Evaluation of Components of Fusarium Head Blight Resistance in Soft Red Winter Wheat Germ Plasm Using a Detached Leaf Assay.

A large environmental influence on phenotypic estimates of disease resistance and the complex polygenic nature of Fusarium head blight (FHB) resistance in wheat (Triticum aestivum) are impediments to developing resistant cultivars. The objective of this research was to investigate the utility of a detached leaf assay, inoculated using inoculum from isolates of Microdochium nivale var. majus, to identify components of FHB resistance among 30 entries of U.S. soft red winter wheat in the 2002 Uniform Southern FHB Nursery (USFHBN). Whole plant FHB resistance of the USFHBN entries was evaluated in replicated, mist-irrigated field trials at 10 locations in eight states during the 2001-2002 season. Incubation period (days from inoculation to the first appearance of a dull gray-green water-soaked lesion) was the only detached leaf variable significantly correlated across all FHB resistance parameters accounting for 45% of the variation in FHB incidence, 27% of FHB severity, 30% of Fusarium damaged kernels, and 26% of the variation in grain deoxynivalenol (DON) concentration. The results for incubation period contrasted with previous studies of moderately resistant European cultivars, in that longer incubation period was correlated with greater FHB susceptibility, but agreed with previous findings for the Chinese cultivar Sumai 3 and CIMMYT germ plasm containing diverse sources of FHB resistance. The results support the view that the detached leaf assay method has potential for use to distinguish between specific sources of FHB resistance when combined with data on FHB reaction and pedigree information. For example, entry 28, a di-haploid line from the cross between the moderately resistant U.S. cultivar Roane and the resistant Chinese line W14, exhibited detached leaf parameters that suggested a combination of both sources of FHB resistance. The USFHBN represents the combination of adapted and exotic germ plasm, but four moderately resistant U.S. commercial cultivars (Roane, McCormick, NC-Neuse, and Pat) had long incubation and latent periods and short lesion lengths in the detached leaf assay as observed in moderately FHB resistant European cultivars. The dichotomy in the relationship between incubation period and FHB resistance indicates that this may need to be considered to effectively combine exotic and existing/adapted sources of FHB resistance.

Effect of Inoculation with Selected Isolates of Stagonospora nodorum on Field Evaluations of Host Resistance in Winter Wheat.

Although Stagonospora nodorum blotch occurs annually in North Carolina, selection for resistance in wheat (Triticum aestivum) breeding nurseries is hampered by the infrequent occurrence of heavy and timely disease pressure. The objective of this study was to compare estimates of host resistance in a population of 147 random winter wheat lines evaluated in epidemics produced by natural infection versus epidemics supplemented by inoculation with selected isolates. Two isolates were chosen from a set of 43 collected in North Carolina based on their aggressiveness on four wheat cultivars in a controlled environment test. Field experiments utilized a split-plot design with three replications. The main plots were inoculation treatments and the subplots were the 147 wheat genotypes. The inoculation treatments were (i) selected isolate A (more aggressive) alone, (ii) selected isolate B (less aggressive) alone, (iii) a combination of isolates A plus B, and (iv) natural infection. Selected isolate treatments were applied at Feekes growth stage 9 to 10.1, and disease intensity was measured two or three times at 14-day intervals postinoculation. The study was conducted at one location in the 1996-97 season and two locations in the 1997-98 season. High levels of natural infection occurred, and no differences were observed among the four inoculation treatments for mean disease intensity in any of the three environments. Within environments, genotype-by-inoculation treatment variance was significant in the two environments inoculated with selected isolates at growth stage 9 but not in the environment inoculated at growth stage 10.1. Magnitudes of genetic variation and heritability for Stagonospora nodorum blotch resistance were not consistently associated with main plot treatments, and inoculation with selected isolates masked genetic variation for resistance in two treatments in one environment. Genotype rank correlations for Stagonospora nodorum blotch resistance between inoculation treatments varied from zero to 0.69 within environments, but only a single correlation between inoculation treatments in different environments was observed. Estimates of host resistance in epidemics supplemented with selected isolates did not consistently agree with estimates in epidemics produced by natural infection. Our results did not support the routine use of supplemental inoculation of wheat breeding nurseries with selected isolates of S. nodorum as a means of increasing genetic gain for host resistance.

Clustering of Environments of Southern Soft Red Winter Wheat Region for Milling and Baking Quality Attributes.

