Dry Matter Partitioning between Plant Organs in Wheat Cultivars Differing in Productivity and Drought Resistance

In microplot field experiments conducted over the course of 12 years, the accumulation of dry matter was recorded in the leaves, stems, and ears of the primary shoots of plants of four cultivars of spring wheat (Triticum aestivumL.) differing in productivity and drought resistance. The contribution of organs to the accumulation of dry matter by the shoot from emergence to anthesis was calculated, and relative changes in the weight of the ear after anthesis were assessed. In all the cultivars, the contribution of the leaves was the same; however, the share of the ear was greater in short-stem cultivars of the intensive type in which the leaves were more active during the time of grain filling. Furthermore, this load increased, and the relative increment in the ear weight after flowering decreased in the years of drought, because the contribution of the leaves did not depend on water supply during the growing season. During these years, the contribution of the ear increased in the plants of all the cultivars. The authors conclude that, in arid regions, in addition to drought resistance, the cultivar should display such a ratio between the ear weight and the weight of leaves that would still ensure satisfactory grain filling     

Effects of Light Spectral Quality on Morphogenesis and Source–Sink Relations in Radish Plants

The accumulation of dry matter and the content of major phytohormones in the aboveground and underground plant parts, as well as light curves and the diurnal course of photosynthesis in the leaves were studied in radish (Raphanus sativusL.) plants of different ages that were grown under red (RL) or blue (BL) light. As seen from the rapid increase in plant biomass, the development of storage organs (hypocotyl or tap root) started on the 14th day after the emergence of seedling of the BL plants and on the 21st day for the RL plants. Conversely, RL stimulated biomass accumulation in the aboveground parts (petioles and stems) already in the early stages of plant development. Light spectral quality only slightly affected the activity and the diurnal course of photosynthesis. The GA content was ten times higher in the aboveground parts of the RL plants than those of the BL plants. The hypocotyl of the BL plants contained much higher amounts of cytokinins and IAA than that of the RL plants. The specific responses of the source–sink relations to the light quality were related to the distribution of various phytohormones between the aboveground and underground parts of the plants: RL increased the content of gibberellins (GA) in the aboveground parts of plants, thus increasing their sink activity, whereas BL stimulated the synthesis of cytokinins and IAA in the hypocotyl and enhanced its development. Light quality-specific morphogenetic responses were reversed when plants were treated with exogenous GA or paclobutrazol, an inhibitor of GA synthesis. The treatment of the BL plants with exogenous GA stimulated petiole and hypocotyl elongation and induced stem formation. The treatment of the BL plants with paclobutrazol led to shortened petioles, the flattening of the storage organ, and the disappearance of the stem.     

Temporal Pattern of Photosynthesis under Continuous Illumination of Radish Plants

Changes in photosynthetic activity, redox state of photosystem I (PSI) and photosystem II (PSII), as well as starch and sucrose content were studied on the source leaves of 18- to 20-day-old radish (Raphanus sativus L.) plants that were dark-adapted for 12 h and then exposed to continuous white light (170 mol quanta/(m² s)). The kinetic pattern of photosynthetic activity comprised three phases. Within the first 6 h of light adaptation (first phase), the maximum photosynthetic rate and the quantum yield of photosynthesis increased 1.6 times in the illuminated leaves compared to the leaves of plants placed in darkness. Further illumination led to the decrease of both photosynthetic indices by about 20% (12 h after the onset of light exposure, second phase) and finally increased them to the level observed after 6-h light exposure (72 h, third phase). The stationary photooxidation level of PSI primary donor was relatively low within the first 6 h of light adaptation, and then it steeply increased. The linear relationship between the amounts of photoreduced PSII primary acceptor and photooxidized PSI primary donor did not change during prolonged light adaptation, showing a highly coordinated functioning of both photosystems. The amount of sucrose in leaves attained its peak after 12 h of light adaptation and did not change further on. The starch content increased to its peak within 24 h of illumination and decreased gradually upon longer exposures. It is concluded that, despite active export of assimilates to the developing storage organ, the source leaves exhibit a nonmonotonic temporal course of endogenously regulated photosynthetic activity, which was related to changes in the effectiveness and, possibly, the number of the components of photosynthetic apparatus.     

Metapopulation dynamics of a diadromous galaxiid fish and potential effects of salmonid aquaculture

1. Direct ecological effects of biological invasions have been widely documented, but indirect genetic effects on native species are poorly known. In many cases, this is because of the lack of information on the genetic structure of species affected by invasions. 2. We used microsatellite DNA loci to estimate the genetic structure and gene flow patterns of Galaxias maculatus, a galaxiid fish endemic to the southern hemisphere, which is increasingly being threatened by salmonid invasions. 3. Analysis of nine diadromous populations of G. maculatus in Chilean Patagonia (an area heavily impacted by farming of non‐native salmonids) indicates that dispersal is mostly a passive process, seemingly driven by wind and currents and resulting in high gene flow and weak population structuring. 4. Gene flow was asymmetrical, with three populations acting as sources and six populations acting as sinks. Sinks had lower habitat quality and had a greater incidence of adults than sources, which consisted mostly of juveniles. 5. Rivers invaded by salmonid escapees experienced significantly higher aquaculture pressure than rivers where salmonid escapees were apparently absent, but no effect on genetic diversity of G. maculatus could be detected. 6. We discuss whether salmonid aquaculture might affect the demography and connectivity of galaxiid metapopulations: indirectly through habitat alteration and directly through escapes of predatory fish.     

