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1.
To minimize application of nematicides in banana fields, crop systems have been developed in the French West Indies that combine fallow or rotation crops and nematode-free in vitro plants. After two to four years, populations of the burrowing nematode Radopholus similis have developed enough to cause economic losses, leading banana growers to use nematicides. To understand how banana fields are recontaminated, we studied the dissemination of R. similis by water flow. At a 1-m scale, we analyzed the dispersion of R. similis under a rainfall simulator: we isolated a 1-m2 study plot, placed a R. similis suspension on the upstream soil surface, and simulated a 60 mm/h rainfall for 72 min. We collected soil samples every 10 cm downstream after 12 min of rainfall, and subsequently at 20-min intervals, and extracted the nematodes using a Seinhorst elutriator and then a Baermann funnel. Our results showed that the nematode dissemination follows an inverse exponential law, and depends more on soil moisture at the beginning of rainfall than on the length of rainfall: in fresh soil, 69–80% of the R. similis recovered were found less than 10 cm downstream from the nematode inoculation line, whereas in wetted soil, 76–85% of the recovered individuals were collected in the outlet tub located downstream from the apparatus. This passive dissemination model partially explains the distance covered by individual nematodes but not the low percentage of motile nematodes recovered in the outlet tub (10% and 36% in fresh and wet soils) compared to the percentage of motile nematodes found in the soil (80% and 84% in fresh and wet soils). Indeed, water runoff is likely to disseminate R. similis over long distances only when soil moisture is close to field capacity.  相似文献   

2.
Radopholus similis is a worldwide endoparasitic nematode that greatly hampers banana (Musa acuminata, Cavendish subgroup) productivity. Earthworms are known to closely interact with above-ground and under-ground soil biota and particularly with plants and microfaunal communities. This study was aimed at investigating, under greenhouse conditions, the effects of the earthworm Pontoscolex corethrurus on banana growth and nutrient uptake, and assessing the influences of this earthworm on the development of an inoculated population of R. similis. Six-week-old tissue culture banana plants were submitted to four treatments: with P. corethrurus, R. similis, P. corethrurus+R. similis, and a control with no earthworms or nematodes. At the end of the experiment, the P. corethrurus treatments showed significantly higher leaf surface areas, shoot dry root weights, and root fresh weights than those without earthworms. This root growth enhancement probably contributed to the evident but non-significant decrease in the density of nematodes in the roots, even though earthworms did not reduce the total number of nematodes per whole root system. Moreover, the presence of earthworms slightly alleviated the severity of root damage. N bioavailability in the soil, along with N, Ca, and Mg content of banana plants, were also significantly increased in the presence of earthworms. Our results demonstrated that banana plant growth and nutrition were positively influenced by earthworms. Cropping practices that boost the development of earthworm communities in soil should therefore be promoted to enhance sustainability and to naturally alleviate nematode impact.  相似文献   

3.
Mycorrhiza-induced resistance has been observed against a broad range of mainly soil-borne pathogens, including plant-parasitic nematodes, but the modes of action involved remain unclear. In this study the role of mycorrhiza-induced resistance was investigated during the pre-infectional phase of nematode host finding and penetration. Banana plants were colonized by Glomus mosseae or Glomus intraradices, two arbuscular mycorrhizal fungi. The plant-parasitic nematode Radopholus similis was inoculated after establishment of the mycorrhizal colonization. Nematode attraction and penetration were assessed within a 12-day period. In root exudate experiments, root exudates collected from both control and mycorrhizal plants were added both to control and mycorrhizal plants to assess their direct impact on the nematode penetration. In an in vitro chemotaxis bio-assay, the chemotactic behavior of R. similis was determined towards isolated root exudates of control and mycorrhizal plants. The penetration experiments clearly showed lower nematode penetration in mycorrhizal plants and the important contribution of differential root exudation by mycorrhizal plants was demonstrated in the exudate experiments as well as in the in vitro chemotaxis bio-assay, with the largest impact on juveniles. The root exudate experiments and in vitro chemotaxis bio-assay point towards a reduced attraction of the nematodes to the mycorrhizal plant roots. The results demonstrate that a water-soluble compound in mycorrhizal root exudates is at least partially responsible for the mycorrhiza-induced resistance at the pre-infectional level of R. similis infection.  相似文献   

