Testing variability in pathogenicity of Phytophthora cinnamomi

Forty eight isolates of Phytophthora cinnamomi from various host plants in France (35 isolates) and in other countries were tested for pathogenicity. Seedlings of chestnut, northern red oak, pine and eucalyptus were infected by soil contamination. Taproots, stems and bark strips of plants of chestnut and different oak species were inoculated with mycelium agar disks. Results of the different experiments were in good agreement. All isolates appeared pathogenic to all the different test species but with variable levels of virulence. Isolates with consistent low or high level of virulence, which could be used as standards in further studies, were identified. Interaction between P. cinnamomi isolates and host plant species was significant in terms of lesion lengths. These interactions could not be related to host from which P. cinnamomi was isolated. Consistent with this, in Quercus rubra, the isolate-provenance interaction was not significant. This feature is encouraging for provenance screening for resistance to P. cinnamomi in this species. The variation in virulence was not related to other isolate characteristics (mating type, electrophoretic type, age).     

Phytophthora species in oak ecosystems in Turkey and their association with declining oak trees

From 1999 to 2001, a survey on the occurrence of Phytophthora spp. in the rhizosphere soil of healthy and declining oak trees was conducted in 51 oak stands in Turkey. Seven Phytophthora spp. were recovered from six out of the nine oak species sampled: P .  cinnamomi , P .  citricola , P .  cryptogea , P .  gonapodyides , P .  quercina , Phytophthora sp. 1 and Phytophthora sp. 2. The most frequently isolated species, P .  quercina , was very common on slopes susceptible to drought. It occurred in four different climatic zones and on six Quercus spp., suggesting that it is native to oaks. The second most common species, P .  citricola , was separated into three subgroups: type C was recovered only in Anatolia, whereas A and B occurred only in the European part of Turkey. Phytophthora cinnamomi was recovered at one site only, and may not be involved in oak decline in Turkey. The other four species were recovered sporadically. On affected sites there was a significant association between deteriorating crown status and the presence of Phytophthora spp., particularly P .  quercina . The occurrence of Phytophthora species was significantly influenced by soil pH. Stem inoculation tests on oak seedlings revealed that Q .  petraea was the most susceptible species.     

西北旱区制种玉米灌浆期气孔导度与叶水势对水氮胁迫的响应

为了探究西北旱区制种玉米水氮胁迫条件下的生理节水机理,设置充分灌溉(FI)与亏缺灌溉(DI)处理,以及施氮(N150)与不施氮(N0)处理,分析了不同水氮模式下叶水势与气孔特征的变化规律。结果表明:充分灌溉条件下,施氮主要通过增加气孔开度和气孔数量提高气孔导度,与不施氮处理相比,气孔开度提高19.46%,气孔数量增加8.46%;亏缺灌溉条件下则通过增加单位面积开放气孔数量和降低气孔开度来应对水分消耗,提高抗旱能力,与充分灌溉相比,亏缺灌溉气孔开度平均降低21.38%,气孔数量平均增加14.08%;增加灌水量能极显著增大气孔开度(P<0.05)和提高玉米叶片黎明和正午叶水势值,促进作物蒸腾作用;FIN150处理较DIN150处理黎明和正午叶水势均值分别上升了3.30%和2.00%,FIN0处理较DIN0处理黎明和正午叶水势均值分别上升了13.41%和14.29%;亏缺灌溉条件下,施氮处理黎明和正午叶水势均值分别上升了2.87%和0.17%。干旱导致气孔导度及正午叶水势的降低可以通过增施氮肥得到部分补偿。     

Histopathology of infection and colonization of Quercus ilex fine roots by Phytophthora cinnamomi

