Relative importance of components affecting the leaf rust progress curve in wheat

Summary The relative importance of various components (latent period, uredium area, infectious period, infectibility, sporulation and weather) affecting the progress of leaf rust (Puccinia recondita) in wheat, in the field, was determined by calculating the effect of equivalent changes in the individual components. The effect was calculated in terms of the period of delay in the epidemic reaching 100 per cent severity of leaf rust. Only four components viz., latent period, infectibility, sporulation and weather were found to be important. Further, it was found that these four components were equally important and that they collectively determined the rate of progress of leaf rust in an additive manner.     

Components of Pink Snow Mould Resistance in Winter Wheat are Expressed Prior to Cold Hardening and in Detached Leaves

Resistance to pink snow mould, caused by Microdochium nivale, was investigated in four resistant winter wheat lines from the USDA World Cereal Collection (CI9342, CI14106, PI173440 and PI181268) and three Nordic wheat lines (Bjørke, Rida and V1004). Pink snow mould resistance was tested in non‐hardened and cold‐hardened plants incubated under artificial snow cover and in detached leaf segments mounted on water agar and incubated at either 3°C in darkness or at room temperature with light during the day. The wheat lines CI9342, CI14106 and PI181268 were more resistant than the Nordic lines, both before and after cold hardening. Thus, although cold hardening strongly increases the level of snow mould resistance in all the wheat lines, some resistance mechanisms are also present prior to cold hardening in some of the resistant lines. CI9342, CI14106 and PI181268 also had a higher level of resistance than the other lines in the detached leaf assay, indicating that these lines have some resistance mechanisms acting in the leaves. The resistance of PI173440 was expressed only in intact hardened plants and not in non‐hardened plants or in detached leaves. This indicates that this line relies on cold hardening‐related changes in the crown for its resistance. In the detached leaf assay the rate of lesion development varied greatly between leaves of different order. The highest correlation with the whole plant test was obtained when using secondary leaves and incubation at 3°C in the dark.     

Changes in endogenous growth regulators in nasturtium leaves during senescence

Summary By use of lettuce-hypocotyl and wheat-coleoptile bioassay, the presence of both gibberellin (GA)-like and abscisic-acid(ABA)-like components in acidic ethyl-acetate extracts of fully expanded nasturtium (Tropaeolum majus) leaves has been shown. During senescence of detached leaves there was a progressive decline in GA-like components and an increase in ABA-like components. Pretreatment of detached leaves with GA₃ or kinetin prevented changes in the levels of endogenous growth regulators and delayed senescence. The observations provide experimental verification for the concept that senescence is associated with changes in endogenous growth regulators.     

Physiological and Molecular Changes of Detached Wheat Leaves in Responding to Various Treatments

Leaf senescence is induced or accelerated when leaves are detached. However, the senescence process and expression pattern of senescence-associated genes (SAGs) when leaves are detached are not clearly understood. To detect senescence-associated physiological changes and SAG expression, wheat (Triticum aestivum L.) leaves were detached and treated with light, darkness, low temperature (4 C), jasmonic acid (JA), abscisic acid (ABA), and salicylic acid (SA). The leaf phenotypes, chlorophyll content, delayed fluorescence (DF), and expression levels of two SAGs, namely, TaSAG3 and TaSAG5, were analyzed. Under these different treatments, the detached leaves turned yellow with different patterns and varying chlorophyll content. DF significantly decreased after the dark, ABA, JA and SA treatments. TaSAG3 and TaSAG5, which are expressed in natural senescent leaves, showed different expression patterns under various treatments. However, both TaSAG3 and TaSAG5 were upregulated after leaf detachment. Our results revealed senescence-associated physiological changes and molecular differences in leaves, which induced leaf senescence during different stress treatments.     

Invasion of naturally senescing root cortices of cereal and grass seedlings byMicrodochium bolleyi

Summary In glasshouse experiments, invasion of wheat and grass (Dactylis glomerata) seedling roots byMicrodochium bolleyi was strongly correlated with the pattern and rate of natural senescence of the root cortex. The fungus did not enhance cortical senescence and did not damage roots except in a few instances when it invaded and killed their tips.M. bolleyi behaved as a weak parasite, largely restricted to invasion of naturally senescing cortices of cereal and grass roots.     

