High Diversity of Bradyrhizobial Species Fix Nitrogen with Woody Legume Spartocytisus supranubius in a High Mountain Ecosystem

The symbiosis between rhizobia and legumes is of pivotal importance in nitrogen-poor ecosystems. Furthermore, as it is a specific process (most legumes only establish a symbiosis with certain rhizobia), it is of great interest to know which rhizobia are able to nodulate key legumes in a specific habitat. This study describes the diversity of the rhizobia that are able to nodulate the shrub legume Spartocytisus supranubius in the harsh environmental conditions of the high mountain ecosystem of Teide National Park (Tenerife). The diversity of microsymbionts nodulating S. supranubius was estimated from a phylogenetic analysis of root nodule bacteria isolated from soils at three selected locations in the park. The results showed that a high diversity of species of Bradyrhizobium and two symbiovars can nodulate this legume. Phylogenies of ribosomal and housekeeping genes showed these strains distributed into three main clusters and a few isolates on separate branches. These clusters consist of strains representing three new phylogenetic lineages of the genus Bradyrhizobium. Two of these lineages belong to the B. japonicum superclade, which we refer to as B. canariense-like and B. hipponense-like, as the type strains of these species are the closest species to our isolates. The third main group was clustered within the B. elkanii superclade and is referred to as B. algeriense-like as B. algeriense is its closest species. This is the first time that bradyrhizobia of the B. elkanii superclade have been reported for the canarian genista. Furthermore, our results suggest that these three main groups might belong to potential new species of the genus Bradyrhizobium. Analysis of the soil physicochemical properties of the three study sites showed some significant differences in several parameters, which, however, did not have a major influence on the distribution of bradyrhizobial genotypes at the different locations. The B. algeriense-like group had a more restrictive distribution pattern, while the other two lineages were detected in all of the soils. This suggests that the microsymbionts are well adapted to the harsh environmental conditions of Teide National Park.

When time is not of the essence: constraints to the carbon balance of bryophytes

The data available so far indicate that the photosynthetic and relative growth rates of bryophytes are 10% of those reported for tracheophytes. By examining the existing literature and reanalysing data published in over 100 studies, this review examines the ecophysiological, biochemical, and structural reasons behind this phenomenon. The limiting Rubisco content and surface for gas exchange are the internal factors that can explain the low photosynthetic and growth rates of bryophytes. The role of the thicker cell walls of bryophytes in limiting CO2 diffusion is unclear, due to the current uncertainties regarding their porosity and permeability to CO2. From this review, it is also evident that, despite bryophytes having low photosynthetic rates, their positive carbon balance is tightly related to their capacity to deal with extreme conditions. Contributing factors include their capacity to deal with large daily temperature oscillations, and their capacity to delay the cessation of photosynthesis under water deficit (or to tolerate desiccation in extreme situations). Although further studies on bryophytes are needed before more solid conclusions can be drawn, it seems that their success relies on their remarkable tolerance to a highly variable environment, possibly at the expense of their maximum photosynthetic rate.

Summit evergreen shrubs living at a semi-arid treeline: photoprotection systems activation in an open vs an understory site

High-mountain-ecosystems in the Mediterranean-type climate are exceptional because of their outstanding biodiversity but also because of their characteristic drought stress in summer. Still, plant functioning in these habitats has been largely understudied. Here, morphological, photochemical, and biochemical traits were seasonally assessed in six shrubs characterized by contrasting morphological traits, in the Teide mountain in the Canary Islands. Two adjacent populations, the first located in an open site and the second in the understorey of Pinus canariensis treeline forest, were evaluated. We aimed at disentangling (1) the role of morphological and biochemical photoprotective strategies and of their seasonal plasticity to cope with changing environmental conditions in this semiarid ecosystem, (2) how the interspecific differences in biochemical photoprotection are related to leaf morphology and phenology and (3) how living in the understory of the treeline may affect those responses. Our results indicate that both morphological and biochemical traits (particularly leaf habit, morphology and carotenoids from the β-branch) play an intricate role in photoprotection, and that a high interspecific variability exists. According to the down-regulation of photochemical activity and the upregulation of photoprotective molecules, species could be grouped into three types: (1) those more responsive to summer stress (e.g. Descurainia bourgeauana); (2) those more responsive to winter stress (e.g. Pterocephalus lasiospermus, Scrophularia glabrata and Adenocarpus viscosus); and (3) those showing rather constant behavior across seasons (e.g. Spartocytisus supranubius and Erysimum scoparium). In all the species, plants in the open site showed a marked seasonal physiological response in most of the studied parameters. Pinus canariensis canopy buffers environmental abiotic constrains. On a global change scenario, and provided further functional studies are needed, our results pinpoints heterogeneity in the sensitivity of these species against for instance late-frost or summer-heat/drought events, which could easily shift current species distribution in the coming years.