomiflorum vary across environments.

1. This study examines the distribution of Anabaena affinis filament lengths under natural conditions as a function of depth and season, and in the laboratory as a function of growth phase. Because Anabaena affinis is only toxic when consumed, both its filament length and position in the water column are important determinants of its potential impact on zooplankton populations. 2. Star Lake (Norwich, Vermont, U.S.A.), a natural, eutrophic pond, remained thermally stratified throughout the Anabaena bloom. Filament number and length differed significantly with both sampling date and water depth. Most filaments occurred at 0.5 m, particularly at the height of the bloom. Throughout the entire water colunm average filament length decreased from approximately 0.53 mm in May to 0.14 mm in July. The shortest filaments occurred at the 2.5 m depth. Filament length distributions (combined for all depths) for 29 May, 12 June and 3 July, corresponding to the beginning, middle and end of the bloom, respectively, differed significantly among the three dates. These patterns most likely reflect variable growth conditions, both during the season and in the water colunon. 3. In the laboratory, Anabaena filament length was affected by medium composition and growth phase. Filaments were significantly longer when grown in MBL than in ASM medium. Also, the average length of Anabaena filaments grown in MBL changed significantly as cultures aged; by day 13 filament length (2.01 ± 0.38 mm, mean ± SD) was twice that on day 0 (0.97 ± 0.71 mm). As cell concentration continued to increase, mean filament length gradually decreased.

No abstract

No abstract