4.
The burrowing nematode, Radopholus similis (Cobb.) Thorne, causes the most damage to bananas. To minimize nematicide applications, cropping systems that use fallow, crop rotation and clean planting material have been developed in the French West Indies. In order to optimize the benefit of the intercropping period, we studied the survivorship of R. similis in different soil types and conditions. We monitored the survivorship of calibrated populations of R. similis in the laboratory on a Nitisol and on an Andosol, two soils derived from volcanic ashes and pumices. We studied water potentials ranging from 0 to ?700 kPa on undisturbed soil and on soil previously frozen to get rid of living nematodes. Mortality of adult R. similis decreased regularly, and was fairly well described by Teissier's model. In the previously frozen soils, R. similis survived longer in wet soils (half-life of 21–46 days at 0 to ?5 kPa) than in dry soils (half-life of less than 10 days between ?80 and ?250 kPa). In contrast, in undisturbed soils, R. similis survived longer in dry soils: half-lives ranged from 57 days at ?273 kPa to 17 days at water saturation in the Andosol, and 36 days at ?660 kPa to 14 days at water saturation in the Nitisol. These results are consistent with the absence of anhydrobiosis in R. similis, unlike Pratylenchus coffeae. P. coffeae survivorship curves over time do not follow a model derived from exponential decrease like Teissier's model. These results also show that the recommended one year host-free period required to sanitize soils cannot be shortened without risk, even if flooding the soil could improve it.  相似文献   

5.
施用生物有机肥抑制香蕉镰刀菌萎蔫病的研究   总被引：2，自引：0，他引：2  
Fusarium wilt is one of the most serious diseases of banana plants caused by soil-borne pathogen Fusarium oxysporum f.sp. cubense(FOC). In this study a pot experiment was conducted to evaluate the effects of different bio-organic fertilizers(BIOs) on Fusarium wilt of banana, including the investigations of disease incidence, chitinase and β-1,3-glucanase activities of banana plants, and FOC populations as well as soil rhizosphere microbial community. Five fertilization treatments were considered, including chemical fertilizer containing the same N, P and K concentrations as the BIO(control), and matured compost mixed with antagonists Paenibacillus polymyxa SQR-21 and Trichoderma harzianum T37(BIO1), Bacillus amyloliquefaciens N6(BIO2), Bacillus subtilis N11(BIO3), and the combination of N6 and N11(BIO4). The results indicated that the application of BIOs significantly decreased the incidence rate of Fusarium wilt by up to 80% compared with the control. BIOs also significantly promoted plant growth, and increased chitinase andβ-1,3-glucanase activities by 55%–65% and 17.3%–120.1%, respectively, in the banana roots. The population of FOC in the rhizosphere soil was decreased significantly to about 104 colony forming units g-1with treatment of BIOs. Serial dilution plating and denaturing gradient gel electrophoresis analysis revealed that the application of BIOs increased the densities of bacteria and actinomycetes but decreased the number of fungi in the rhizosphere soil. In general, the application of BIOs revealed a great potential for the control of Fusarium wilt disease of banana plants.  相似文献   

