•Similar yield ranking of white clover populations in monocultures and mixtures.•Relative performance of white clover population depended on site conditions.•The mixture benefit was higher at the ...fertile than at the more marginal site.•Including chicory into the mixtures increased the between-year yield stability.
Due to complementary resource use of the components, grass-clover mixtures are usually higher yielding compared to unfertilized pure grass stands. The number and identity of species in a mixture are known as important factors for the mixture’s productivity. How intraspecific variability of component species affects the productivity of mixtures has received less attention. We established an experiment to investigate how and to what extent different populations of white clover (Trifolium repens L.) affect mixture performance. Eight novel populations of white clover and one variety of perennial ryegrass (Lolium perenne L.) and chicory (Cichorium intybus L.) were grown as monocultures and in two- and three-species mixtures at two sites differing in soil fertility. Aboveground herbage was cut twice in the establishing year and four times in each of the three following years. The accumulated dry matter yield was calculated as the sum of the four annual dry matter yields. There was no significant interaction between white clover population and crop stand (i.e., white clover monoculture or mixtures with partner species) on dry matter yield accumulated over four years; the white clover population that performed well in monoculture also did so in mixtures. At both sites, the binary mixtures of white clover and chicory produced significantly higher dry matter yields than the white clover monocultures and other mixtures. Inclusion of chicory in mixtures significantly increased the stability of yield production as well. Site conditions strongly affected the performance of the forage species mixtures. We found that the benefit of mixtures over monocultures was more related to the identity of species in mixture than to the white clover population.
In order to investigate the physiological effects of exogenous γ-aminobutyric acid (GABA) on drought tolerance in white clover (Trifolium repens), GABA shunt, polyamines (PAs), and proline (Pro) ...metabolism were examined after plants pretreated with or without GABA (8 mM) and then exposed to water or 15% PEG-induced drought stress in growth chamber. In this study, exogenous application of GABA effectively alleviated drought-induced damage in leaves, as reflected by significantly higher relative water content, lower electrolyte leakage, lipid peroxidation, and leaf wilt. Exogenous GABA further promoted drought-induced increases in GABA transaminase and alpha ketone glutarate dehydrogenase activities, but inhibited glutamate decarboxylase activity under both control and drought conditions, resulting in an increase in endogenous glutamate (Glu) and GABA content. Besides, exogenous GABA could well accelerated PAs synthesis and suppressed PAs catabolism, which lead to the extremely enhanced different types of PAs content (free Put and Spd, insoluble bound Spd and Spm, soluble conjugated Spd and Spm, and total Put, Spd and Spm) under drought stress. In addition, exogenous GABA application further activated drought-induced Δ
-pyrroline-5-carboxylate synthetase and proline dehydrogenase activities, but suppressed drought-facilitated ornithine -δ-amino transferase activities, leading to a higher Pro accumulation and metabolism in GABA-pretreated plants in the middle and last period of drought. The results suggested that increased endogenous GABA by exogenous GABA treatment could improve drought tolerance of white clover associated with a positive regulation in the GABA-shunt, PAs and Pro metabolism.
Since the most important deficiency of forage stands is the inability to maintain adequate legume participation in mixture, it is the purpose of this paper to examine persistence in red clover in ...Târgovişte Plain eco-climatic conditions, together with the factors that affect it. Six red clover cultivars (Napoca-Tetra, Dacia Tetra, Vesna – tetraploids; Flora, Roxana, Start – diploids) and one white clover diploid cultivar (Karina) were used in pure culture and in mixture (50:50) with hybrid ryegrass (Zefir – tetraploid) in a randomized block design with three replicates. Ground cover assessment in early spring was a suggestive indicator of the stand persistence to define the stability and sustainability boundaries of a reliable intensive system. In the beginning of the third year of cropping, ground cover was 54.33% for tetraploid cultivars (CV = 43.25%), and 67% for diploid cultivars (CV = 6.83%) in pure stands. Same ground cover average of 27% was established both for tetraploid cultivars (CV = 36.47%), and for diploid cultivars (CV = 16.97%) in mixtures with hybrid ryegrass.
Increased emissions of N2O, a potent greenhouse gas (GHG), from agricultural soils is a major concern for the sustainability of grassland agriculture. Emissions of N2O are closely associated with the ...rates and forms of N fertilisers applied as well as prevailing weather and soil conditions. Evidence suggests that multispecies swards require less fertiliser N input, and may cycle N differently, thus reducing N loss to the environment. This study used a restricted simplex-centroid experimental design to investigate N2O emissions and soil N cycling following application of urea-N (40 kg N ha−1) to eight experimental swards (7.8 m2) with differing proportions of three plant functional groups (grass, legume, herb) represented by perennial ryegrass (PRG, Lolium perenne), white clover (WC, Trifolium repens) and ribwort plantain (PLAN, Plantago lanceolata), respectively. Swards were maintained under two contrasting soil moisture conditions to examine the balance between nitrification and denitrification. Two N2O peaks coincided with fertiliser application and heavy rainfall events; 13.4 and 17.7 g N2O-N ha−1 day−1 (ambient soil moisture) and 39.8 and 86.9 g N2O-N ha−1 day−1 (wet soil moisture). Overall, cumulative N2O emissions post-fertiliser application were higher under wet soil conditions. Increasing legume (WC) proportions from 0% to 60% in multispecies swards resulted in model predicted N2O emissions increasing from 22.3 to 96.2 g N2O-N ha−1 (ambient soil conditions) and from 59.0 to 219.3 g N2O-N ha−1 (wet soil conditions), after a uniform N application rate. Soil N dynamics support denitrification as the dominant source of N2O especially under wet soil conditions. Significant interactions of PRG or WC with PLAN on soil mineral N concentrations indicated that multispecies swards containing PLAN potentially inhibit nitrification and could be a useful mitigation strategy for N loss to the environment from grassland agriculture.
