Time course of nodule development in the Discaria trinervis (Rhamnaceae) –Frankia symbiosis

The time course of initiation and development of root nodules was investigated in the South American actinorhizal shrub Discaria trinervis (Rhamnaceae). A local strain of Frankia (BCU110501) which was isolated from D. trinervis nodules, was used as inoculum. Inoculated seedlings were periodically studied under the light microscope after clearing with aqueous NaClO. In parallel, semithin and ultrathin sections were analysed by light and electron microscopy. Infection by Frankia BCU110501 involved intercellular penetration among epidermal and cortical root cells. Nodule primordia were detected from 6 d after inoculation, while bacteria were progressing through intercellular spaces of the outer layers of cortical cells. Invasion of host cells by the symbiont occurred 7–9 d after inoculation, and hypertrophy of the primordium cells was associated with Frankia penetration. Root hairs were not deformed during the early events of nodule formation. From 13 to 16 d after inoculation, the proximal cellular zone of the primordia behaved differently from the other tissues after NaClO treatment and remained darkly pigmented. At the same time, differentiation of Frankia vesicles started to occur inside already infected cells. By 16 d after inoculation, spherical vesicles of BCU110501 were homogeneously distributed in the host cells. These vesicles were septate and surrounded by void space. Frankia spores or sporangia were not observed in the nodule tissue. This study has clarified the mode of Frankia penetration in D. trinervis , one of the Rhamnaceae which also includes Ceanothus . The events involved in infection, nodule induction, host-cell infection and vesicle differentiation have been characterized and identified as time-segregated developmental processes in the ontogeny of D. trinervis root nodules.     

Tetraploid Rangpur lime rootstock increases drought tolerance via enhanced constitutive root abscisic acid production

Whole‐genome duplication, or polyploidy, is common in many plant species and often leads to better adaptation to adverse environmental condition. However, little is known about the physiological and molecular determinants underlying adaptation. We examined the drought tolerance in diploid (2x) and autotetraploid (4x) clones of Rangpur lime (Citrus limonia) rootstocks grafted with 2x Valencia Delta sweet orange (Citrus sinensis) scions, named V/2xRL and V/4xRL, respectively. Physiological experiments to study root–shoot communication associated with gene expression studies in roots and leaves were performed. V/4xRL was much more tolerant to water deficit than V/2xRL. Gene expression analysis in leaves and roots showed that more genes related to the response to water stress were differentially expressed in V/2xRL than in V/4xRL. Prior to the stress, when comparing V/4xRL to V/2xRL, V/4xRL leaves had lower stomatal conductance and greater abscisic acid (ABA) content. In roots, ABA content was higher in V/4xRL and was associated to a greater expression of drought responsive genes, including CsNCED1, a pivotal regulatory gene of ABA biosynthesis. We conclude that tetraploidy modifies the expression of genes in Rangpur lime citrus roots to regulate long‐distance ABA signalling and adaptation to stress.     

Effects of drought on mesophyll conductance and photosynthetic limitations at different tree canopy layers

In recent years, many studies have focused on the limiting role of mesophyll conductance (g_m) to photosynthesis (A_n) under water stress, but no studies have examined the effect of drought on g_m through the forest canopy. We investigated limitations to A_n on leaves at different heights in a mixed adult stand of sessile oak (Quercus petraea) and beech (Fagus sylvatica) trees during a moderately dry summer. Moderate drought decreased A_n of top and lowest beech canopy leaves much more than in leaves located in the mid canopy; whereas in oak, A_n of the lower canopy was decreased more than in sunlit leaves. The decrease of A_n was probably not due to leaf‐level biochemistry given that V_Cmax was generally unaffected by drought. The reduction in A_n was instead associated with reduction in stomatal and mesophyll conductances. Drought‐induced increases in stomatal limitations were largest in leaves from the top canopy, whereas drought‐induced increases in mesophyll limitations were largest in leaves from the lowest canopy. Sensitivity analysis highlighted the need to decompose the canopy into different leaf layers and to incorporate the limitation imposed by g_m when assessing the impact of drought on the gas exchange of tree canopies.     