Quercus ilex is one of the European forest species most susceptible to root rot caused by the oomycete Phytophthora cinnamomi. This disease contributes to holm oak decline, a particularly serious problem in the ‘dehesas’ ecosystem of the southwestern Iberian Peninsula. This work describes the host–pathogen interaction of Q. ilex and P. cinnamomi, using new infection indices at the tissue level. Fine roots of 6‐month‐old saplings inoculated with P. cinnamomi were examined by light microscopy and a random pool of images was analysed in order to calculate different indices based on the measured area of pathogen structures. In the early stages of invasion, P. cinnamomi colonizes the apoplast and penetrates cortical cells with somatic structures. On reaching the parenchymatous tissues of the central cylinder, the pathogen develops different reproductive and survival structures inside the cells and then expands through the vascular system of the root. Some host responses were identified, such as cell wall thickening, accumulation of phenolic compounds in the middle lamella of sclerenchyma tissues, and mucilage secretion blocking vascular cells. New insights into the behaviour of P. cinnamomi inside fine roots are described. Host responses fail due to rapid expansion of the pathogen and a change in its behaviour from biotrophic to necrotrophic.     

Characterization of accessions and species of Macadamia to stem infection by Phytophthora cinnamomi

Phytophthora cinnamomi is a major pathogen in most macadamia plantations worldwide. Due to stem lesions, stem cankers and leaf defoliation, it results in loss of productivity and tree death. This study examined accessions of the four Macadamia species and their hybrids, produced via rooted stem cuttings or germinated seeds, for susceptibility to stem canker and necrotic lesions caused by P. cinnamomi. Plants were wound‐inoculated with agar containing P. cinnamomi. The symptoms produced in inoculated plants were used to characterize host susceptibility variation within and among the population. Lesion length and severity of stem canker were recorded. The four species and hybrids differed significantly in stem canker severity (P < 0.001) and lesion length (P = 0.04). Macadamia integrifolia and M. tetraphylla hybrids were the most susceptible. Macadamia integrifolia had the greatest stem canker severity and the most extensive lesions above and below the site of inoculation. Restricted lesion sizes were observed in M. ternifolia and M.  jansenii. The effects of basal stem diameter and the method of propagation either from cuttings or from seed were not significant. The genetic variation in the reaction of macadamia accessions to stem infection by P. cinnamomi is discussed.     

Response of stomatal conductance of two tree species to vapor pressure deficit in three climate zones

Stomatal behavior is a central topic of plant ecophysiological research under global environmental change. However, the physiological mechanism controlling the response of stomata to vapor pressure deficit(VPD) or relative humidity(RH) has been inadequately understood till now. In this study, responses of stomatal conductance(gs) to VPD in two species of trees(Fraxinus chinensis Roxb., Populus alba L. var. pyramidalis Bge.) in three different climate zones(Jinan with typical warm humid/semi-humid climate, Urumqi with temperate continental arid climate and Turpan with extreme arid desert climate) were measured. Levels of two phytohormones(abscisic acid, ABA; indole-3-acetic acid, IAA) in the leaves of the two tree species at these three sites were also measured by high performance liquid chromatography. The results showed that the responses of gs to an increasing VPD in these two tree species at the three sites had peak curves which could be fitted with a Log Normal Model(gs=a·exp(–0.5(ln(D/c)/b)2). The VPD/RH values corresponding to the maximum gs can be calculated using the fitting models for the two tree species in the three sites. We found that the calculated gs-max-VPD correlated negatively with relative air humidity in the three sites during the plant growth period(April to October 2010), which showed the values of gs-max-VPD were related to the climate conditions. The prevailing empirical stomatal model(Leuning model) and optimal stomatal behavior model could not properly simulate our measured data. The water use efficiency in the two tree species did not show obvious differences under three very different climatic conditions, but the highest gs, photosynthetic and transpiration rates occurred in P. alba var. of Turpan. The sensitivity in response of gs to VPD in leaves of the two trees showed positive correlations with the concentration of ABA, which implied that ABA level could be used as an indicator of the sensitivity of stomatal response to VPD. Our results confirmed that the prediction of the response of gs to VPD might be incomplete in the two current popular models. Therefore, an improved gs model which is able to integrate the results is needed. Also, the stomatal response mechanism of single peak curves of gs to VPD should be considered.     

