Relationships between photosynthesis, nitrogen and leaf structure in 14 grass species and their dependence on the basis of expression

The relationships between leaf structure, nitrogen concentration and CO₂ assimilation rate ( A ) were studied for 14 grass species grown in the laboratory under non-limiting nutrient conditions. Structural features included leaf thickness and density, and the proportion of leaf volume occupied by different types of tissue (mesophyll, epidermis, vessels and sclerenchyma). Relationships were assessed for data expressed per unit leaf area and fresh mass. The latter was found to be closely related to leaf volume, which allowed us to use A per unit leaf fresh mass ( A _fm) as a surrogate of A per unit leaf volume. Assimilation rate per unit leaf area ( A _a) was positively correlated with leaf thickness and with the amount of mesophyll per unit leaf area; the relationship with leaf nitrogen content per unit area was only marginally significant. A _fm was negatively correlated with leaf thickness and positively with fresh mass-based leaf organic nitrogen concentration. A multiple regression involving these two variables explained 81% of the variance in A _fm. The value of A _fm was also significantly related to the proportion of mesophyll in the leaf volume, but surprisingly the correlation was negative. This was because thin leaves with high A _fm and nitrogen concentration had proportionally more mechanically supportive tissues than thick ones; as a consequence, they also had a lower proportion of mesophyll. These data suggest that, in addition to leaf nitrogen, leaf thickness has a strong impact on CO₂ assimilation rate for the grass species studied.     

Importance of denitrifiers lacking the genes encoding the nitrous oxide reductase for N2O emissions from soil

Analyses of the complete genomes of sequenced denitrifying bacteria revealed that approximately 1/3 have a truncated denitrification pathway, lacking the nosZ gene encoding the nitrous oxide reductase. We investigated whether the number of denitrifiers lacking the genetic ability to synthesize the nitrous oxide reductase in soils is important for the proportion of N₂O emitted by denitrification. Serial dilutions of the denitrifying strain Agrobacterium tumefaciens C58 lacking the nosZ gene were inoculated into three different soils to modify the proportion of denitrifiers having the nitrous oxide reductase genes. The potential denitrification and N₂O emissions increased when the size of inoculated C58 population in the soils was in the same range as the indigenous nosZ community. However, in two of the three soils, the increase in potential denitrification in inoculated microcosms compared with the noninoculated microcosms was higher than the increase in N₂O emissions. This suggests that the indigenous denitrifier community was capable of acting as a sink for the N₂O produced by A. tumefaciens. The relative amount of N₂O emitted also increased in two soils with the number of inoculated C58 cells, establishing a direct causal link between the denitrifier community composition and potential N₂O emissions by manipulating the proportion of denitrifiers having the nosZ gene. However, the number of denitrifiers which do not possess a nitrous oxide reductase might not be as important for N₂O emissions in soils having a high N₂O uptake capacity compared with those with lower. In conclusion, we provide a proof of principle that the inability of some denitrifiers to synthesize the nitrous oxide reductase can influence the nature of the denitrification end products, indicating that the extent of the reduction of N₂O to N₂ by the denitrifying community can have a genetic basis.     

The importance of cytotaxonomy in understanding the biogeography of African rodents: Lake Chad murids as an example

1. Due to the existence of numerous morphologically sibling species among African rodent genera, cytogenetical analyses are essential for correct taxonomic identifications. As an illustration, karyotypes of rodents captured in the Lake Chad area were systematically conducted, to determine them unambiguously. 2. Thirteen species were found, giving a list of at least 18 species in total for the whole Lake Chad region when previously published data were critically re‐examined. Gerbilline rodents dominate in this Sahelian region, with five species of Gerbillus, four of Taterillus, two of Tatera and Desmodilliscus braueri, and six species of murine rodents were also counted. Moreover, two species of Taterillus appeared characterized by unique and hitherto unknown karyotypes, and undoubtedly represent new species. 3. The Lake Chad area, although poorly studied from a biogeographical point of view, occupies a central position in the northern half of Africa which makes its zoogeographical affinities interesting to evaluate. Analysis of the biogeographical characteristics of the rodent species recorded here leads to the conclusion that the murid rodent community of the Lake Chad region is of mixed origin, with a dominant West African component. However, the presence of two species considered as endemics (Mastomys verheyeni and Taterillus lacustris), combined with our discovery of two new chromosomal species of Taterillus, suggest that this region may have acted as a centre of differentiation in relatively recent times, perhaps reflecting the extensive variations of the Lake Chad basin associated with Plio‐Pleistocene cycles of transgression–regression. 4. Without chromosomal data, most of these findings would have remained weakly supported, or even undiscovered, as between five and eight species could have been overlooked.     

Cancer as a consequence of the rising level of oxygen in the Late Precambrian

The origin of multicelled animal life required collagen-family molecules whose own formation depended on the availability of molecular oxygen. Cancers, by contrast, are characterized by their low use of oxygen. In discussing the relationship between the origin of multicelled life and the origin of cancer, it is useful to think in terms of tissues rather than individual cells or complete animals. When animal tissues are disturbed, their constituent cells may be partially released from the constraints of multicellularity. This permits or obliges cells to reactivate anaerobic metabolic ways used by their single-celled ancestors in the oxygen-deficient Precambrian seas. Inhibition or loss of cell respiration under such circumstances may cause reversion to glycolytic fermentation, a less efficient metabolic style that generates waste products that are retained, thereby producing excess cell-growth. Distortion of tissue architecture may ensue with impairment of cell-to-cell adhesion, thereby liberating individual cells. Cells freed from tissue constraints undergo Darwinian variation which leads to loss of differentiation and produces cell types that are incompatible with the normal functioning of tissues. These steps, which may manifest themselves as carcinogenesis, are not reversible by restoration of oxygen and in effect constitute a demergence from the metazoan state. The existence of cancer among diverse phyla and especially among domesticated animals, suggests that the risk of cancer may be an initial condition of complex multicellular life and that it remains preferentially associated with newly modified designs. If so, there would be therapeutic strategies that have not yet been adequately considered. □ Cambrian explosion, cancer, cell differentiation, collagen, glycolysis, hard parts, metazoan origins.