Biological studies and phenology of the slug Deroceras reticulatum (Müller, 1774) (Pulmonata: Stylommatophora)

Summary

In the southeast of Buenos Aires Province (Argentina), slugs cause reductions in crop yield by killing seeds and seedlings, by destroying stems and growing points, and by reducing leaf area. Deroceras reticulatum (Müller, 1774) was introduced into Argentina a long time ago, but the biology and ecology of this species in this country are still unknown. The aim of this study was to determine how temperature affects growth rate, reproduction and survival of D. reticulatum and also to assess the seasonal phenology of this species. The life cycle of D. reticulatum was investigated at 12°C, 20°C and at a temperature which alternated between 12°C for 16 h and 20°C for 8 h. The light/dark regime during the experiment was LD: 8/16 h. Slug populations were sampled from grassland located at the Experimental Station of INTA Balcarce. Eggs hatched after 16.3 to 39.8 days and fertility varied between 83.9% and 91.1%. The mean time from hatching to oviposition was longer and fecundity lower at 20°C than 12°C and 12/20°C (p <0.05). The net reproductive rate was 1.49 to 70.53 and the mean generation time varied between the 27 and 30 weeks. In natural conditions, eggs of D. reticulatum were found from early winter until the end of the spring, when soil temperature was 8–15°C and humidity was above 15%. These results indicate that D. reticulatum has one generation per year, univoltine phenology, with peak densities of slugs from the beginning of winter to the end of spring, which coincides with the sowing and emergence of winter and summer crops.     

Noncorrelated effects of seed predation and pollination on the perennial herb Ruellia nudiflora remain spatially consistent

By simultaneously manipulating both seed predator and pollinator effects on the perennial herb Ruellia nudiflora at two sites in Yucatan (Mexico), the present study evaluated (1) whether a correlation (interaction) existed between seed predator and pollinator effects on R. nudiflora seed production and (2) whether such an interaction varied geographically. We used three populations per site, and a total of 20 plants per population ( N  = 120). Groups of five plants were randomly chosen at each population to simultaneously receive one of two seed predator and pollinator exclosure levels (present or excluded in each case). These two factors were fully crossed, resulting in each group being subjected to one of four possible combinations: pollinators excluded/herbivores present; herbivores excluded/pollinators present; herbivores excluded/pollinators excluded; or control (neither excluded). Response variables were the number of seeds produced per plant and the proportion of attacked fruits by seed predators per plant. Seed predators had a large impact on R. nudiflora seed production but did not show any preference for fruits from plants not excluded from pollinators. In addition, the pollination treatment was not significant, indicating no effect of pollinators on reproductive success. These findings resulted in a nonsignificant herbivory × pollination interaction, which was consistent across sites, indicating lack of correlated selection of these two guilds on R. nudiflora seed production.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 800–807.     

Fine scale genetic structure in the wild ancestor of maize (Zea mays ssp. parviglumis)

Analysis of fine scale genetic structure in continuous populations of outcrossing plant species has traditionally been limited by the availability of sufficient markers. We used a set of 468 SNPs to characterize fine‐scale genetic structure within and between two dense stands of the wild ancestor of maize, teosinte (Zea mays ssp. parviglumis). Our analyses confirmed that teosinte is highly outcrossing and showed little population structure over short distances. We found that the two populations were clearly genetically differentiated, although the actual level of differentiation was low. Spatial autocorrelation of relatedness was observed within both sites but was somewhat stronger in one of the populations. Using principal component analysis, we found evidence for significant local differentiation in the population with stronger spatial autocorrelation. This differentiation was associated with pronounced shifts in the first two principal components along the field. These shifts corresponded to changes in allele frequencies, potentially due to local topographical features. There was little evidence for selection at individual loci as a contributing factor to differentiation. Our results demonstrate that significant local differentiation may, but need not, co‐occur with spatial autocorrelation of relatedness. The present study represents one of the most detailed analyses of local genetic structure to date and provides a benchmark for future studies dealing with fine scale patterns of genetic diversity in natural plant populations.     