6.
《Soil biology & biochemistry》2001,33(7-8):931-941
The influence of host plants on Frankia populations was investigated using soils from Ceanothus velutinus (Dougl.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands. Population sizes of Ceanothus-infective Frankia in the soils were measured using plant bioassays with C. velutinus, C. sanguineus (Pursh), and C. integerrimus (H. & A.) as trap plants. The Frankia population in soil from the C. velutinus stand soil was about 10 times higher than that from the Douglas-fir stand. This result supports previous reports that, although the presence of host plants increases Frankia populations, Frankia persist without host plants. Nodulation capacities of the three trap plants were not significantly different. All nodules showed N2 fixation activity using the acetylene reduction assay. The diversity of Frankia that nodulated trap plants was examined using repetitive intergenic DNA and the polymerase chain reaction (rep-PCR). A newly designed, direct repeat sequence and a BOX sequence were used as rep-PCR primers. The results showed that infective Frankia in the two soils contained a common group of Frankia as well as some Frankia strains unique to each soil. The level of host specificity of the infective Frankia was low; however, one group of Frankia nodulated only C. integerrimus seedlings. Taken together, the results suggest that the higher populations in the soil from the C. velutinus stand may be due to preferential increases in particular groups of Frankia.  相似文献   

7.
Varietal mixture is a cultural technique in which the genetic and functional diversities of varieties are used to manage pests and diseases. This strategy is commonly used on grass crops such as rice, barley, maize, and wheat to mitigate some windborne and soilborne pathogens. The effects of varietal mixtures on the number and community structure of pests, including plant-feeding nematodes (PFNs), however, have rarely been studied. In experiments conducted in Martinique, we evaluated the effect of varietal mixtures of bananas on PFN communities. A growth chamber experiment was used to measure the susceptibility of three banana cultivars dessert banana cv. 902 (Musa AAA, Cavendish subgroup); a new synthetic hybrid cv. FB924 (Musa AAA); and a plantain cv. Creole blanche (Musa AAB, French Horn) to the two major PFNs. The multiplication rates of Radopholus similis and Pratylenchus coffeae were substantially different on the three varieties; for example, the multiplication rate was up to 10 times greater on plantain cv. Creole blanche than on hybrid cv. FB924. In a field experiment, we planted the three varieties in pairs that included all six possible combinations. Banana varietal mixtures significantly affected both PFN densities and community composition. Differences in community composition among the pairs involved a shifting equilibrium among nematode species and an interspecific competition for food resources. The relative abundance of the spiral nematode Helicotylenchus multicinctus increased while that of the burrowing nematode R. similis, which is the most damaging species on bananas, decreased. The use of a varietal mixture in which one variety supports a low PFN multiplication rate appears to have practical relevance, especially in systems based on very susceptible cultivars such as plantains. The use of varietal mixtures should not create management problems, especially for plantations that produce bananas for local markets.  相似文献   

8.
Thus far, there is little knowledge of the genetic diversity, structure and gene flow dynamics in rare wild and semi-wild soybean mixed populations, and such information is vital for understanding of the origin of semi-wild soybean (Glycine gracilis) and the biosafety protection of wild soybean from transgenic soybeans. Population eco-genetic data are necessary to provide a more coherent and comprehensive understanding of the genetic events that occurred in the natural habitats of wild soybean (Glycine soja). We tested genetic diversity and structure of 11 wild mixed populations of wild soybean (Glycine soja) and semi-wild soybean (G. gracilis), 1 wild soybean population, and 1 cultivated soybean variety population were studied using 20 nuclear microsatellite markers (SSRs). We found based on microsatellite polymorphisms that the mixed populations were characterized by higher mean heterozygosity (H o = 0.029) and outcrossing rate (t m = 6.35 %), and lower fixation index (F is = 0.891), and the semi-wild plants had distinctly higher heterozygosity (H o = 0.081) than that of the wild plants (H o = 0.007). The occurrence of semi-wild plants influenced population genetic structure but not geographical population differentiation. These mixed populations exhibited strong ecogeographical differentiation, which suggests that their original populations were colonized over a long phytogeographical history. The introgression occurred through pollen gene flow from the soybean fields into wild populations and created the semi-wild plants, with significant genetic differentiation from the typical wild ones. Introgressive genes could become established by two possible modes in wild soybean populations by both self-segregation and/or intrapopulation secondary hybridization. The latter deserves attention because of the possibility of rapid transgene escape.  相似文献   

