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The relationship between yield loss and disease severity can differ widely between crops. This has given rise to the concept of disease tolerance, with some crops exhibiting a smaller yield loss under a given severity of disease than others. Genetic improvement to minimise yield loss under disease is an attractive goal, as it exerts little or no selection pressure on pathogen populations, and could form a useful component of durable disease management programmes. However, progress towards this end requires a thorough understanding of the phenotypic traits that influence the response of yield to disease, their genetic control and the possible trade-offs involved with other desirable agronomic characteristics. This paper examines the candidate crop traits that may confer tolerance of foliar disease in wheat and barley and reviews evidence of genetic variation in their expression. In wheat grown under the relatively low light conditions of North-West Europe, post-anthesis source (assimilate supply) and grain sink capacity (capacity for dry matter accumulation) appear to be closely balanced. Traits associated with maintaining post-anthesis radiation interception and radiation use efficiency in spite of disease may confer tolerance. The most promising traits include a larger flag leaf and compensatory increases in photosynthetic rate in non-infected parts of leaves. In barley, yield is often more strongly sink limited, and early-season disease management is required to protect the formation of potential grain sites. A wider range of potential traits may influence tolerance including compensatory adjustments in leaf growth and morphology, and differences in the sensitivity of tiller and spikelet mortality to photoassimilate supply. Different methods for quantifying tolerance are suggested depending on the trait of interest. [ABSTRACT FROM AUTHOR]

Treatment of barley ( Hordeum vulgare) with 3 mm saccharin, applied as a foliar treatment to the first leaf or as a soil drench, provided significant control of powdery mildew ( Blumeria graminis f.sp. hordei) on first and second leaves. This was unlikely to be the result of a direct effect of saccharin on the fungus, as application of the chemical to first leaves 2 h before inoculation did not affect conidial germination or formation of appressoria. Saccharin treatment had no significant effect on plant growth, except for a reduction in total leaf area in plants treated with a saccharin drench 14 days before inoculation with mildew. Phenylalanine ammonia-lyase activity was reduced significantly in second leaves 18 and 48 h after inoculation in plants treated with saccharin 14 days earlier. Peroxidase activity increased significantly in plants challenged with mildew within 6 days of saccharin application, although changes were not apparent until 48 h after pathogen challenge. On these occasions, treatment with saccharin resulted in a 33% increase in peroxidase activity compared with controls. In plants inoculated 10 or 14 days after saccharin application, cinnamyl alcohol dehydrogenase (CAD) activity increased prior to, and 18, 24 and 48 h after, inoculation of the barley plants with mildew. CAD activity increased approximately twofold compared with controls. However, in contrast to peroxidase, CAD activity was significantly higher in saccharin-treated plants prior to and after inoculation with powdery mildew, suggesting that saccharin primes CAD activity prior to pathogen challenge. [ABSTRACT FROM AUTHOR]

Abstract In the incompatible interaction between the barley cultivar Hart (mlo allele) and the powdery mildew fungus Blumeria graminis DC. Speer f. sp. hordei Marchal, free spermidine and conjugated forms of putrescine and spermidine were found to accumulate 1–3 days following inoculation. The increase in free spermidine could be accounted for by increased activities of the biosynthetic enzymes ornithine decarboxylase (EC 4.1.1.17) and S-adenosylmethionine decarboxylase (EC 4.1.1.50). Activities of the transferase enzymes putrescine hydroxycinnamoyl transferase (EC 2.3.1.-) and tyramine feruloyl-CoA transferase (EC 2.3.1.-) were also increased in leaves of Hart inoculated with powdery mildew, as were activities of the polyamine catabolic enzymes diamine oxidase (DAO, EC 1.4.3.6) and polyamine oxidase (PAO, EC 1.4.3.4). The possible role of conjugated polyamines, and increases in DAO and PAO, in penetration based resistance to powdery mildew infection is discussed. [ABSTRACT FROM AUTHOR]