Involvement of soilborne Phytophthora species in Central European oak decline and the effect of site factors on the disease

A survey was made on the occurrence of soilborne Phytophthora species in 35 oak stands on a range of geologically different sites in Bavaria. The most widespread species were P. quercina , P. cambivora and P. citricola . Seven other Phytophthora species were isolated infrequently. The fine root systems of 106 healthy and 111 declining mature trees of Quercus robur and Q. petraea were intensively investigated. The results indicate that, depending on the site conditions, at least two different complex diseases are referred to under the name 'oak decline'. On sites with a mean soil pH (CaCl₂)  3·5 and sandy-loamy to clayey soil texture Phytophthora spp. were commonly isolated from rhizosphere soil, and highly significant correlations existed between crown transparency and various root parameters. Oaks with P. quercina or other Phytophthora spp. in their rhizosphere had markedly higher levels of fine root damage than oaks without Phytophthora spp., and were subject to a relative risk of severe crown symptoms of 2·1 and 2·8, respectively. In contrast, in stands with sandy to sandy-loamy soils and a mean soil pH  3·9, Phytophthora spp. were not found. In these stands, correlations between crown transparency and various root parameters were either less significant or not significant. It is concluded that Phytophthora species are strongly involved in oak decline on sandy-loamy to clayey sites with a mean soil-pH (CaCl₂)  3·5.     

Hydraulic resistance partitioning between shoot and root system and plant water status of Haloxyolon ammodendron growing at sites of contrasting soil texture

Hydraulic resistance components and water relations were studied on Haloxyolon ammoden-dron,a small xeric tree,growing at sites significantly differed in soil texture.Soil water content,leaf water potential(ψl),xylem water potential(ψx),root water potential(ψroot),leaf transpiration rate(TR) and stomatal conductance(gs) were measured at the two sites during the growing season of 2005 and 2006.Leaf spe-cific hydraulic resistance(Rplant) during the whole growing season,hydraulic resistance of plants(Rp),shoots(Rshoot) and roots(Rroot) in the August of both years were calculated and expressed on leaf area basis.The results showed the proportion of the hydraulic resistance of the aerial part(Rshoot) to the Rp was the same to the proportion of the hydraulic resistance of the soil part(Rroot) to the Rp,indicating that both parts were equivalent important to plant water hydraulic system from soil to leaf.Positive significant corre-lations were found between Rp and Rroot,suggesting that root hydraulics resistance was a major determinant of plant hydraulic resistance(Rp) and transpiration rate.The integrated effect of stomatal control,hy-draulic regulation and morphology adjustment enabled plants at heavy soil site surviving the extreme water deficit period.     

植物水分运转影响因子的研究进展

影响植物水分运转的主要因子有气象因子(包括太阳辐射、大气温度、大气湿度、风速、CO2浓度等)、土壤的水分状况、气孔导度、植物叶面积、水分在植物体内的传输阻力等。大量研究表明:植物蒸腾的日变化主要是由气象因子的日变化引起的,而蒸腾的年变化主要与土壤的水分状况、叶面积动态和气候条件有关。此外,组织贮存水和导管的空穴对植物的水分运转也有一定的影响。     

Water adaptive traits of deep-rooted C3 halophyte（Karelinia caspica（Pall.） Less.） and shallow-rooted C4 halophyte（Atriplex tatarica L.） in an arid region,Northwest China

This paper focused on the water relations of two halophytes differing in photosynthetic pathway,phenotype,and life cycle:Karelinia caspica (Pall.) Less.(C3,deep-rooted perennial Asteraceae grass) and A...     