Growth and sporulation of Oidium begoniae: development of a deterministic model

Growth and sporulation of Oidium begoniae (begonia powdery mildew) on detached leaves was observed daily under a stereomicroscope. A mathematical model of growth and sporulation was constructed based on the patterns observed. The essential components of the model are three genetically fixed components of colony growth and sporulation and six components which changed with environmental and physiological conditions. The genetically fixed components were: the ultimate number of germ tubes, the hyphal growth unit (HGU or mean length supporting one branch), and the diurnal periodicity of sporulation. These components were shown to have approximately the same values at 15 and 21 °C. Components which varied with temperature were: the latent periods of germination, branching and sporulation, the time taken to reach the ultimate germ tube number, and the number of conidiophores per HGU, The derived equations were tested against observed sporulation and resulted in highly significant correlations. Similar results were obtained with published observations for Erysiphe graminis f. sp. hordei. This colony-level model can be used as units to construct models of disease progress and suggests that it is theoretically possible to model disease in the field from laboratory studies. The model can be used in computer simulations of colony growth as demonstrated by a computer program in the Appendix.     

Diversity of Physiological and Biochemical Characters of Microdochium Fungi

The biological characterization of Microdochium majus, M. nivale, and M. seminicola strains with wide geographical origins showed the diversity of their pathogenic properties and metabolite compounds, allowing them to exist in their habitats. Significant differences in the ability of Microdochium fungi to cause lesions on wheat and oat leaves were found. The intensity of symptoms depended on the species and substrate origin of the strains. On average M. seminicola strains were able to cause less leaf necrosis than M. majus and M. nivale. The volatile organic compound (VOC) profile of Microdochium fungi included 29 putative fungal metabolites. The spectrum of the identified VOCs in M. seminicola strains was much richer than that in M. majus and M. nivale strains. In addition, the strains of M. seminicola emitted at least six sesquiterpenes. Mycotoxin analysis by HPLC/MS/MS revealed that the analyzed Microdochium strains did not produce any toxic metabolites typically produced by filamentous fungi.     

The Significance of Vascular Connection in Regulating Senescence of the Detached Flag Leaf of Wheat

The relationship between the accumulation of soluble substancesand the senescence of a detached flag leaf of wheat in the lightwas investigated. With respect to transport to and from theleaf, a detached flag leaf can be considered as either an ‘open’or a ‘closed’ system. A closed system was obtainedby cutting the flag leaf at the base of the leaf sheath, thussevering the vascular connection between the leaf and the stem.An open system was prepared by excising the wheat stem, belowthe flag leaf insertion node, thereby preserving vascular connectionsbetween the flag leaf, stem and ear. By varying the number ofnodes left on the stem, or having the ear either intact or detached,soluble carbohydrates and soluble nitrogenous compounds wereinduced to accumulate within the leaf blade at different rates.Treatments which restricted transport of carbohydrate and nitrogen,out of the detached leaf were found to promote senescence. Senescencewas most rapid when the leaf system was ‘closed’and was considerably delayed when the system was ‘open’and the ear intact. The onset of senescence was closely associatedwith the attainment of a threshold concentration of ethanol-solublecarbohydrate in the leaf, while the rate of senescence was modifiedby the number of attached nodes or by exogenous cytokinin treatments. Key words: Wheat, Flag leaf, Senescence     

Analysis of Microdochium nivale isolates from wheat in the UK during 1993

A total of 144 isolates of Microdochium nivale from stem bases of winter wheat were taken from 30 sites and 91 isolates from grain were taken from seven sites throughout the UK. Identification by polymerase chain reaction (PCR) amplification of the internal transcribed spacer (ITS) region followed by restriction enzyme digestion of the PCR product revealed that 70% of stem base isolates were M. nivale var. majus and 30% var. nivale while 93% grain isolates were var. majus and 7% var. nivale. Almost all isolates were resistant to the benzimidazole fungicide benomyl. Perithecial production in vitro was more common in var. majus isolates and occurred in almost all grain isolates, but was less common in stem base isolates. The implications of the findings in terms of epidemiology and chemical control of this important cereal pathogen are discussed.     

IN VITRO DEVELOPMENT OF THE ISOLATED SHOOT APICAL MERISTEM OF ANGIOSPERMS

Development of complete plants was achieved from isolated shoot apical meristems of Nicotiana tabacum L., Daucus carota L., Nicotiana glauca Grah., Tropaeolum majus L., and Coleus blumei Benth. The explants consisted of only meristematic dome tissue with no visible leaf primordia. A simple nutrient medium composed of the Murashige and Skoog salt mixture, 100 mg/liter myo-inositol, 0.4 mg/liter thiamin-HCl, 1-2 mg/liter IAA, 30 g/liter sucrose, and 1% agar was adequate. Histologically there occurred principally tissue enlargement during the first 3-6 days, followed by appearance of bipolar organization in 6-9 days and formation of a well-defined root apex and initiation of first leaf primordium by 12 days.     