Abstract Polyamine levels and activities of enzymes of polyamine biosynthesis and catabolism were examined in the barley cultivar Delibes (Ml1al + Ml(Ab) ) reacting hypersensitively to the powdery mildew fungus, Blumeria graminis f. sp. hordei (race CC220). Levels of free putrescine and spermine and of conjugated forms of putrescine, spermidine and spermine were greatly increased 1–4 d following inoculation of barley with the powdery mildew. These changes in polyamine levels were accompanied by elevated activities of the polyamine biosynthetic enzymes ornithine decarboxylase (ODC), arginine decarboxylase (ADC) and S -adenosylmethionine decarboxylase (AdoMetDC) and the polyamine catabolic enzymes diamine oxidase (DAO) and polyamine oxidase (PAO). Activities of two enzymes involved in conjugating polyamines to hydroxycinnamic acids, putrescine hydroxycinnamoyl transferase (PHT) and tyramine feruloyl-CoA transferase (TFT) were also examined and were found to increase significantly 1–4 d after inoculation. The possibility that the increased levels of free spermine, increased polyamine conjugates, and increased DAO and PAO activities are involved in development of the hypersensitive response of Delibes to powdery mildew infection is discussed. [ABSTRACT FROM AUTHOR]

Effects were examined of barley powdery mildew (Erysiphe graminis f. sp. bordei) the major leaf protein, ribulose-1,5-bisphosphate carboxylase (RuBPCase), and other enzymes of CO2 assimilation, phosphoenolpyruvate carboxylase (PEPcase), and malic enzymes, and enzymes associated with RuBPCase in the reductive pentose phosphate pathway. Activity of RuBPCase per unit fresh weight of leaf was inhibited by infection from the first sample, 3 days after inoculation, to the last sample, 24 days after inoculation, when healthy control leaves were visibly senescing. The inhibition occurred because the amount of RuBPCase protein (measured specifically by an immunological technique) was reduced from 6 days after inoculation, at which time there was little protein remaining. Activity of PEPcase per unit fresh weight of leaf was initially stimulated by infection, but, thereafter, it was inhibited. Inhibition, like that also affecting malic enzymes (NAD) and (NADP), 3-phosphglycerate kinase, and glyceraldehydes-3-phosphate dehyrogenases (NAD) and (NADP), was associated with a decline in amounts of total soluble minus RuBPCase protein per unit fresh weight of leaf. Reduced amounts of leaf protein may be associated with reduced nitrate uptake by roots and fungal demand for nitrogen in mildewed plants. [ABSTRACT FROM AUTHOR]

Polyamine metabolism, lipoxygenase and catalase activities and ethylene production were examined in green-islands induced to form on detached barley leaves infected with the powdery mildew fungus, Erysiphe graminis f. sp. hordei Marchal. Several-fold increases in free polyamines were detected in green-islands, which could not be accounted for by changes in polyamine biosynthetic and degradative enzymes: a small increase in ornithine decarboxylase (ODC) activity, greatly decreased arginine decarboxylase (ADC) activity, virtually unchanged activity of S-adenosylmethionine decarboxylase (AdoMetDC) and greatly increased polyamine oxidase (PAO) activity. Changes in conjugated forms of polyamines suggest that elevated free polyamine concentrations might result from reductions in this pool. Green-island tissues displayed a reduced activity of lipoxygenase, unchanged catalase activity and a greatly lowered rate of ethylene production. Chlorophyll concentration and rates of photosynthesis and dark respiration were slightly increased in these tissues. In senescing regions of infected leaves, there was a small increase in free putrescine and spermine and a decrease in spermidine concentration, while concentrations of conjugated polyamines were substantially reduced. This was accompanied by an overall decrease in ODC and ADC activities and an increased activity of both AdoMetDC and PAO. These regions exhibited lowered lipoxygenase activity, greatly enhanced catalase activity and reduced rates of ethylene production. In addition, senescing regions exhibited a substantial reduction in chlorophyll concentration and the rate of photosynthesis, although dark respiration was similar to control values. These results support the view that green-islands represent areas of the infected leaf where senescence is retarded, biosynthetic activity is retained and in which polyamines may play an important role. [ABSTRACT FROM AUTHOR]

