Can the biomass-ratio hypothesis predict mixed-species litter decomposition along a climatic gradient?

Background and Aims

The biomass-ratio hypothesis states that ecosystem properties are driven by the characteristics of dominant species in the community. In this study, the hypothesis was operationalized as community-weighted means (CWMs) of monoculture values and tested for predicting the decomposition of multispecies litter mixtures along an abiotic gradient in the field.

Methods

Decomposition rates (mg g⁻¹ d⁻¹) of litter from four herb species were measured using litter-bed experiments with the same soil at three sites in central France along a correlated climatic gradient of temperature and precipitation. All possible combinations from one to four species mixtures were tested over 28 weeks of incubation. Observed mixture decomposition rates were compared with those predicted by the biomass-ratio hypothesis. Variability of the prediction errors was compared with the species richness of the mixtures, across sites, and within sites over time.

Key Results

Both positive and negative prediction errors occurred. Despite this, the biomass-ratio hypothesis was true as an average claim for all sites (r = 0·91) and for each site separately, except for the climatically intermediate site, which showed mainly synergistic deviations. Variability decreased with increasing species richness and in less favourable climatic conditions for decomposition.

Conclusions

Community-weighted mean values provided good predictions of mixed-species litter decomposition, converging to the predicted values with increasing species richness and in climates less favourable to decomposition. Under a context of climate change, abiotic variability would be important to take into account when predicting ecosystem processes.     

Influence of short‐term drought conditions and subsequent re‐watering on the physiology and proteome of Lolium multiflorum/Festuca arundinacea introgression forms,with contrasting levels of tolerance to long‐term drought

Festuca arundinacea is a drought tolerant species. Lolium multiflorum has better forage quality but lower tolerance to abiotic stresses. Their hybrids offer an opportunity to perform research on the molecular basis of tolerance to drought. The aim of this work was to recognise the mechanisms of response to short‐term drought (11 days) in a glasshouse in two L. multiflorum/F. arundinacea introgression forms with distinct levels of tolerance to long‐term drought (14 weeks) in the field. Measurements of physiological parameters, analyses of protein accumulation profiles using two‐dimensional gel electrophoresis, and mass spectrometry identification of proteins, which were accumulated differentially between the selected genotypes during short‐term drought, were performed. Genotype 7/6, with lower yield potential during 14 weeks of drought, and lower ability to re‐grow after watering, had a higher capacity for photosynthesis during 11 days of drought. Genotype 4/10, more tolerant to long‐term drought, was able to repair damaged cell membranes after watering and was also characterised by lower transpiration during short‐term drought. A total of 455 proteins were analysed, and the 17 that were differentially accumulated between the two genotypes were identified. The results of physiological and proteomic research led to a hypothesis that the higher photosynthetic capacity of genotype 7/6 could be due to a more efficient Calvin cycle, supported by higher accumulation of crucial proteins involving chloroplast aldolase.     

Soil moisture variations affect short‐term plant‐microbial competition for ammonium,glycine, and glutamate

We tested whether the presence of plant roots would impair the uptake of ammonium (), glycine, and glutamate by microorganisms in a deciduous forest soil exposed to constant or variable moisture in a short‐term (24‐h) experiment. The uptake of ¹⁵NH₄ and dual labeled amino acids by the grass Festuca gigantea L. and soil microorganisms was determined in planted and unplanted soils maintained at 60% WHC (water holding capacity) or subject to drying and rewetting. The experiment used a design by which competition was tested in soils that were primed by plant roots to the same extent in the planted and unplanted treatments. Festuca gigantea had no effect on microbial N uptake in the constant moist soil, but its presence doubled the microbial uptake in the dried and rewetted soil compared with the constant moist. The drying and rewetting reduced by half or more the uptake by F. gigantea, despite more than 60% increase in the soil concentration of . At the same time, the amino acid and ‐ N became equally valued in the plant uptake, suggesting that plants used amino acids to compensate for the lower acquisition. Our results demonstrate the flexibility in plant‐microbial use of different N sources in response to soil moisture fluctuations and emphasize the importance of including transient soil conditions in experiments on resource competition between plants and soil microorganisms. Competition between plants and microorganisms for N is demonstrated by a combination of removal of one of the potential competitors, the plant, and subsequent observations of the uptake of N in the organisms in soils that differ only in the physical presence and absence of the plant during a short assay. Those conditions are necessary to unequivocally test for competition.     