9.
Data from microsatellite markers have been extensively used for both in situ and ex situ conservation strategies by determining the level of genetic diversity of natural populations that can widen the gene pool of cultivated plants. Such conservation practices are based on understanding of the between and within population genetic variations and partitioning populations on the basis of geographic origin. Therefore, the objective of this study was to assess the genetic diversity of Oryza longistaminata Chev. et Roehr and how this variation is partitioned within and between the eight O. longistaminata populations found in the different geographic regions of Ethiopia using simple sequence repeat markers. Five microsatellite markers in 320 samples generated 64 alleles that revealed the presence of large amount of genetic variability (Ho = 0.225; He = 0.768; Na = 7.375; Ne = 6.565 and P = 0.744). The F-statistics detected by the microsatellite loci showed Fst = 0.064 and Fis = 0.743 and there was no population in Hardy–Weinberg equilibrium. The genetic diversity results obtained from this data indicated that there are high levels of genetic diversity in the populations of O. longistaminata studied and it is higher within than between populations. Among the eight populations sampled, five populations were identified as priorities for conservation strategies. Thus, national collection and conservation strategies need to consider these populations.  相似文献   

10.
A perennial version of grain sorghum [S. bicolor (L.) Moench] would create opportunities for greatly reducing tillage and preventing soil degradation. Efforts to select for perenniality and grain production among progeny of hybrids between S. bicolor (2n = 20) and the weedy tetraploid perennial S. halepense (L.) Pers. (2n = 40) are complicated in that F1 hybrids produced by diploid × tetraploid sorghum crosses are usually tetraploid. In 2013, a set of random pollinations between 19 diploid cytoplasmic male-sterile inbred lines and 43 tetraploid perennial plants produced 165 F1 hybrid plants, more than 75% of which had highly atypical plant, panicle, and seed phenotypes. Phenotypic segregation in F2 populations derived from atypical hybrids was also anomalous. Examination of mitotic metaphase cells in F1 or F2 root tips revealed that 129 of the 165 hybrids were diploid. Parentage of the diploid progenies was confirmed using simple-sequence repeat analysis. The mechanism by which diploid hybrids arise from diploid × tetraploid crosses is unknown, but it may involve either production of monohaploid (n = 10) pollen by the tetraploid parent or chromosome elimination during early cell divisions following formation of the triploid zygote. The ability to produce diploid germplasm segregating for S. bicolor and S. halepense alleles could have great utility, both for the development of perennial sorghum and for the improvement of conventional grain sorghum.  相似文献   

11.
Soil samples taken from 28 sites following varying periods of cropping in a crop-pasture rotation contained very low populations of Rhizobium trifolii. Populations were less than 103g?1R. trifolii of soil for 89% of the sites and were significantly correlated with soil pH. Application of lime resulted in a build-up of R. trifolii in the absence of the host legume, subterranean clover, but when inoculated clover seed was sown the populations built up to satisfactory levels after the first season's growth, regardless of soil pH.The number of nodules per plant was increased by the application of lime, but the plants growing in unlimed soil had fewer, larger nodules. The increase in nodulation with lime on these low-calcium acid soils persisted to the third growing season.  相似文献   

12.
American ginseng (Panax quinquefolius) is a native North American medicinal plant that is becoming increasingly vulnerable despite government harvest restrictions. To better understand the genetic diversity and gene flow of American ginseng, we studied RAPD variation in cultivated and wild populations. Classical and Bayesian analogues of genetic diversity statistics were estimated in seven wild and two cultivated populations. The wild populations were more highly structured (G stβ  = 0.41) than the cultivated populations (G stβ  = 0.24). The genetic diversity within populations ranged from H eβ  = 0.05 to 0.38. Based on genetic pairwise distances, six of the wild populations clustered with the locally-derived cultivated population, while one wild population was more similar to the non-local cultivated population than the local populations. This wild population was highly diverse (P = 1.0; U = 1.0) suggesting that it was supplemented from exotic seed. A set of eight RAPD markers was identified that differentiated plants of local and non-local origin. As a conservation strategy, we recommend that regional gene banks be established based on molecular and geographic diversity to preserve the locally adapted germplasm. These regional gene banks would serve as a conservation tool and also provide a source of genes for genetic improvement of cultivated ginseng.  相似文献   

