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Materials and methods

Various accessions of four wild Triticum and nine Aegilops species from the germplasm collection maintained at the Punjab Agricultural University, Ludhiana were used for this study. Four to ten accessions each of four wild Triticum and six Aegilops species were tested for seedling reaction to five to six Indian isolates of leaf rust (Puccinia recondita). The eight Indian leaf rust isolates used in this study were selected on the basis of their avirulence/virulence on the leaf rust resistance (Lr) genes so as to have large diversity for pathogenicity among them (Nayar et al. 1997). The standard procedure for inoculation of seedlings (Nayar et al. 1997) was followed and the seedling reactions were recorded two weeks after inoculation according to the scale developed by Mains and Jackson (1926). The reactions 0; 0 and 2 were classified as resistant whereas reactions 3 and 4 were categorized as susceptible. Similarly, two to twelve accessions each of three wild Triticum and nine Aegilops species were tested with three diverse isolates of stripe rust (P. stiriiformis) by following the standard inoculation procedure (Nayar et al. 1997). The resistant and susceptible categories were made the same way as for the leaf rust.

Thirty one intraspecific crosses were made among different accessions of each of seven Aegilops and three wild Triticum species. Forty to ninety F2 seedlings of each of 30 intraspecific crosses were tested with an individual isolate of leaf rust. In three of these crosses, the tests were made with two or three individual rust isolates. The chi-square test was used to assess the goodness of fit to the expected ratios of resistant and susceptible F2 segregants. In the case of intraspecific crosses of T. urartu, where one parent exhibited an intermediate reaction (i.e. 2+ to 3- or X) and the other parent was resistant, three categories (resistant, intermediate and susceptible) were made to test the goodness of fit to the expected ratio. For genetic analysis of resistance to stripe rust, F2 seedlings of another intraspecific cross between two accessions of T. dicoccoides, exhibiting intermediate (Acc 4667) and resistant (Acc 13985) reactions to isolate N of stripe rust, were tested with this isolate.


Results and discussion

Seedling tests of different accessions of wild Triticum and Aegilops species with individual P. recondita pathotypes showed large intraspecific diversity for rust resistance. Seven accessions of T. boeoticum (Ab) showed seven different reaction patterns to six pathotypes (Table 1). Similarly, there was large variability for reaction patterns among small number of accessions of Ae. longissima (Sl) and Ae. squarrosa (D). However, only two distinct patterns were observed among ten accessions of Ae. speltoides (S). Variability in reaction pattern was also high among tetraploid species (Table 2). Six accessions of T. dicoccoides (AB) showed six different reaction patterns. Five accessions of T. araraticum (AG) had at least four different reaction patterns. Eight lines of Ae. ovata (UMo) had four reaction patterns. Testing of diploid (Table 3) and tetraploid (Table 4) wild wheats and Aegilops species with individual pathotypes of P. striiformis also revealed significant intraspecific diversity for seedling response.

The study of segregation for reaction to individual pathotypes of P. recondita in the F2 generations of 30 intraspecific crosses further supported the existence of significant intraspecific diversity in the diploid and tetraploid species (Table 5). Segregation for rust resistance was observed in the F2 of all the crosses among five accessions of T. urartu (A) tested with P. recondita pathotype 77A. Similarly, of nine crosses among five accessions of Ae. triuncialis (UC) having resistant reactions to pathotype 77-2, eight crosses segregated in 15 resistant : 1 susceptible ratio. This suggested at least four different dominant genes for resistance to leaf rust among the five accessions. Therefore, this species could be a large reservoir of leaf rust resistance genes. Furthermore, segregation in limited number of crosses between different resistant accessions of Ae. longissima (Sl), Ae. triaristata (UMt) and Ae. ovata (UMo) supported the prevalence of considerable intraspecific diversity within wild Aegilops species. However, no susceptible plant was observed in F2 generation of crosses among different accessions of Ae. speltoides (S). This is in agreement with the observation that there were only two reaction patterns among the accessions of Ae. speltoides (S). Since there are two major groups of P. recondita ('Group I' and 'Group II') based on aecial and telial host range (Anikster 1997) and one of the types ('Type C') belonging to one group ('Group II') is specific to Aegilops species having S genome, the larger resistance of Ae. speltoides (S) accessions to a number of leaf rust pathotypes from wheat may be a case of non-host resistance (Niks and Dekens 1991).

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