不同灌水量下限对高羊茅绿期及抗寒性生理指标的影响

采用盆栽试验的方法,研究了秋末冬初不同灌水量下限\[分别占田间持水量(FC)的80%、70%、60%、50%\]对高羊茅绿期及抗寒性生理指标的影响.结果表明：在冬季低温条件下,80%和70%FC灌水处理使高羊茅叶片相对含水量、保护酶(SOD、POD和CAT)活性、叶绿素、可溶性糖和游离脯氨酸含量维持在较高水平,丙二醛含量和电解质外渗率降低,高羊茅的抗寒性增强.80%FC灌水处理分别较70%、60%和50%FC处理的草坪草绿期延长4、22和28 d,到达枯黄休眠的时间最晚,完成返青的时间最早.综合考虑节水和提高水分利用效率等多种因素,70%FC灌水处理为高羊茅秋末冬初季节最佳的灌水下限.     

Invasive Grass Alters Litter Decomposition by Influencing Macrodetritivores

The results of nitrogen (N) fertilization experiments have shown inconsistent rates of plant litter decomposition, a phenomenon that may be explained by dispropotionate influence of animal detritivores (macro-detritivores) on litter mass loss versus that of microbial decomposers, whose activity may be dependent on inorganic N. In turn, macrodetritivores may be influenced by plant species composition via their selection of optimal food resources and habitats. In our experiment, fertilizer had no apparent effect on litter decomposition, suggesting that microbial decomposers did not use the additional inorganic N and/or that macrodetritivores had a greater influence on decomposition. Manipulation of macrodetritivores suggested that plant species composition—dominated in this study by Festuca arundinacea, an exotic, invasive grass, and Aster ericoides, a native forb—caused shifts in detrivore communities and/or feeding patterns that tended to increase litter mass loss. Canopy cover of F. arundinacea and A. ericoides ranged from 0% to 11%, suggesting that low-intensity invasion may produce significant changes in ecosystem function, such as decomposition.     

Changes in plasmalemma K+Mg2+ -ATPase dephosphorylating activity and H+ transport in relation to seasonal growth and freezing tolerance of Festuca pratensis Huds

Changes in plasmalemma K⁺Mg²⁺-ATPase dephosphorylating activity and H⁺ transport were examined in freezing-tolerant and non-tolerant genotypes of the perennial grass species Festuca pratensis Huds. Enzyme activity and ΔμH⁺ were measured in plasmalemma fractions isolated from basal nodes and roots. Three types of experiments were undertaken: (i) a field experiment, utilizing the seasonal growth and cessation cycle of a perennial plant; (ii) a cold acclimation experiment in hydroponics; and (iii) an instant freezing test. A specific fluctuation in K⁺Mg²⁺-ATPase activity was found throughout the seasonal growth of the plants (i). The K⁺Mg²⁺-ATPase activity peaks for both the basal node and the root plasmalemma were determined early in the spring before the renewal of growth. The lowest activity values in roots occurred at the time approaching flowering, and in basal nodes at the transition into the growth cessation. The K⁺Mg²⁺-ATPase activity was approximately 50% lower in the basal node plasmalemma of freezing-tolerant plants than of non-tolerant ones, when assessed at the optimal growth stage in hydroponics. In hydroponics (ii) and in the freezing test (iii), temperature stress was followed by a more pronounced change in the level of K⁺Mg²⁺-ATPase activity than in that of H⁺ transport, and this change was more clearly differentiated in the basal node plasmalemma of contrasting genotypes than in the roots. Stress response was manifested differently in freezing-tolerant and non-tolerant plants at cold acclimation (4–2 °C) and at freezing (−8 °C) temperatures. Proton transport regulation via coupled changes in the hydrolysed ATP/transported proton ratio, as an attribute of freezing-tolerant plants, is discussed.     