13.
Growing-season populations of rhizobia associated with annual host-plant roots and nearby soil were examined in a field soil showing a nodulation problem in the second year after establishment. Rhizobium lupini reached higher populations at a faster rate than R. trifolii. A sharp drop in the population of R. trifolii associated with subterranean clover roots early in the growing season was followed by a recovery to high numbers. No such phenomenon occurred with R. lupini. The numbers of rhizobia under patches of non-nodulated plants in second-year stands were very low, usually <5/g soil, whereas the numbers under healthy plants in problem stands were similar to those under established stands. Differences in the colonization of both root and soil by R. trifolii in the first year were reflected in the second-year nodulation.  相似文献   

14.
Plant‐pathogenic nematodes are a major cause of crop damage worldwide, the current chemical nematicides cause environmental damage, but alternatives such as biological control are less effective, so further understanding of the relationship between nematodes, nematicides, biological control agents and soil and rhizosphere microorganisms is needed. Microbial populations from roots of cabbage and tomato plants infested with the root‐knot nematode Meloidogyne incognita were compared with those from plants where the nematode was controlled by the nematicide aldicarb, or a nematophagous fungus with biological control potential, Pochonia chlamydosporia. The total numbers of culturable bacteria and fungi in rhizosphere soil were similar in all three treatments for both plants, around 100‐fold more than in control soil in which there were no plants. However, there were clear differences in the catabolic diversity, assessed by Biolog EcoPlate? carbon substrate utilization assays, between microbial populations from unplanted soil and the rhizosphere. In cabbage, a poor host for M. incognita, the rhizosphere population from P. chlamydosporia‐treated plants was distinct from the population from untreated and aldicarb‐treated plants. In tomato, a host susceptible to the nematode, the catabolic diversity of populations from aldicarb‐ and P. chlamydosporia‐treated plants was similar and differed from the untreated, nematode‐infested plants. The genetic diversity of the fast‐growing heterotrophic bacteria in the tomato rhizosphere, indicated by PCR fingerprinting with ERIC primers, was very different in the infested roots, whereas the profiles of isolates from both aldicarb‐ and P. chlamydosporia‐treated roots were similar. Evidently, nematodes have a greater impact on the rhizosphere population of a susceptible host, tomato, than a poor one, cabbage, and nematode‐infested roots are colonized by a different subpopulation of soil microbes from that on plants where infection is controlled, illustrating differences in root morphology and physiology.  相似文献   

15.
To preserve populations of endangered bee species, sound knowledge of their maximum foraging distance between nest and host plants is crucial. Previous investigations predicted maximum foraging distances of 100-200 m for small bee species and up to 1100 m for very large species based on mainly indirect methods. The present study applied a new and direct approach to experimentally investigate maximum foraging distances in solitary bees. One endangered and two common species of different body sizes, all of which restrict pollen foraging to a single plant genus, were established in a landscape lacking their specific host plants. Females were forced to collect pollen on potted host plants that were successively placed in increasing distance from fixed nesting stands. The maximum foraging distance recorded for the small Hylaeus punctulatissimus was 1100 m, for the medium sized Chelostoma rapunculi 1275 m and for the large Hoplitis adunca 1400 m, indicating that maximum foraging distances at species level have been underestimated. However, the capability to use resources on such a large spatial scale applied only to a small percentage of individuals as 50% of the females of H. punctulatissimus and H. adunca did not forage at distances longer than 100-225 m and 300 m, respectively. This finding suggests that a close neighbourhood of nesting and foraging habitat within few hundred meters is crucial to maintain populations of these species, and that threshold distances at which half of the population discontinues foraging are a more meaningful parameter for conservation practice than the species specific maximum foraging distances.  相似文献   

