Claudino Rodriguez-Barrueco

Plant species diversity as a driver of early succession in abandoned fields: a multi-site approach

Abstract Succession is one of the most studied processes in ecology and succession theory provides strong predictability. However, few attempts have been made to influence the course of succession thereby testing the hypothesis that passing through one stage is essential before entering the next one. At each stage of succession ecosystem processes may be affected by the diversity of species present, but there is little empirical evidence showing that plant species diversity may affect succession. On ex-arable land, a major constraint of vegetation succession is the dominance of perennial early-successional (arable weed) species. Our aim was to change the initial vegetation succession by the direct sowing of later-successional plant species. The hypothesis was tested that a diverse plant species mixture would be more successful in weed suppression than species-poor mixtures. In order to provide a robust test including a wide range of environmental conditions and plant species, experiments were carried out at five sites across Europe. At each site, an identical experiment was set up, albeit that the plant species composition of the sown mixtures differed from site to site. Results of the 2-year study showed that diverse plant species mixtures were more effective at reducing the number of natural colonisers (mainly weeds from the seed bank) than the average low-diversity treatment. However, the effect of the low-diversity treatment depended on the composition of the species mixture. Thus, the effect of enhanced species diversity strongly depended on the species composition of the low-diversity treatments used for comparison. The effects of high-diversity plant species mixtures on weed suppression differed between sites. Low-productivity sites gave the weakest response to the diversity treatments. These differences among sites did not change the general pattern. The present results have implications for understanding biological invasions. It has been hypothesised that alien species are more likely to invade species-poor communities than communities with high diversity. However, our results show that the identity of the local species matters. This may explain, at least partly, controversial results of studies on the relation between local diversity and the probability of being invaded by aliens.

Growth promotion of chickpea and barley by a phosphate solubilizing strain of Mesorhizobium mediterraneum under growth chamber conditions

The efficacy of a strain of Mesorhizobium mediterraneum to enhance the growth and phosphorous content in chickpea and barley plants was assessed in a soil with and without the addition of phospates in a growth chamber. The results obtained show that the strain PECA21 was able to mobilize phosphorous efficiently in both plants when tricalcium phosphate was added to the soil. In barley and chickpea growing in soils treated with insoluble phosphates and inoculated with strain PECA21 the phosphorous content was significantly increased in a 100 and 125%, respectively. Also, the dry matter, nitrogen, potassium, calcium and magnesium content in both plants was significantly increased in inoculated soil added with insoluble phosphate. These results show that the inoculation of a soil with rhizobia should not be based only on the effectiveness of the strains with respect to their nitrogen fixation potential, since these microorganisms can increase the growth of plants by means of other mechanisms, for example the phosphate solubilization. q 2001 Elsevier Science Ltd. All rights reserved.

Climate vs. soil factors in local adaptation of two common plant species

Evolutionary theory suggests that divergent natural selection in heterogeneous environments can result in locally adapted plant genotypes. To understand local adaptation it is important to study the ecological factors responsible for divergent selection. At a continental scale, variation in climate can be important while at a local scale soil properties could also play a role. We designed an experiment aimed to disentangle the role of climate and (abiotic and biotic) soil properties in local adaptation of two common plant species. A grass (Holcus lanatus) and a legume (Lotus corniculatus), as well as their local soils, were reciprocally transplanted between three sites across an Atlantic-Continental gradient in Europe and grown in common gardens in either their home soil or foreign soils. Growth and reproductive traits were measured over two growing seasons. In both species, we found significant environmental and genetic effects on most of the growth and reproductive traits and a significant interaction between the two environmental effects of soil and climate. The grass species showed significant home site advantage in most of the fitness components, which indicated adaptation to climate. We found no indication that the grass was adapted to local soil conditions. The legume showed a significant home soil advantage for number of fruits only and thus a weak indication of adaptation to soil and no adaptation to climate. Our results show that the importance of climate and soil factors as drivers of local adaptation is species-dependent. This could be related to differences in interactions between plant species and soil biota.