Parallel evolution of insular Olea europaea subspecies based on geographical structuring of plastid DNA variation and phenotypic similarity in leaf traits

Strong geographical isolation within the distribution of a species may result in differentiated lineages exhibiting conspicuous phenotypic differences. In the present paper, we investigate whether plastid and phenotypic variation is geographically structured within the Olea europaea complex in Macaronesia, which comprises three subspecies separated by oceanic barriers: maroccana (south‐west Morocco), guanchica (Canary Islands) and cerasiformis (Madeira archipelago). Plastid variation showed a significant pattern of geographical structure (N_ST > G_ST = 0.56), because of the lack of shared haplotypes among subspecies and the presence of a single and private haplotype in the eastern Canary Islands. Such a clear molecular structure, however, was not reflected in a congruent pattern of phenotypic differentiation among taxa in leaf morpho‐functional traits. Despite the substantial genetic differentiation observed between the subspecies from Madeira and the Canary Islands, they displayed both higher leaf size (leaf area) and specific leaf area (leaf surface area‐to‐mass ratio) than their continental counterparts, probably as a result of oceanic conditions in subtropical environments. Unlike most of the plant groups previously studied in the Macaronesian region, the lineages of Olea illustrate how low phenotypic differentiation can be also related to a clear molecular differentiation in oceanic island enclaves. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 162 , 54–63.     

Exceptionally preserved tadpoles from the Miocene of Libros,Spain: ecomorphological reconstruction and the impact of ontogeny upon taphonomy

McNamara, M.E., Orr, P.J., Kearns, S.L., Alcalá, L., Anadón, P. & Peñalver‐Mollá, E. 2010: Exceptionally preserved tadpoles from the Miocene of Libros, Spain: ecomorphological reconstruction and the impact of ontogeny upon taphonomy. Lethaia, Vol. 43, pp. 290–306. The Libros exceptional biota from the Upper Miocene of NE Spain includes abundant frog tadpoles (Rana pueyoi) preserved in finely laminated lacustrine mudstones. The tadpoles exhibit a depressed body, short tail, low tail fins, dorso‐laterally directed eyes and jaw sheaths; these features identify the Libros tadpoles as members of the benthic lentic ecomorphological guild. This, the first ecomorphological reconstruction of a fossil tadpole, supports phylogenetic evidence that this ecology is a conserved ranid feature. The soft‐tissue features of the Libros tadpoles are characterized by several modes of preservation. The space occupied previously by the brain is defined by calcium carbonate, the nerve cord is defined by calcium phosphate, and jaw sheaths and bone marrow are preserved as organic remains. Gut contents (and coprolites adjacent to specimens) comprise ingested fine‐grained sedimentary detritus and epiphyton. The body outline and the eyespots, nares, abdominal cavity, notochord, caudal myotomes and fins are defined by a carbonaceous bacterial biofilm. A similar biofilm in adult specimens of R. pueyoi from Libros defines only the body outline, not any internal anatomical features. In the adult frogs, but not in the tadpoles, calcium phosphate and calcium sulphate precipitated in association with integumentary tissues. These differences in the mode of preservation between the adult frogs and tadpoles reflect ontogenetic factors. □Anuran, ecology, soft‐tissue, tadpoles, taphonomy.     

Daily variations in the optical properties of a small lake

1. The major components of the underwater light field (ULF: vertical attenuation, absorption, scattering and suspensoids including plankton fractions) of Las Madres Lake, a small wind‐sheltered, oligohumic lake in Central Spain, were investigated daily over a period of 3 months at the onset of vernal circulation. 2. Gilvin, arising mostly from the decomposition of reeds in the littoral in autumn, was the main component of vertical attenuation, and its variability explained the highest fraction of absorption variability. Tripton appeared to be the main factor responsible for scattering, and might have resulted from dust deposition from the surrounding mining land. The plankton community played a minor role in attenuation, absorption or scattering throughout the investigation period. 3. Vertical and horizontal mixing dynamics may control the ULF to a certain extent, as most optical properties changed within different mixing periods and poor advective exchanges may have resulted in uneven distribution of water colour in this small lake. 4. Time series analysis showed that most autocorrelations were shorter than a week, inherent properties (absorption, scattering) being delayed longer than apparent properties (attenuation, transparency) as a result of their lower dependence on solar irradiance. A 2‐day lag was observed in cross‐correlations between either gilvin and absorption or tripton and scattering. When different mixing periods at early circulation were considered, however, ULF components changed their relationships and delays with suspensoids and dissolved substances over such periods, probably tracking the dynamics of their controlling factors. 5. Our study, and others at daily, weekly, seasonal, interannual and long‐term scales, demonstrates that ULF is a system upon which different processes are operating at different time scales. Contrary to expectations, however, the variability in the ULF does not increase with time scale and depends partly upon the trophic status of lakes.