16.
Banana (Musa AAA) and plantain (Musa AAB) are important dietary staples for over 70 million people in sub-Saharan Africa. Banana weevil (Cosmopolites sordidus) and nematodes (Radopholus similis, Helicotylenchus multicinctus) are often the largest constraints to production. Leguminous crops in the genera Canavalia, Mucuna and Tephrosia have been reported as having repellent or insecticidal properties. We assessed: weevil and nematode populations and damage; banana growth and development; soil and foliar nutrient concentrations; and soil moisture status in banana intercropped with three legumes (Canavalia ensiformis, Mucuna pruriens and Tephrosia vogelii). The research took place at Kawanda, Uganda, on a field site infested with weevils and nematodes. The legume intercrops did not affect weevil populations or damage. The legumes also provided no benefit in terms of nematode control; in fact, banana root necrosis was higher in the T. vogelii intercrop than in the banana monoculture. Banana fruit yield was not adversely affected by the legume intercrops. The low yield noted across treatments was attributed to the high level of damage by the weevils and the nematodes. Significantly more NO3-N accumulated from 0- to 0.3-m depth in the M. pruriens intercrop than in the other treatments. There was no treatment difference in soil moisture. In summary, there was no significant advantage in banana production associated with the legume intercrops. However, the lack of a significant difference in banana fruit production between the monoculture and the intercrops suggests that land use efficiency may be increased by incorporating food and or/fodder legumes into the banana cropping system.  相似文献   

17.
Rubus parvifolius L. and R. coreanus Miq. are the two morphologically distinct, endemic wild brambles in East Asia that may be useful in raspberry breeding and cultivar development. In this study, genetic relationships among a total of nine populations of these two species, collected from Sichuan Province in southwestern China, were investigated by cytological and RAPD analyzing. With the exception of ploidy levels, R. parvifolius and R. coreanus had similar cytological features. Six R. parvifolius populations included diploids, triploid, tetraploid, and mixed diploid-tetraploid cytotype plants; among them, the population that produced the fewest seeds was the triploid, while the three R. coreanus populations were all diploid. But obvious genetic differentiation between R. parvifolius and R. coreanus was revealed by RAPD markers, the Euclidean’s GS coefficient generated by UPGMA cluster analysis between R. parvifolius and R. coreanus was only 0.442, about half of that observed within species. Both genetic relationships among these populations and the origin of polyploidy in R. parvifolius populations are also discussed.  相似文献   

18.
Collection of plants and seeds from wild populations threatens a large number of cycad species. We investigated to what extent individual life history stages contribute to population growth (λ) and compared two species with major differences in life histories in the African genus Encephalartos: Encephalartos cycadifolius, a highly persistent grassland species that resprouts after fire, and Encephalartos villosus, a relatively fast growing, non-sprouting forest species. Several harvesting scenarios impacting different sized individuals were simulated to determine the sensitivity of the two functional types to harvesting. In both species λ was most sensitive to changes in abundance of adult plants. The harvesting of seeds had minimal impact on population growth rates, whereas harvesting of adult plants led to rapid population decline. This response from two very different functional types suggests that the conservation of adult plants is critical for all cycad species. Despite similar responses to adult mortality, the two species had substantially different population growth rates. This determined recovery time after harvesting of adult individuals. Encephalartos cycadifolius is typical of highly persistent plant species associated with low levels of recruitment and unable to recover from even small losses of adults within a reasonable conservation time frame (<100 years). Our results suggest that the ability to recover from loss of individuals is an important factor that should be considered when assessing the vulnerability of wild populations to threats.  相似文献   

