Plastic morphological response to density in Spergularia marina (L.) Griseb.: Leaf demography and meristem activity

In order to explain, in terms of a morphologically based plastic response, the absence of density-dependent mortality, and the regulation of reproduction in the halophilous annual Spergularia marina , leaf demography and the use of meristems were studied in populations of three different densities. Leaf birth and death rates were almost constant at the higher densities, whereas at the lowest density both increased throughout most of the observation period. This increase was caused by the addition of new vegetative shoots, each one resembling the individuals growing at higher densities in having an almost constant leaf birth and death rate. The capacity to adjust leaf death rate to birth rate with density was high. There was no difference in the proportions of meristems used for flowering as the density increased. After the first flower had been produced, each following node produced a flower. The proportions of meristems used for vegetative shoots were strongly density-dependent; more shoots were produced at lower densities. The regulation of flower production was almost entirely caused by the addition of vegetative shoots.     

The relationship between Rb+ influx and K+-stimulated Mg2+-ATPase activity in oat roots of different K+ status flexible coupling?

The relationship between Rb⁺ influx and microsomal ATPase activity stimulated by K⁺ and Mg²⁺+ K⁺ was investigated for roots of 7-day-old seedlings of oat (Avena sativa L., cv. Brighton). Different concentrations of K⁺ in the roots, K⁺_root were produced by cultivating plants in complete nutrient solutions of different dilutions and dFifferent K⁺ concentrations at various temperatures. Experiments were performed in both light and darkness. The range of the influx/ATPase ratios was large with a factor of 5 or more between the highest and lowest values. In most cases, the highest ratios were obtained at low K⁺_root and at high temperatures, and the lowest at high K⁺_root and at low temperatures. At high temperatures (20 and 25°C) in the light, the influx/ATPase ratio was constant, independently of K⁺_root, if K⁺ in the medium was kept constant but the bulk of the nutrient solution diluted. If K⁺ was varied and the other components of the medium kept constant, the normal relation of decreasing influx/ATPase ratio at increasing K⁺_root was found; thus, Rb⁺ influx appears regulated by both the internal and external potassium conditions. Also in darkness, at 15°C and with K ⁺ in the medium varied, the influx/ATPase ratio was independent of K⁺_root but in the corresponding light experiments, ratio and K⁺_root had the normal, inverse relationship. The difference between light and dark conditions appears to indicate that growth rate is of importance for the relationship between energy input and transport. Our data lead to the concept of “flexible coupling” between transducers) of energy and ion carrier. Without excluding other possibilities, this may be one of the mechanisms for ecological adaptation to variations in the root medium.     

Density-dependent reproductive output in relation to a drastically varying food supply: getting the density measure right

When a limiting resource (e.g. food) varies drastically between years, and population density is measured in the conventional way as individuals per area, demographic processes such as productivity and survival may erroneously be considered density-independent. We tested the hypothesis that if the variation in a limiting resource is not taken into account in the density measure, this may lead to erroneous conclusions about the density-dependence of demographic variables. We studied the food-related variation in productivity of bramblings Fringilla montifringilla , an insectivorous passerine bird, using 19 years of standardised insect censusing, bird censusing and mist-netting of birds in subalpine birch forest in Swedish Lapland. The yearly variation in our measure of brambling per capita productivity (numbers of juveniles per adult trapped) was explained to 30–40% by the larvae abundance of the moth Epirrita autumnata . Taking larvae density into account, no other environmental variable (inferred predation pressure, breeding phenology, and summer temperature) was significantly related to variation in reproductive output. There was no effect of brambling population density on per capita productivity, that is, when density was measured the conventional way, productivity seemed density-independent. However, per capita productivity was significantly and negatively correlated to the food-related population density (population density divided by larval density), supporting the hypothesis that not including a limiting resource into the density measure may indeed lead to erroneous conclusions about the density-dependence of demographic variables.     

Interactions between two rust fungi and their host plant Anemone nemorosa

In a field study we found that two rust fungi. Tranzchelia anemones and Ochropsora ariae. on the shared host Anemone nemorosa varied in abundance among populations. Tranzchelia anemones was favoured in high light habitats and was particularly common in mixed deciduous forests as well as productive grasslands, whereas the abundance and occurrence of O. ariae was dependent on the presence of the alternate host, Sorbus aucuparia When found in the same population, the two rusts rarely attacked the same ramet. Both T. anemones and O. ariae had a negative effect on plant performance by reducing flower production, and rhizome length. Diseased shoots were also elongated.     

Acoustic microscopy of cultured cells

The scanning acoustic microscope (SAM) allows one to measure mechanical parameters of living cells with high lateral resolution. By analyzing single acoustic images’ sound attenuation and sound velocity, the latter corresponding to stiffness (elasticity) of the cortical cytoplasm can be determined. In this study, measurements of stiffness distribution in XTH-2 cells were compared with the organization of F-actin and microtubules. Single XTH-2 cells exhibit relatively high stiffness at the free margins; toward the cell center this value decreases and reaches a sudden minimum where the slope of the surface topography enlargens at the margin of the dome-shaped cell center. The steepness of the increase in slope is linearly related to the decrease in sound velocity at this site. Thus, a significant determinant of cell shape is paralleled by an alteration of stiffness. In the most central parts, no interferences could be distinguished, therefore, this region had to be excluded from the calculations. Stiffness distribution roughly coincided with the distribution of F-actin, but no correlation to microtubule arrangement was found. Following the treatment of XTH-2 cells with ionomycin in the presence of calcium (in the culture medium), the cell cortex first contracted as indicated by shape changes and by a marked increase in stiffness (deduced from sound velocity). This contraction phase was followed by a phase of microtubule and F-actin disassembly. Concomittantly, sound velocity decreased considerably, indicating the loss of elasticity in the cell cortex. No structural equivalent to sound attenuation has been identified.     

Consequences of differential utilization of meristems in the annual Spergularia marina and the perennial S. media

The differences in vegetative expansion and in reproduction associated with the annual Spergularia marina and the perennial S. media were investigated and explained as a function of their differential utilization of meristems. Plant size and the lime of first reproduction depended on at which node transition of the apical meristems from growth to reproduction took place. Leaf birth-rate and number of flowers depended on the time of activation and the proportion of axillary meristems used in producing shoots.