Ecology and population structure of the bayou darter, <Emphasis Type="Italic">Etheostoma rubrum</Emphasis>: disjunct riffle habitats and downstream transport of larvae

The bayou darter, Etheostoma rubrum (Percidae), is endemic to the Bayou Pierre system in Mississippi. Adult and juvenile E. rubrum occupy swift, shallow riffles or runs over coarse gravel and pebble substrata. Habitat requirements of larval and post-larval stages, and the role of downstream dispersal of larvae in colonizing riffles are poorly known. The potential for movement and the high level of habitat specificity for the discontinuous riffle habitat suggest that E. rubrum may comprise a metapopulation of linearly arranged local populations. The greatest population densities of E. rubrum occur in the upper reaches of Bayou Pierre. We hypothesized that metapopulation structure of E. rubrum may include source–sink dynamics, whereby downstream areas are a sink for larvae/early juveniles originating upstream. We tested hypotheses that a transport mechanism, larval drift, occurred in E. rubrum, and that downstream riffles showed characteristics of population sinks. We captured larval stages of E. rubrum in cross-sectional and longitudinal drift nets, and rates of drift tended to increase during the day. Larval E. rubrum (n=19) occurred in samples above and below riffle areas, with no differences among longitudinal drift nets placed above and below riffles. Thus, larval drift is a viable transport mechanism. Relative abundance of adults and juveniles declined from upstream to downstream, and inter-riffle distances increased with cumulative downstream distance. However, both predictions of the source–sink hypothesis were not supported. The distribution of size classes did not change between upstream and downstream riffles nor did the mean size-adjusted body mass.     

Source–sink dynamics: how sinks affect demography of sources

Models of source–sink population dynamics have to make assumptions about whether, and eventually how, demographic parameters in source habitats are dependent on the demography in sink habitats. However, the empirical basis for making such assumptions has been weak. Here we report a study on experimental root vole populations, where estimates of demographic parameters were contrasted between source patches in source–sink (treatment) and source–source systems (control). In the presence of a sink patch (simulated by a pulsed removal of immigrants), source‐patch populations failed to increase over the breeding season, mainly due to a high spatially density‐dependent dispersal rate from source to sink patches. The per capita recruitment rate was almost two times higher in source–sink than in the source–source systems, but this did not compensate for the loss rate due to dispersal from source to sink patches. Sex ratio in the source–sink systems became less female biased, probably as a result of an enhanced frequency of dispersal movements in females. Good knowledge of the degree of density‐and habitat‐dependent dispersal is critical for predicting the dynamics of source–sink populations.     

HOW DOES POLLEN VERSUS SEED DISPERSAL AFFECT NICHE EVOLUTION?

In heterogeneous landscapes, the genetic and demographic consequences of dispersal influence the evolution of niche width. Unless pollen is limiting, pollen dispersal does not contribute directly to population growth. However, by disrupting local adaptation, it indirectly affects population dynamics. We compare the effect of pollen versus seed dispersal on the evolution of niche width in heterogeneous habitats, explicitly considering the feedback between maladaptation and demography. We consider two scenarios: the secondary contact of two subpopulations, in distinct, formerly isolated habitats, and the colonization of an empty habitat with dispersal between the new and ancestral habitat. With an analytical model, we identify critical levels of genetic variance leading to niche contraction (secondary contact scenario), or expansion (new habitat scenario). We confront these predictions with simulations where the genetic variance freely evolves. Niche contraction occurs when habitats are very different. It is faster as total gene flow increases or as pollen predominates in overall gene flow. Niche expansion occurs when habitat heterogeneity is not too high. Seed dispersal accelerates it, whereas pollen dispersal tends to retard it. In both scenarios very high seed dispersal leads to extinction. Overall, our results predict a wider niche for species dispersing seeds more than pollen.     

Where do you come from,where do you go? Directional migration rates in landscape genetics

Identifying landscape elements that influence gene flow and migration in wild species is the current main topic of landscape genetics. Most landscape genetic studies infer gene flow and migration from genetic distances among populations or individuals and statistically relate these measurements to landscape composition and configuration. This approach assumes symmetrical gene flow between pairs of populations. Such an assumption, however, will often be violated, especially in source–sink systems. Source populations provide more emigrants than they receive immigrants, and sink populations get many immigrants, but release few emigrants. Source–sink dynamics cannot be explored using common landscape genetic approaches relying on genetic distances. In this issue of Molecular Ecology, Andreasen et al. ( 2012 ) apply an alternative approach allowing them to infer asymmetrical migration. They use a Bayesian assignment test among objectively defined populations of mountain lions (Puma concolor) in western USA to estimate recent and directional migration rates. The study shows that an area with a high amount of wildlife refuges and low hunting pressure harbours a source population for mountain lion dispersal, while areas with high hunting pressures form sink populations; a result helpful in making informed decisions in conservation management.     

Effects of a Marine Reserve on Recruitment of Grunts (Pisces: Haemulidae) at Barbados, West Indies

The effects of a non-extractive marine reserve on the recruitment dynamics of haemulid fishes and their predators on Barbados coral reefs were studied using visual census and mark-recapture methods. Size and abundance of piscivores (including large adult grunts) known to prey on grunts were greater within the reserve than on adjacent reefs, whereas size and abundance of older juvenile grunts did not differ between protected and exploited reefs. Recruitment and early juvenile abundance were lower within the reserve and were inversely related to predator density (including adult conspecifics). Patterns in density of new recruits may also have been influenced by oceanographic patterns of supply of larvae. Thus, although protection has a significant positive effect on the size and abundance of large carnivorous fishes, higher predation pressure within a reserve may serve to reduce juvenile recruitment within the reserve. At some size/age, cumulative recruitment due to lower size-specific predation mortality results in higher density within the reserve. This increased density is maintained by the absence of fishing mortality within the reserve. Despite maintaining high spawning biomass of several large, commercially exploited species that may export larvae to downstream areas, the Barbados Marine Reserve appears to be a local sink for juvenile grunts.