Uptake of the polyamine biosynthesis inhibitor, α-difluoromethylornithine (DFMO), by barley seedlings was examined. DFMO was taken up by intact roots and approached saturation at a rate of 780 mol m-12 g-1 d.wt. The uptake as a function of DFMO concentration in the external medium was biphasic, with Eadie-Hofstee transformations giving Km values for system 1 uptake of 1.61 mol m-3 and for system 2 uptake of 53.31 mol m-3. DFMO uptake was reduced in darkness and was greatly influenced by transpiration rate. The inhibitor was taken up from host cells by the powdery mildew fungus, Erysiphe graminis f.sp. hordei Marchal, and DFMO fed to roots, or to excised leaves at different times after inoculation, gave substantial control of mildew infection. [ABSTRACT FROM AUTHOR]

The effects of the inhibitors of polyamine biosynthesis, 2-difluoromethylornithine (DFMO), (E)-2-(fluoromethyl) dehydroornithine (Δ-MFMO), (E)-2-(fluoromethyl) dehydroornithine methyl ester (Δ-MFMO CH3), 2-hydrazinoornithine, (2R,5R)-6-heptyne-2,5-diamine (RR-MAP), 2-difluoromethylarginine (DFMA), cyclohexylamine (CHA) and methylglyoxal-bis (guanylhydrazone) (MGBG) on infection of barley leaves (Hordeum vulgare L.) by Erysiphe graminis f.sp. hordei Marchal, were examined. Various concentrations of the inhibitors were sprayed onto barley leaves cv. Golden Promise as post-inoculation treatments. DFMO, MGBG and a combination of the two were also applied as pre-inoculation treatments. Each inhibitor substantially reduced mildew infection. DFMO was as efficient as the more recently developed ornithine analogues. With the exception of the DFMA treatments, post-inoculation sprays were more effective than pre-inoculation ones. When DFMO, MGBG and DFMA were sprayed onto leaves at different times, DFMO and MGBG most effectively controlled mildew when sprayed on the third day after inoculation. DFMA was more efficient as a pre-inoculation treatment. Addition of polyamines to DFMO sprays increased mildew infection, compared to that resulting from the DFMO treatment alone, but infection was less than in the controls. [ABSTRACT FROM AUTHOR]

The concentrations of three amines (putrescine, spermidine and spermine) were much increased in first leaves of barley infected with powdery mildew, whilst the concentrations of putrescine and spermidine in roots of mildewed barley were significantly decreased. Although no significant change in the activities of three polyamine biosynthetic enzymes (ADC, arginine decarboxylase; ODC, ornithine decarboxylase; and SAMDC, S-adenosyl methionine decarboxylase) could be detected in roots of mildewed barley, the activities of ADC, ODC and SAMDC were increased in infected first leaves. The significance of these alterations in polyamines and their biosynthetic enzymes in mildewed barley is discussed. [ABSTRACT FROM AUTHOR]

First leaves of barley seedlings grown in solution culture were inoculated with powdery mildew when the second leaf was fully emerged. 14C-labelled carbon dioxide was fed to either first or second leaves of infected and non-infected plants. Translocation of labelled photoassimilates into roots, and partitioning into soluble, storage and structural fractions were studied in tip, mid and basal regions of primary roots 24 h after feeding. Mildew reduced the total activity in the plant, but had little effect on assimilate distribution since the percentage activity translocated to roots was only transiently reduced, approximately 7 days after infection. Reduced import led to a reduction in the specific activity of different fractions within roots, reductions being greatest in root tips. These changes were similar whichever leaf was fed. Changes in specific activity became progressively more pronounced in the 10 days following infection, and were paralleled by a reduction in the mitotic index of root tips. It is concluded that meristematic activity, and thus the growth, of primary roots of barley is particularly sensitive to reductions in photoassimilation caused by powdery mildew infection. [ABSTRACT FROM AUTHOR]

Abstract This study aimed at examining the effects of the saturated fatty acid lauric acid on mycelial growth of Rhizoctonia solani and Pythium ultimum and on infection of barley seedlings with Blumeria graminis f. sp. hordei . Mycelial growth of R. solani and P. ultimum in agar culture was significantly reduced by lauric acid at concentrations of 100 μm and above, while no fungal growth occurred in liquid culture at concentrations above 50 μm. Application of lauric acid at concentrations ranging from 250 to 1000 μm to barley leaves before or after inoculation with B. graminis f. sp. hordei led to significant reductions in infection. This study provides the first report of the activity of lauric acid against plant pathogenic fungi and indicates the need for investigation of its mechanism of action. [ABSTRACT FROM AUTHOR]