Gas exchange of Populus euphratica leaves in a riparian zone

Riparian vegetation belts in arid regions of Central Asia are endangered to lose their ecosystem services due to intensified land use.For the development of sustained land use,management knowledge of plant performance in relation to resource supply is needed.We estimated productivity related functional traits at the edges of the habitat of Populus euphratica Oliv.Specific leaf area(SLA) and carbon/nitrogen(C/N) ratio of P.euphratica leaves growing near a former river bank and close to moving sand dunes in the Ebinur Lake National Nature Reserve in Xinjiang,Northwest China(near Kazakhstan) were determined and daily courses of CO2 net assimilation(PN),transpiration(E),and stomatal conductance(gs) of two consecutive seasons were measured during July-August 2007 and June-July 2008.Groundwater level was high(1.5-2.5 m below ground) throughout the years and no flooding occurred at the two tree stands.SLA was slightly lower near the desert than at the former river bank and leaves contained less N in relation to C.Highest E and g s of P.euphratica were reached in the morning before noon on both stands and a second low maximum occurred in the afternoon despite of the unchanged high levels of air to leaf water vapor pressure deficit(ALVPD).Decline of g s in P.euphratica was followed by decrease of E.Water use efficiency(WUE) of leaves near the desert were higher in the morning and the evening,in contrast to leaves from the former river bank that maintained an almost stable level throughout the day.High light compensation points and high light saturation levels of PN indicated the characteristics of leaves well-adapted to intensive irradiation at both stands.In general,leaves of P.euphratica decreased their g s beyond 20 Pa/kPa ALVPD in order to limit water losses.Decrease of E did not occur in both stands until 40 Pa/kPa ALVPD was reached.Full stomatal closure of P.euphratica was achieved at 60 Pa/kPa ALVPD in both stands.E through the leaf surface amounted up to 30% of the highest E rates,indicating dependence on water recharge from the ground despite of obviously closed stomata.A distinct leaf surface temperature(Tleaf) threshold of around 30℃ also existed before stomata started to close.Generally,the differences in gas exchange between both stands were small,which led to the conclusion that micro-climatic constraints to E and photosynthesis were not the major factors for declining tree density with increasing distance from the river.     

Seasonal changes in susceptibility of Quercus suber to Botryosphaeria stevensii and Phytophthora cinnamomi

Monthly inoculations of both intact plants and excised shoots of Quercus suber with the pathogenic species Botryosphaeria stevensii and Phytophthora cinnamomi were performed to investigate seasonal changes in susceptibility of this forest tree species in relation to environmental parameters and plant water status. Infection symptoms were mainly detected on seedlings inoculated from spring to autumn (April through October) with either pathogen. Mean canker sizes also showed a seasonal pattern, the higher values being recorded in the same period as above. Lesion lengths were significantly ( P  < 0·001) related to environmental minimum temperature. Mean daily minimum temperatures within the range of 5–12°C clearly inhibited lesion development of P. cinnamomi , whereas B. stevensii showed a less pronounced decrease in canker expansion at the same temperature range. In excised shoots of Q. suber inoculated monthly with B. stevensii , a negative linear relationship was found between the studied range of plant relative water content (81–91%) and canker length. In contrast, the lesions caused by P. cinnamomi were not significantly ( P  = 0·32) related to any seasonal change in water content. Some control measures for the diseases caused by both pathogens are discussed on the basis of the seasonal changes in host susceptibility observed in this study.     

荒漠植物白麻气孔导度特征及其影响因子研究

依据2010年对干旱荒漠区白麻的观测资料,对白麻气孔运动及其影响因子进行了分析研究并揭示了荒漠植物白麻的水分利用特征。结果表明:(1)白麻气孔导度的整体下降过程中有少许周期性波动,与净光合速率、蒸腾速率和胞间CO2浓度的转折点基本一致。(2)白麻气孔导度与影响因子之间进行偏相关分析,多元回归分析,通径分析,最终得出白麻气孔导度主要受光合有效辐射(PAR)、水汽压亏缺(VPD)、叶气温差(ΔT)和空气温度(Ta)的影响。(3)白麻水分利用效率和气孔限制值的关系可以看出:水分利用效率随着气孔限制值的增大而增大,随着气孔限制值的减小而减小。     