High temperature effects on ammonium assimilation in leaves of two Festuca arundinacea cultivars with different heat susceptibility

The aim of this study was to determine the effects of high temperature stress on ammonium assimilation in leaves of two tall fescue cultivars (Festuca arundinacea), Jaguar 3 brand (J3) (heat-tolerant) and TF 66 (T6) (heat-sensitive). High temperature stress for either 10 d or 20 d, and particularly the 20 d stress, produced dramatic changes in ammonium assimilation. After 20 d of stress treatment, the accumulations of total nitrogen, nitrate, soluble protein and total free amino acid (20 amino acids) decreased in both cultivars. Moreover, the activities of main regulatory enzymes, such as nitrate reductase, glutamine synthetase (GS), NADH-dependent glutamate synthase (GOGAT), as well as Δ¹-pyrroline-5-carboxylate reductase (P5CR), also decreased in both cultivars when exposed to 20 d stress. Heat stress had little influence on ammonium accumulation in J3, but this was not the case with T6. The accumulations of nitrate, ammonium, soluble protein, and total free amino acid between the two cultivars were different. This suggests that accumulations of these nitrogen forms were associated with heat tolerance in both tall fescue cultivars. Changes of both NADH-glutamate dehydrogenase (NADH-GDH) activity and Glx (glutamine and glutamic acid) concentration in both cultivars indicated that there is an alternative system for assimilation of nitrogen through glutamate dehydrogenase (GDH) in T6 during longer high temperature stress periods. Our results provide an insight to further selection and breeding of heat-tolerant tall fescue turfgrass cultivars.     

Comparative analysis of transgenic tall fescue (<Emphasis Type="Italic">Festuca arundinacea</Emphasis> Schreb.) plants obtained by <Emphasis Type="Italic">Agrobacterium</Emphasis>-mediated transformation and particle bombardment

Agrobacterium-mediated transformation and particle bombardment are the two most widely used methods for genetically modifying grasses. Here, these two systems are compared for transformation efficiency, transgene integration and transgene expression when used to transform tall fescue (Festuca arundinacea Schreb.). The bar gene was used as a selectable marker and selection during tissue culture was performed using 2 mg/l bialaphos in both callus induction and regeneration media. Average transformation efficiency across the four callus lines used in the experiments was 10.5% for Agrobacterium-mediated transformation and 11.5% for particle bombardment. Similar transgene integration patterns and co-integration frequencies of bar and uidA were observed in both gene transfer systems. However, while GUS activity was detected in leaves of 53% of the Agrobacterium transformed lines, only 20% of the bombarded lines showed GUS activity. Thus, Agrobacterium-mediated transformation appears to be the preferred method for producing transgenic tall fescue plants.     

Genome size and GC content evolution of Festuca: ancestral expansion and subsequent reduction

Background and Aims: Plant evolution is well known to be frequently associated withremarkable changes in genome size and composition; however,the knowledge of long-term evolutionary dynamics of these processesstill remains very limited. Here a study is made of the finedynamics of quantitative genome evolution in Festuca (fescue),the largest genus in Poaceae (grasses). Methods: Using flow cytometry (PI, DAPI), measurements were made of DNAcontent (2C-value), monoploid genome size (Cx-value), averagechromosome size (C/n-value) and cytosine + guanine (GC) contentof 101 Festuca taxa and 14 of their close relatives. The resultswere compared with the existing phylogeny based on ITS and trnL-Fsequences. Key Results: The divergence of the fescue lineage from related Poeae waspredated by about a 2-fold monoploid genome and chromosome sizeenlargement, and apparent GC content enrichment. The backwardreduction of these parameters, running parallel in both mainevolutionary lineages of fine-leaved and broad-leaved fescues,appears to diverge among the existing species groups. The mostdramatic reductions are associated with the most recently andrapidly evolving groups which, in combination with recent intraspecificgenome size variability, indicate that the reduction processis probably ongoing and evolutionarily young. This dynamicsmay be a consequence of GC-rich retrotransposon proliferationand removal. Polyploids derived from parents with a large genomesize and high GC content (mostly allopolyploids) had smallerCx- and C/n-values and only slightly deviated from parentalGC content, whereas polyploids derived from parents with smallgenome and low GC content (mostly autopolyploids) generallyhad a markedly increased GC content and slightly higher Cx-and C/n-values. Conclusions: The present study indicates the high potential of general quantitativecharacters of the genome for understanding the long-term processesof genome evolution, testing evolutionary hypotheses and theirusefulness for large-scale genomic projects. Taken together,the results suggest that there is an evolutionary advantagefor small genomes in Festuca.