The Effects of Temperature on the Incubation and Latent Periods of Powdery Mildew (Erysiphe polygoni) on Clematis

The effects of temperature on the length of the incubation and latent periods of clematis powdery mildew, caused by Erysiphe polygoni , were studied. At constant temperatures over the range of 10–29°C, the length of the incubation period ranged from 4 to 11 days, and the length of the latent period ranged from 4 to 13 days; no visible colonies developed at 30°C after 19 days. The relationship between temperature and the rates of fungal development within the incubation and latent periods (expressed as the reciprocal of the lengths of the incubation and latent periods) under constant temperature were described well by a non-linear model. The resulting curves were not the usual form of an asymmetrically bell-shaped type; instead, they were of exponential type with the development rate increasing with increasing temperatures. The relationship between temperature and the development rate during the post-incubation (before sporulation) period was non-linear with an optimum temperature of approximately 20°C.     

High levels of auto-infection in plant pathogens favour short latent periods: a theoretical approach

Auto-infection (infection arising from inoculum produced on the same host unit) is common in polycyclic plant pathogens, but often neglected in experimental and theoretical studies, which focus instead on infection of new hosts (allo-infection). Here we test the hypothesis that high auto-infection, as observed for leaf infecting fungal pathogens, could select for short latent periods. An individual-based simulation model keeps track of lesions, resulting from the spread of spores, between and within individual leaves. Linked to a trade-off between latent period and spore production capacity, as observed for Puccinia triticina on wheat, the adaptation of the latent period is analysed for different levels of auto-infection using the methods of pairwise invasibility plots. Results suggest that increased auto-infection selects for reduced latent periods. A reduction in leaf longevity also selects for reduced latent periods, which is most obvious for a relatively low ratio of auto- to allo-infection. This study is the first to consider the effect of auto-infection on the evolution of pathogen life history traits. The fact that auto-infection could drastically reduce pathogen latent periods highlights the need for more research in this area.     

Leaf appearance, tillering and their coordination in response to NxP fertilization in barley

The aim of this work was to study the effects of nitrogen (N) and phosphorus (P) deficiencies and their interaction on the appearance of leaves and tillers of barley (Hordeum vulgare spp. distichum L.) by analyzing the rate and duration of the appearance period. Three microcrops experiments were carried out in 200 L containers using malting barley cv. Quilmes Palomar. Treatments were a factorial combination of two levels on N and two or three levels of P fertilization. Both N and P deficiencies delayed leaf and tiller appearance rates but leaf appearance was less susceptible to nutrient deficiencies than tiller appearance. P deficiency effects on leaf emergence differed from N effects. While N deficiency diminished the rate of leaf appearance (RLA), it has no significant effect on the duration of leaf appearance or on the duration of the period from emergence to flowering. On the other hand, P deficiency diminished RLA, and increased both the duration of leaf appearance and the duration of the period from emergence to flowering.     

The effects of light,cycloheximide and incubation medium on the senescence of detached seagrass leaves

The senescence of detached leaves of the Australian seagrassZostera muelleri Irmisch ex Aschers was associated with decreased levels of chlorophyll, soluble protein, and alkaline pyrophosphatase and catalase activities. Senescence was faster in artificial seawater than in distilled water. Irradiance has a retarding effect on senescence while cycloheximide did not retard senescence appreciably. The action of cycloheximide was studied usingCyperus rotundus L.,Hydrangea macrophylla (Thunb.) Ser.,Solandra maxima (Sesśe & Moc.) P. S. Green, andDatura suaveolens Humb. & Bonpl., and the cycloheximide sensitivity correlated with low chlorophyllase activity of the leaf. An erratum to this article is available at .     

1H NMR-based metabolite profiling as a potential selection tool for breeding passive resistance against Fusarium head blight (FHB) in wheat

¹H NMR measurements on protein-free extracts from wheat leaf and stem tissue were used to investigate the biochemical correlates of partial resistance to fungal species implicated in the Fusarium head blight (FHB) disease complex. The wheat genotypes included five commercial wheat cultivars and 116 wheat genotypes, from the CIMMYT international FHB breeding programme in Mexico, that had been bred for FHB disease resistance, utilizing exotic, typically Chinese, resistance sources. Principal component analysis of the nuclear magentic resonance (NMR)-derived metabolite profiles revealed distinct separation of the commercial wheat cultivars from the majority of the CIMMYT wheat genotypes with the commercial cultivars exhibiting markedly higher carbohydrate concentrations. A cross-validated partial least squares (PLS) regression model of the metabolite profile against the partial disease resistance component latent period (delay in sporulation of the fungus on the plant tissue) predicted latent periods that were significantly correlated with the experimentally determined values ( R ² = 0.34, P  < 0.001). Identified metabolites that were found in plants with shorter latent periods (higher disease susceptibility) included choline, the single most influential metabolite in the PLS model, betaine, the amino acids glutamine, glutamate and alanine, trans -aconitate and sucrose. Metabolites related to increased disease resistance included glucose and unassigned resonances in the carbohydrate and aromatic regions of the NMR spectra. The current study has demonstrated the potential of metabolite profiling as a tool for marker-assisted selection in commercial breeding for resistance to FHB in wheat.     