分根交替渗透胁迫下玉米幼苗叶片中游离氨基酸的变化

用Hoagland溶液培养玉米幼苗,研究分根交替渗透胁迫对其叶片水势、可溶性糖、NH4 和游离氨基酸的影响。实验表明,当玉米根系一半处于高水势,另一半处于低水势溶液时,在24h间隔交替过程中,叶片的可溶性糖、NH4 和游离氨基酸的含量都比同浓度聚乙二醇(PEG)不分根胁迫下的处理低,但叶水势则相反。说明分根交替渗透胁迫能够发挥根系吸水的补偿效应,提高叶水势。     

Gene and Genotypic Diversity of Phytophthora cinnamomi in South Africa and Australia Revealed by DNA Polymorphisms

Phytophthora cinnamomi isolates from South Africa and Australia were compared to assess genetic differentiation between the two populations. These two populations were analysed for levels of phenotypic diversity using random amplified polymorphic DNAs (RAPDs) and gene and genotypic diversity using restriction fragment length polymorphisms (RFLPs). Sixteen RAPD markers from four decanucleotide Operon primers and 34 RFLP alleles from 15 putative loci were used. A few isolates from Papua New Guinea known to posses alleles different from Australian isolates were also included for comparative purposes. South African and Australian P. cinnamomi populations were almost identical with an extremely low level of genetic distance between them (D_m=0.003). Common features for the two populations include shared alleles, low levels of phenotypic/genotypic diversity, high clonality, and low observed and expected levels of heterozygosity. Furthermore, relatively high levels of genetic differentiation between mating type populations (D_m South Africa=0.020 and D_m Australia=0.025 respectively), negative fixation indices, and significant deviations from Hardy–Weinberg equilibrium, all provided evidence for the lack of frequent sexual reproduction in both populations. The data strongly suggest that both the South African and Australian P. cinnamomi populations are introduced.     

Evidence on the response of Patagonian forage grasses to the mulching effect of recent tephra deposits in Argentina

In Patagonia, arid and semiarid lands are being affected both by inappropriate management practices, which are leading to degradation, and by volcanic activity, whose effects are still unclear. This study aimed to test whether superficial deposition of volcanic tephra could benefit two of the most prominent Patagonian forage grass species (Poa ligularis and Pappostipa speciosa var. speciosa). Pots with P. ligularis and P. speciosa were kept under wet (W) and dry (D) conditions in the presence (T+) or absence (T?) of tephra for 105 days, and then were all well-watered. We determined the effects of tephra on soil water retention and conservation, soil moisture content (% v/v), plant growth, stomatal conductance (gs), and gs recovery capacity. The water regime significantly affected both species performance and gs. The presence of tephra increased soil water conservation, soil moisture content in wet conditions, and P. ligularis gs in wet conditions, and decreased senescence in dry conditions (9% in P. ligularis and 16% in P. speciosa). The presence of tephra allowed roots to grow in 8/10 and 2/10 pots in W conditions for P. ligularis and P. speciosa, respectively, and in only 1/10 pots in D conditions, only for P. ligularis. Tephra was also associated with gs recovery after dry conditions. Poa ligularis was more positively affected by tephra than P. speciosa, probably because P. ligularis has higher phenotypic plasticity. The positive effects of tephra may increase the resilience and resistance of P. speciosa and P. ligularis to periods of water shortage.     

用双涌源能量守恒模型计算浑善达克沙地羊草群落蒸腾量的研究

文章利用浑善达克沙地南沙梁试验站两年的羊草群落气孔导度及相关的环境因子观测数据,用多元逐步回归法建立了羊草群落单一牧草与环境因子间响应关系模型,并用此模型计算出单一牧草的冠层导度。羊草群落冠层的导度用叶宽、株高加权单一牧草气孔导度获得。利用羊草群落冠层阻力及其他阻力按S-W能量守恒模型计算了两个不同水文年的羊草群落蒸腾量。2004年是丰水年(生育期雨量R=274.6mm;P=5%),计算的羊草群落的蒸腾量为430.1mm,实测土壤蒸发169.0mm;2005年(生育期雨量R=178.8mm;P=50%)计算的羊草群落的蒸腾量为378.4mm,实测土壤蒸发124.2mm。     