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•Measurement of N2O emissions and N cycling from varying sward compositions.•Post N application (40 kg N ha−1) N2O loss increased with white clover proportion.•N2O emissions from PRG were 2.5 fold higher in wet soil (WFPS > 60%) compared to ambient.•Soil N dynamics suggest denitrification as dominant N2O source when WFPS > 60%.•Plantago lanceolata (forage herb) potentially regulates N cycling pathways.
Spermidine is well known to accumulate in plants exposed to drought, but the regulatory network associated with its biosynthesis and accumulation and the underlying molecular mechanisms remain ...unclear. Here, we demonstrated that the Trifolium repens TrMYB33 relayed the ABA signal to modulate drought-induced spermidine production by directly regulating the expression of TrSAMS1, which encodes an S-adenosylmethionine synthase. This gene was identified by transcriptome and expression analysis in T. repens. TrSAMS1 overexpression and its pTRV-VIGS-mediated silencing demonstrated that TrSAMS1 is a positive regulator of spermidine synthesis and drought tolerance. TrMYB33 was identified as an interacting candidate through yeast one-hybrid library screening with the TrSAMS1 promoter region as the bait. TrMYB33 was confirmed to bind directly to the predicted TAACCACTAACCA (the TAACCA MYB binding site is repeated twice in tandem) within the TrSAMS1 promoter and to act as a transcriptional activator. Additionally, TrMYB33 contributed to drought tolerance by regulating TrSAMS1 expression and modulating spermidine synthesis. Additionally, we found that spermidine accumulation under drought stress depended on ABA and that TrMYB33 coordinated ABA-mediated upregulation of TrSAMS1 and spermidine accumulation. This study elucidated the role of a T. repens MYB33 homolog in modulating spermidine biosynthesis. The further exploitation and functional characterization of the TrMYB33–TrSAMS1 regulatory module can enhance our understanding of the molecular mechanisms responsible for spermidine accumulation during drought stress.
The practice of inoculating forage legumes with rhizobia strains is widespread. It is assumed that the inoculated strain determines the performance of the symbiosis and nitrogen fixation rates. ...However, native-naturalized strains can be competitive, and actual nodule occupancy is often scarcely investigated. In consequence, failures in establishment, and low productivity attributed to poor performance of the inoculant may merely reflect the absence of the inoculated strain in the nodules. This study lays out a strategy followed for selecting a
sv. trifolii strain for white clover (
) with competitive nodule occupancy. First, the competitiveness of native-naturalized rhizobia strains selected for their efficiency to fix N
in clover and tagged with
A was evaluated in controlled conditions with different soils. Second, three of these experimental strains with superior nodule occupancy plus the currently recommended commercial inoculant, an introduced strain, were tested in the field in 2 years and at two sites. Plant establishment, herbage productivity, fixation of atmospheric N
(
N natural abundance), and nodule occupancy (ERIC-PCR genomic fingerprinting) were measured. In both years and sites, nodule occupancy of the native-naturalized experimental strains was either higher or similar to that of the commercial inoculant in both primary and secondary roots. The difference was even greater in stolon roots nodules, where nodule occupancy of the native-naturalized experimental strains was at least five times greater. The amount of N fixed per unit plant mass was consistently higher with native-naturalized experimental strains, although the proportion of N derived from atmospheric fixation was similar for all strains. Plant establishment and herbage production, as well as clover contribution in oversown native grasslands, were either similar or higher in white clover inoculated with the native-naturalized experimental strains. These results support the use of our implemented strategy for developing a competitive inoculant from native-naturalized strains.
Urban ecosystems are an increasingly dominant feature of terrestrial landscapes. While evidence that species can adapt to urban environments is accumulating, the mechanisms through which urbanization ...imposes natural selection on populations are poorly understood. The identification of adaptive phenotypic changes (i.e. clines) along urbanization gradients would facilitate our understanding of the selective factors driving adaptation in cities. Here, we test for phenotypic clines in urban ecosystems by sampling the frequency of a Mendelian-inherited trait—cyanogenesis—in white clover (Trifolium repens L.) populations along urbanization gradients in four cities. Cyanogenesis protects plants from herbivores, but reduces tolerance to freezing temperatures. We found that the frequency of cyanogenic plants within populations decreased towards the urban centre in three of four cities. A field experiment indicated that spatial variation in herbivory is unlikely to explain these clines. Rather, colder minimum winter ground temperatures in urban areas compared with non-urban areas, caused by reduced snow cover in cities, may select against cyanogenesis. In the city with no cline, high snow cover might protect plants from freezing damage in the city centre. Our study suggests that populations are adapting to urbanization gradients, but regional climatic patterns may ultimately determine whether adaptation occurs.
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