Division of regional nursery test sites into homogenous subregions contributes to more efficient evaluation and better differentiation of cultivars. Data from the Uniform Southern Soft Red Winter Wheat Nursery (USSRWWN) were analyzed to group testing sites into relatively homogenous subregions for milling and baking quality (MBQ) attributes. Environmental effects due to years accounted for over 50% of the total variation for protein content (P) and 42% for alkaline water retention capacity (AWRC). Genotype effect accounted for 63% of the total variation for softness equivalence (SE), and 37% for flour yield (FLY). A significant genotype x location (GxL) interaction occurred for FLY and P. However, the GxL variance component accounted for a small proportion of the total phenotypic variance, suggesting that clustering would be more beneficial for resource efficiency than for increasing differentiation of genotypes. A hierarchical cluster analysis was used to group locations on the basis of GxL interaction effects for FLY, P, AWRC, and SE. Cluster analysis divided the USSRWWN into two main subregions within which the GxL interaction was reduced by over 90% for FLY and by 60% for P. Although this classification is not entirely consistent with the geographic distribution of locations, clusters do follow general geographic-climatic-disease regions. Our results suggest that the USSR-WWN can be divided into subregions to reduce the resources expended on evaluation of MBQ attributes. This classification of locations could be useful in breeding for specific adaptability within subregions.

Wheat vegetative nitrogen compositional changes in response to reduced reproductive sink strength.

N redistribution patterns and the N composition of vegetative tissues above the peduncle node of wheat (Triticum aestivum L.) plants with altered reproductive sink strength were evaluated to determine the role of vegetative storage proteins in the temporary storage of excess N destined for export. The degree of leaf senescence symptoms (loss of chlorophyll, total N, and ribulose-1,5-bisphosphate carboxylase/oxygenase) were initially reduced, but the complete senescence of vegetative tissues proceeded even for plants completely lacking reproductive sinks. Plants with 50% less sink strength than control plants with intact spikes redistributed vegetative N to the spike almost as effectively as the control plants. Plants without reproductive sinks exported less N from the flag leaf and had flag leaf blades and peduncle tissues with higher soluble protein and alpha-NH(2) amino acid levels than control plants. An abundant accumulation of polypeptides in the soluble protein profiles of vegetative tissues was not evident in plants with reduced sink strength. Storage of amino acids apparently accommodates any excess N accumulated by vegetative tissues during tissue reproductive growth. Any significant role of vegetative storage proteins in the N economy of wheat is unlikely.

Main stem sink manipulation in wheat : effects on nitrogen allocation to tillers.

The role of main stem (MS) sink size on N use by field-grown soft red winter wheat (Triticum aestivum L. cv Hart) was determined. At Feeke's growth stage 8 (last leaf just visible), 100 micromoles of 99 atom% (15)N-ammonium was injected into the lower MS. At anthesis, MS sink size was adjusted by removal of 0, 33, 66, and 100% of the MS spikelets; tiller spikes were left intact. The MS and tiller average kernel size was unaffected by MS sink manipulations. The MS kernel N concentration increased when MS spikelets were removed. Tiller kernel N concentrations were unaffected except when the entire MS reproductive sink was removed, which caused an increase in tiller kernel N concentration. Net losses of MS vegetative N during grain fill were similar for all treatments except for plants lacking MS spikelets, which mobilized 30% less N from the MS. Labeled N was predominately (>90%) associated with the insoluble reduced N fraction of plant tissues at anthesis. Allocation of labeled N to tillers was not proportional to reduction in MS sink size. These results indicate that the reproductive sink on an individual culm has first priority for vegetative N mobilized during grain fill even when sink demand is reduced substantially.

Postanthesis nitrate assimilation in winter wheat : in situ flag leaf reduction.

When adequate levels of soil NO(3) (-) are available, concurrent NO(3) (-) absorption and assimilation, and mobilization of vegetative N reserves accumulated prior to anthesis, may be used to supply N to developing wheat (Triticum aestivum L.) kernels. Vegetative wheat components (stems, leaves, spike) are known to possess NO(3) (-) reductase activity, but the in situ utilization of NO(3) (-) translocated to the shoot has not been studied. Assimilation and partitioning of (15)N was determined in winter wheat ;Doublecrop.' At 7 days after anthesis, the stem immediately above the peduncle node was heat girdled to block phloem export from the flag leaf. Control plants were not girdled. One day later, 50 micromoles of (15)NO(3) (-) (98 atom percent (15)N) was injected into the penultimate internodal lacuna, after which (15)NO(3) (-) utilization was determined sequentially over a 5 day period. Based on differences in spike accumulation of reduced (15)N excess between treatments and the amount of reduced (15)N excess remaining in the flag leaf, it was estimated that the flag leaf contributed 37% of the total reduced (15)N excess in the injected shoot. The lower shoot contribution was 18% and that of the peduncle plus spike was 45%.

Variation in nitrogen use efficiency among soft red winter wheat genotypes.

Nitrogen use efficiency (NUE), defined as grain dry weight or grain nitrogen as a function of N supply, was evaluated in 25 soft red winter wheat genotypes for two years at one location. Significant genotypic variation was observed for NUE, nitrogen harvest index, and grain yield. Genotype x environment interaction for these traits was not significant. Several variables including N uptake efficiency (total plant N as a function of N supply), grain harvest index, and N concentration at maturity were evaluated for their role in determining differences in NUE. Nitrogen uptake efficiency accounted for 54% of the genotypic variation in NUE for yield and 72% of the genotypic variation in NUE for protein. A path coefficient analysis revealed that the direct effect of uptake efficiency on NUE was high relative to indirect effects.