气象因子对黄土高原间套作豌豆和春小麦叶片水势的影响

在甘肃省农业科学院会宁农业试验站,对春小麦和豌豆间套作模式下,不同气象因子对春小麦和豌豆叶水势的影响进行了田间对比观测。结果表明:在半干旱地区,春小麦和豌豆的叶水势与气象因子的关系在不同生育期的表达式不同,与大气温度呈线性关系,与太阳辐射、大气相对湿度、大气水势均呈二次方程模型,与综合气象因子也呈良好的线性关系。不同气象因子对作物叶水势影响的通径分析表明,影响春小麦叶水势日变化作用最强的气象因子是大气水势,其次是大气相对湿度、大气温度和太阳辐射。大气相对湿度、大气温度和太阳辐射对春小麦叶水势日变化的直接影响小于它们通过大气水势的间接影响。对豌豆叶水势日变化直接影响最大的气象因子是大气温度,其次是大气水势、太阳辐射和大气相对湿度。大气水势、太阳辐射和大气相对湿度的直接通径系数均小于各自通过大气温度的间接通径系数,这三者对豌豆叶水势日变化的直接影响小于它们通过大气温度的间接影响。     

Annual and herbaceous perennial native Australian plant species are symptomless hosts of Phytophthora cinnamomi in the Eucalyptus marginata (jarrah) forest of Western Australia

Resistant annual and herbaceous perennial plant species were identified as key hosts which allow Phytophthora cinnamomi to persist on severely impacted black gravel sites within the Eucalyptus marginata (jarrah) forest of southwest Western Australia. Of the annual and herbaceous perennial plant species present on black gravel sites, 15 out of 19 species were found to be hosts of P. cinnamomi, and 10 of these were symptomless hosts. In particular, the native annual Trachymene pilosa and the two native herbaceous perennials Stylidium diuroides and Chamaescilla corymbosa were commonly found to be hosts of the pathogen. Species from 12 new genera including three from new families (Crassulaceae, Droseraceae and Primulaceae) are reported for the first time to be hosts of P. cinnamomi. The species from which P. cinnamomi was recovered were the native species: Chamaescilla corymbosa, Crassula closiana, Drosera erythrorhiza, Hydrocotyle callicarpa, Levenhookia pusilla, Paracaleana nigrita, Podotheca angustifolia, Pterochaeta paniculata, Rytidosperma caespitosum, Siloxerus multiflorus, Stylidium diuroides and Trachymene pilosa, and the introduced annual weeds Hypochaeris glabra, Lysimachia arvensis and Pentameris airoides.     

Metabarcoding and development of new real‐time specific assays reveal Phytophthora species diversity in holm oak forests in eastern Spain

The evergreen holm oaks (Quercus ilex subsp. ilex and Q. ilex subsp. ballota) are the most representative tree species in the Iberian peninsula and the main tree species in oak‐rangeland ecosystems (dehesas). Oak decline in western, central and southern parts of Spain has been associated with root rot caused by Phytophthora cinnamomi for decades. However, Phytophthora species such as P.  quercina and P. psychrophila have recently been found associated with Quercus decline in eastern Spain where calcareous soils are predominant. Soil and root samples from two Quercus forests presenting decline symptoms in two different geographical areas in eastern Spain (Carrascar de la Font Roja and Vallivana) were analysed by amplicon pyrosequencing. Metabarcoding analysis showed Phytophthora species diversity, and revealed that an uncultured Phytophthora taxon, named provisionally Phytophthora taxon ballota, was the predominant species in both areas. In addition, a real‐time PCR assay, based on the pyrosequencing results, was developed for the detection of this uncultured Phytophthora taxon, and also for the detection of P. quercina. TaqMan assays were tested on soil and root samples, and on Phytophthora pure cultures. The new assays showed high specificity and were consistent with metabarcoding results. A new real‐time PCR protocol is proposed to evaluate the implication of different Phytophthora spp. in oak decline in eastern Spain.