Senescence and protein degradation in leaf segments of young winter wheat: influence of leaf age

Leaf senescence in intact wheat plants can be strongly influencedby altered source/sink relations. Interactions with other plantparts are no longer possible in detached leaves and thereforedifferences in their senescence behaviour reflect the physiologicalstatus of the leaf before cutting. The net degradation of chlorophyllsand of selected enzyme proteins (detected by SDS-PAGE and immunoblotting)was delayed in detached young leaves as compared to senescingor mature leaves excised from the same field-grown wheat plants.The physiological leaf age was therefore decisive for the velocityof artificial senescence. Net degradation rates of the enzymesinvestigated varied in detached leaves. The protein quantitiesof plastidial glutamine synthetase, phosphoribulokinase andphosphoglycolate phosphatase decreased more rapidly than thoseof ferredoxin-dependent glutamate synthase and nitrite reductase.Differences were also detected between two enzymes involvedin the same metabolic pathway (photorespiratory carbon cycle)but located in different subcellular compartments: the plastidialenzyme phosphoglycolate phosphatase was lost more rapidly thanglycolate oxidase (peroxisomal enzyme). Key words: Detached leaves, senescence, proteolysis, leaf age, wheat     

晚播小麦叶片衰老代谢和粒重变化的比较研究

对７个小麦品种在晚播条件下的叶片衰老生理特性和粒重变化进行了比较研究。根据小麦叶片的衰老特征相差差异,将７个小麦品种区分为３个类型;后健型、早衰型和中间型。在小麦旗叶的衰老过程中,后健型小麦品种旗叶叶绿素和类胡萝卜素含量显著高于早衰型,脂质过氧化产物ＭＤＡ含量显著低于早衰型,小麦粒重降幅依次为早衰型〉中间型〉后健型,并讨论了活性氧代谢在小麦叶片衰老过程中可能作用。     

Peroxisomal enzyme activities in attached senescing leaves

Recently it has been demonstrated that detached leaves show glyoxysomal enzyme activities when incubated in darkness for several days. In this report glyoxylate-cycle enzymes have been detected in leaves of rice (Oryza sativa L.) and wheat (Triticum durum L.) from either naturally senescing or dark-treated plants. Isolated peroxisomes of rice and wheat show isocitrate lyase (EC 4.1.3.1), malate synthase (EC 4.1.3.2) and -oxidation activities. Leaf peroxisomes from dark-induced senescing leaves show glyoxylic-acid-cycle enzyme activities two to four times higher than naturally senescing leaves. The glyoxysomal activities detected in leaf peroxisomes during natural foliar senescence may represent a reverse transition of the peroxisomes into glyoxysomes.This work was supported by CNR Italy, special grant RAISA, subproject 2, paper no. 26.     

Le Paraquat, un Outil pour la Révélation Rapide d'Infections Fongiques Latentes et de Champignons Endophytes

Paraquat: a tool for the quick development of latent fungal infections and endophytic fungi.
Detached winter wheat leaves without any symptom of fungal infection were thoroughly washed, surface desinfected, treated with paraquat (0.03–0.32% a.i.) and incubated on nutrient agar or in moist chambers under sterile conditions. Microscopic examinations showed that hypae and yeast cells emerged from several stomata within 48–72 h after treatment. Fungal colonies developed on agar within the same period of time. More than 27 filamentous fungal genera and yeasts were identified during a wheat cropin healthy, green leaves. Non-treated leaves showed very few early emergences of fungi from stomata, but senescent leaves eventually yielded several fungal taxa observed after paraquat treatment. Similar observations were made on non-detached wheat leaves, and on detached sugarbeet leaves. Paraquat also induced development of Fusarium spp. from detached healthy winter wheat ears before or after heading. In grape berries, paraquat induced the development of latent infections of Botrytis cinerea , a few days after treatment.
The usefulness of paraquat for the detection of endophytes and for the diagnosis of early and latent fungal infections in plants, and the possible involvement of endophytes in tissue senescence are discussed.