Differential effects of coinoculations with Pseudomonas jessenii PS06 (a phosphate-solubilizing bacterium) and Mesorhizobium ciceri C-2/2 strains on the growth and seed yield of chickpea under greenhouse and field conditions

In the course of a project carried out in two regions of Spain, Castilla y Leon and Andalucia, aiming to find useful biofertilizers for staple grain-legumes, an efficient rhizobia nodulating chickpea (termed as C-2/2) and a powerful in vitro phosphate-solubilizing bacterial strain (termed as PS06) were isolated. Analyses of their 16S rDNA sequence indicated that they belong to the bacterial species Mesorhizobium ciceri and Pseudomonas jessenii, respectively. Greenhouse and field experiments were carried out in order to test the effect of single and dual inoculations on chickpea (ecotype ILC-482) growth. Under greenhouse conditions, plants inoculated with Mesorhizobium ciceri C-2/2 alone had the highest shoot dry weight. The inoculation treatment with P. jessenii PS06 yielded a shoot dry weight 14% greater than the uninoculated control treatment, but it was not correlated with shoot P contents. However, the co-inoculation of C-2/2 with PS06 resulted in a decrease in shoot dry weight with respect to the inoculation with C-2/2 alone. Under field conditions, plants inoculated with M. ciceri C-2/2, in single or dual inoculation, produced higher nodule fresh weight, nodule number and shoot N content than the other treatments. Inoculation with P. jessenii PS06 had no significant effect on plant growth. However, the co-inoculation treatment ranked the highest in seed yield (52% greater than the uninoculated control treatment) and nodule fresh weight. These data suggest that P. jessenii PS06 can act synergistically with M. ciceri C-2/2 in promoting chickpea growth. The contrasting results obtained between greenhouse and field experiments are discussed.

Growth promotion of common bean (Phaseolus vulgaris L.) by a strain of Burkholderia cepacia under growth chamber conditions

We isolated a strain of Burkholderia cepacia (SAOCV2), which solubilizes inorganic phosphates and antagonizes Fusarium oxysporum f. sp. phaseoli and Fusarium solani in vitro, from soil. However, this strain does not have antibiotic activity against the bacteria tested in this study, which included bacterial plant pathogens and rhizobia. The efficacy of this strain to enhance the growth and P content in common bean was assesed in a soil traditionally cultivated with this species. In this soil, the common bean is affected by Fusarium. Our results show that the strain SAOCV2 was able to mobilize P efficiently in the common bean, so that its P content was increased by 44% with respect to uninoculated plants, whereas it was not significantly different with respect to the plants treated with fungicide. Also, the N content in plants inoculated with the strain SAOCV2 was significantly higher than in uninoculated plants. This result is correlated with a larger number of nodules in plants inoculated with SAOCV2 and in plants treated with fungicide and indicates that the inhibition of fungal growth enhances the bacterial community in the plant rhizosphere, including rhizobia. Our results suggest that the inoculation with strain SAOCV2 promotes the growth of common bean by several mechanisms, that include P mobilization, antagonism towards pathogenic species of Fusarium and, indirectly, by an increase in nodulation that may lead to an increase in N2 fixation.

The Effects of Spatial Scale on Trophic Interactions

Food chain models have dominated empirical studies of trophic interactions in the past decades, and have lead to important insights into the factors that control ecological communities. Despite the importance of food chain models in instigating ecological investigations, many empirical studies still show a strong deviation from the dynamics that food chain models predict. We present a theoretical framework that explains some of the discrepancies by showing that trophic interactions are likely to be strongly influenced by the spatial configuration of consumers and their resources. Differences in the spatial scale at which consumers and their resources function lead to uncoupling of the population dynamics of the interacting species, and may explain overexploitation and depletion of resource populations. We discuss how changed land use, likely the most prominent future stress on natural systems, may affect food web dynamics by interfering with the scale of interaction between consumers and their resource.

