Nitrogen Uptake and Plant Growth II. An Empirical Model of Vegetative Growth and Partitioning

An empirical model of vegetative plant growth is presented.The model is based on experimental data on Polygonum cuspidatum,which showed (1) that the partitioning of dry matter and nitrogenamong organs was linearly related to the nitrogen concentrationof the whole plant and (2) that leaf thickness was negativelycorrelated with leaf nitrogen concentration. The model properlydescribes the behaviour of plants. Steady-state solutions ofthe model give the relative growth rate, specific leaf weight,and partitioning of dry matter and nitrogen among organs withthe net assimilation rate and the specific absorption rate asenvironmental variables. The effect of nitrogen removal on drymatter and nitrogen partitioning was examined as non-steady-statedynamic solutions of the model. The model predicted not onlyreduced leaf growth and enhanced root growth but also a fluxof nitrogen from the leaf to the root, which agreed with theexperimental results. Mathematical model, partitioning of dry matter and nitrogen, plant nitrogen, relative growth rate, shoot: root ratio, specific leaf weight     

Synchronization of Mating Reactivity Rhythms in Populations of Paramecium bursaria

Cell populations of Paramecium bursaria show arhythmic mating reactivity after exposure to constant light (LL) for more than 2 wk. After this arhythmic population is exposed to darkness for 9 h, the mating reactivity rhythm of the cell population reappears. The phases of rhythms in individual cells are synchronized to each other. When the arhythmic population in constant light is exposed to dark pulses of various durations, the first peak of the recovered mating reactivity rhythm appears 6 h after the end of the dark pulse. Thus, in the case of dark pulses to cells in LL, the transition from dark to light sets the phase of the subsequent mating reactivity rhythm. When an arhythmic population in LL is transferred to constant darkness (DD), a rhythm of mating reactivity also appears and, in this case, the first peak of the rhythm occurs 18 h after the LL to DD transition. Therefore, arhythmic populations of cells in LL can be synchronized by either a dark pulse or by transition to continuous darkness. When the arhythmic populations in LL were transferred to various light/dark (LD) cycles, the mating reactivity rhythms entrained to LD cycles of 18 to 30 h in duration. Finally, mating rhythms can also be synchronized by treatment with puromycin (400 μg/ml for 6–18 h).     

Dual Appearances of Pigment Cells from In Vitro Cultured Embryonic Cells of Japanese Flounder: An Implication for a Differentiation–Associated Clock

The cells dissociated from developing embryos of Japanese flounder (Paralichthys olivaceus) are cultured in vitro to examine the developmental fate of their pigment cells in relation to establishment of bilaterally asymmetric integumental coloration in vivo. When neurula embryos are dissociated using trypsin–EDTA in Dulbecco's modified Ca²⁺–, Mg²⁺–free phosphate buffered saline and then cultured in vitro using L–15–based fetal calf serum–supplemented growth medium at 20°C, numerous pigment cells appear twice in the same culture with an interval of approximately 1 month even under similar culture conditions. The first group of pigment cells, which is relatively larger in cell size (about 70 μm wide) and lower in cell density, emerges within 12 hr after plating, whereas the second, which is far smaller in cell size (about 30 μm) and overwhelmingly higher in cell density than the first, does so about 1 month after plating. The timing of their appearances in vitro is in good accordance, respectively, with that observed for the larvae under normal development in vivo; the first group appears at the period corresponding to hatching, whereas the second at the period corresponding to the completion of metamorphosis. Light microscopic examinations disclose that each group of pigment cells is composed of black melanophores and reflecting leucophores, and that the population density of melanophores and leucophores in the first group at the climax of appearance is approximated as 1:4. Typical xanthophores that are distributed in the skin of the larvae of this species are scarcely observed in culture in vitro. Because of their dual synchronous appearances with about 1 month interval under the similar culture conditions, and because of their low proliferative activity during the periods from the first appearance to the second, it is presumed that both groups of pigment cells are installed with a clock set differently for their differentiation. Light and electron microscopic immunocytochemistry on cultured cells using the HNK–I antibody, which marks avian migratory neural crest cells, both disclose that the antibody cross–reacts with all these pigment cells, and that a certain number of immunoreactive unpigmented cells exist even at the time of the second appearance of pigment cells. These findings would imply that the second group of pigment cells served in a form of undifferentiated propigment cells up to metamorphosis, at which they start to differentiate under control of a clock presumably set during neurulation.     

Accelerated growth rates in late‐hatched Rhinoceros Auklet Cerorhinca monocerata chicks depend on food conditions and growth stage: an experimental approach

In some bird species, the survival of chicks hatching later in the season is lower than those hatched earlier due to increased risk of predation and a seasonal decline in feeding conditions. To reduce these risks, it might be advantageous for late‐hatched chicks to grow faster and hence fledge at younger age. In this experimental study, the growth rates of early‐ and late‐hatched Rhinoceros Auklet Cerorhinca monocerata chicks were compared under average and poor food supplies in captivity. Controlling for potentially confounding effects of chick mass at 10 days old, chick age and nest‐chamber temperature, late‐hatched chicks had higher wing growth rate than early‐hatched chicks before attaining the minimum wing length required for fledgling under both average and poor food supplies. After attaining the minimum wing length, however, late‐hatched chicks had a lower fledging mass, indicating a potential cost that could diminish the early advantage of fast wing growth.     

The immediate and long‐term effects of water drawdown on macrophyte assemblages in a large subtropical reservoir

1. Disturbances play a central role in determining the spatial and temporal dynamics of many plant communities. In our study of macrophyte assemblages at 150 sites in five arms of a large subtropical reservoir (Itaipu Reservoir, Brazil–Paraguay border), we used co‐occurrence null models and spatiotemporal analyses to describe the patterns in the assemblages during a historically large water drawdown in 2000, in comparison with the previous year (1999) and subsequent years (2001–07). A C‐Score co‐occurrence index tested the null hypothesis of random structure during the drawdown period. A detrended correspondence analysis and multiresponse permutation procedure were used to verify whether species composition differed before, during and after the disturbance. 2. In contrast to our expectations, the null models showed that the macrophyte assemblages were spatially structured during the drawdown (2000), although species composition was significantly different from the previous year (1999) and also changed in the following years (2001–07). Significant species co‐occurrence patterns were generated by the drawdown disturbance, with species extinctions and colonisation by new species from propagules and seed bank germination. 3. The randomness we expected in 2000 actually occurred in 2001, probably because the reestablishment of normal water level enabled both submersed and free‐floating species to recolonise the shore that emergent species had inhabited since the drawdown. Biotic interactions appeared to increase during the years after the disturbance and the habitat preferences of the aquatic macrophytes were re‐established, resulting in higher similarities in aquatic macrophyte species composition in the years after the drawdown.