19.
Field pea (Pisum sativum L.) is widely grown in South Australia (SA), often without inoculation with commercial rhizobia. To establish if symbiotic factors are limiting the growth of field pea we examined the size, symbiotic effectiveness and diversity of populations of field pea rhizobia (Rhizobium leguminosarum bv. viciae) that have become naturalised in South Australian soils and nodulate many pea crops. Most probable number plant infection tests on 33 soils showed that R. l. bv. viciae populations ranged from undetectable (six soils) to 32×103 rhizobia g−1 of dry soil. Twenty-four of the 33 soils contained more than 100 rhizobia g−1 soil. Three of the six soils in which no R. l. bv. viciae were detected had not grown a host legume (field pea, faba bean, vetch or lentil). For soils that had grown a host legume, there was no correlation between the size of R. l. bv. viciae populations and either the time since a host legume had been grown or any measured soil factor (pH, inorganic N and organic C). In glasshouse experiments, inoculation of the field pea cultivar Parafield with the commercial Rhizobium strain SU303 resulted in a highly effective symbiosis. The SU303 treatment produced as much shoot dry weight as the mineral N treatment and more than 2.9 times the shoot dry weight of the uninoculated treatment. Twenty-two of the 33 naturalised populations of rhizobia (applied to pea plants as soil suspensions) produced prompt and abundant nodulation. These symbioses were generally effective at N2 fixation, with shoot dry weight ranging from 98% (soil 21) down to 61% (soil 30) of the SU303 treatment, the least effective population of rhizobia still producing nearly double the growth of the uninoculated treatment. Low shoot dry weights resulting from most of the remaining soil treatments were associated with delayed or erratic nodulation caused by low numbers of rhizobia. Random amplified polymorphic DNA (RAPD) polymerase chain reaction (PCR) fingerprinting of 70 rhizobial isolates recovered from five of the 33 soils (14 isolates from each soil) showed that naturalised populations were composed of multiple (5-9) strain types. There was little evidence of strain dominance, with a single strain type occupying more than 30% of trap host nodules in only two of the five populations. Cluster analysis of RAPD PCR banding patterns showed that strain types in naturalised populations were not closely related to the current commercial inoculant strain for field pea (SU303, ≥75% dissimilarity), six previous field pea inoculant strains (≥55% dissimilarity) or a former commercial inoculant strain for faba bean (WSM1274, ≥66% dissimilarity). Two of the most closely related strain types (≤15% dissimilarity) were found at widely separate locations in SA and may have potential as commercial inoculant strains. Given the size and diversity of the naturalised pea rhizobia populations in SA soils and their relative effectiveness, it is unlikely that inoculation with a commercial strain of rhizobia will improve N2 fixation in field pea crops, unless the number of rhizobia in the soil is very low or absent (e.g. where a legume host has not been previously grown and for three soils from western Eyre Peninsula). The general effectiveness of the pea rhizobia populations also indicates that reduced N2 fixation is unlikely to be the major cause of the declining field pea yields observed in recent times.  相似文献   

20.
Germination of wild and managed populations of columnar cacti was compared to analyze differences associated to management intensity. The species studied: Polaskia chende, Escontria chiotilla, Myrtillocactus schenckii, Polaskia chichipe, and Stenocereus pruinosus are in a gradient from lower to higher management intensity, respectively. Within each species wild, silvicultural, and cultivated populations also represent a gradient from lower to higher management intensity. We hypothesized that seeds of plants under higher management intensity would require more water to germinate than others since managed environments are relatively wetter than unmanaged environments. Such pattern could be visualized within and among species according to management intensity. We tested germination of seeds from different populations of the different species to the water potentials: 0.0, ?0.2, ?0.4, ?0.6, ?0.8, and ?1.0 MPa. In all species studied seeds of managed populations had significantly more mass than seeds from wild populations. Each species had different susceptibility to water availability decrease, which could be related to particular adaptations to drought resistance. Stenocereus pruinosus and Polaskia chende were the most and least susceptible species, respectively. In control treatments of all species, seeds of managed populations had higher germination rate than seeds from wild populations, but when water availability decreased the pattern was the contrary. Our results suggest that artificial selection for larger fruits and heavier seeds in plants established in wetter environments have affected germination patterns. Although species have particular critical points of water availability for germination, differences in germination patterns are more pronounced according to management and artificial selection intensity.  相似文献   

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