22 Relationships between the Mycorrhizal and Actinorhizal Symbioses in Non-legumes

Publisher Summary This chapter discusses relationships between the mycorrhizal and actinorhizal symbioses in non-legumes. Some non-leguminous, nitrogen-fixing plants carry dual symbiotic infection involving both mycorrhizal fungi and actinorhizal actinomycetes. The characterization of the genus Frankia, the microsimbiont in the actinorhizal symbiosis, is approaching maturity but the optimal application of methods already developed requires the consideration of the real contribution of mycorrhiza associated with the actinorhizal symbioses. The chapter discusses mycorrhiza in actinorhizal plants. Mycorrhizal species are found in most actinorhizal plant genera so far examined in both ectomycorrhizal and vesicular-arbuscular mycorrhizal symbioses. The presence of a mycorrhizal symbiosis in a plant species depends largely on many soil characteristics. Thus, infection of Myrica gale by both ecto- and vesicular-arbuscular mycorrhiza is restricted to well-drained soils, being absent in wet soils. The actinorhizal symbioses are also highlighted in the chapter.

Analysis of LMW RNA profiles of Frankia strains by staircase electrophoresis.

An optimized technique of polyacrylamide gel electrophoresis, Staircase Electrophoresis (SCE), was applied to determine the stable Low Molecular Weight RNA (LMW RNA) profiles of 25 Frankia strains from diverse geographic origins and host specificity groups as well as species from other actinomycete genera. Application of the technique permits the rapid identification of Frankia strains and their differentiation from other actinomycetes. The isolates used in this study were grouped in eight clusters, each comprising strains with identical LMW RNA profiles. Comparison of these results with others obtained from DNA sequences or DNA hybridization methods suggest a high degree of complexity in the genus Frankia. Application of SCE to profile LMW RNA should in the future facilitate biodiversity studies of Frankia and discrimination of new species.

Paenibacillus castaneae sp. nov., isolated from the phyllosphere of Castanea sativa Miller.

A bacterial strain, designated Ch-32(T), was isolated from the phyllosphere of Castanea sativa in Spain. Phylogenetic analysis based on 16S rRNA gene sequences placed the isolate in the genus Paenibacillus within the same subgroup as Paenibacillus xinjiangensis and Paenibacillus glycanilyticus, with similarities of 96.3 and 96.8 %, respectively. DNA-DNA hybridization values for strain Ch-32(T) with these two species were lower than 20 %. The novel isolate was a Gram-variable, motile, sporulating rod. It produced catalase and oxidase and hydrolysed cellulose, gelatin and aesculin. Acetoin and urease production, nitrate reduction and starch hydrolysis were negative. Growth was supported by many carbohydrates and organic acids as carbon sources. MK-7 was the only menaquinone detected and anteiso-C(15 : 0), iso-C(16 : 0) and C(16 : 0) were the major fatty acids. The DNA G+C content was 46 mol%. Phylogenetic, DNA relatedness and phenotypic analyses showed that strain Ch-32(T) should be classified as a novel species of the genus Paenibacillus, for which the name Paenibacillus castaneae sp. nov. is proposed; the type strain is Ch-32(T) (=CECT 7279(T)=DSM 19417(T)).

The effects of aluminium on nodulation and symbiotic nitrogen fixation in Casuarina cunninghamiana Miq.

In order to investigate the effects of Al on nodule formation and function in the Casuarina-Frankia symbiosis, inoculated plants were grown in sand culture at five nominal Al concentrations (0-880 μM Al) at pH 4.0. There was an Al-free control at pH 6.0 to assess the effects of pH 4.0 treatments. Mean N concentration of nodules was significantly less at pH 4.0 (1.83%) than at pH 6.0 (2.01%). There were nodulated plants at all Al levels, though there were fewer nodulated plants at 440 and 880 μM Al. Dry weights of nodules, shoots and roots were not reduced by Al concentrations at or below 220 μM Al, but were decreased by Al concentrations at or above 440 μM Al. Nodule weight expressed as a percentage of total weight did not differ significantly with respect to an Al-free control at pH 4. N concentrations of shoots and whole plants were significantly reduced at 440 μM Al. Nodular specific acetylene reduction activity (ARA) did not differ significantly among Al treatments. However, N2-fixation efficiency was decreased from 0.20 to 0.10 mg N fixed mg nodule dry weight−1 at 880 μM Al.