Abstract: Annual wild grass species of wheat i.e. Aegilops tauschii, Ae. geniculata, Ae. neglecta, Ae. variables, Ae. speltoids and Ae. triunciallis were screened against leaf rust Puccinia recondita Roberge ex Desmaz.f.sp. tritici (Eriks. &E.henn.) D.M. Henderson . The frequency of immune and resistant accessions was fairly high in Ae. geniculata, Ae. variables and Ae. tauschii while low in the accessions of Ae. neglecta. No accession of Ae. geniculata were found to be susceptible. All the tested accessions of Ae. variables, Ae. speltoides and Ae. triunciallis were immune to the prevalent races of leaf rust in this region.
Introduction
Of the approximately 325 wild species in the tribe Triticeae, about 250 are perennial and 75 are annuals (Dewey, 1984). The annual species, which includes many forage grasses, have the potential to serve as vital genetic reservoir for the improvement of the wheat. Leaf rust caused by Puccinia recondita Roberge ex Desmaz.f.sp. tritici (Eriks. & E.henn.) D.M. Henderson, is one of the major constrain in the wheat production of Pakistan and all over the world. Its importance varies with the cultivars, regions, seasons, biotic and abiotic stresses. Leaf rust disease sometimes becomes epidemic over a wide area when favorable environmental conditions, such as high moisture and relatively high temperature are present.
Wild relatives of wheat with its diverse range of accessions and distribution provides a unique opportunity for exploiting novel genetic variability for wheat improvement associated with biotic/abiotic stress factors. Varying number of accessions of Aegilops species were evaluated against prevalent races of yellow, brown rust and Karnal bunt and identified excellent sources of resistance (Khem et al., 1987).
Most of the annual and perennial species have been screened against salinity (Farooq et al., 1988, 1989) and the identified species are now being used as donor of stress tolerance genes to cultivated wheat varieties (Farooq et al., 1994, 1995). In the present study, some of the results obtained on the screening of annual species against rust resistance are being presented.
Materials and Methods
Material used in this study consisted of different accessions of annual Aegilops species (Table 1). Seeds of Aegilops species were obtained from Plant Breeding Institute (PBI) Cambridge U.K., ICARDA Syria and USDA , ARS, USA . All the species are being maintained under natural environmental conditions in the net house at NIAB Faisalabad, Pakistan and shifted in the month of January 1994 for the screening to the Leaf Rust Screening nursery of Wheat Research Institute, Ayub Agriculture Research Institute Faisalabad, Pakistan.
Maintenance of the species: Seeds of each accession/species were dusted with fungicide (Vitavax Ciba-Giegy, Switzerland) and placed in a petri-plate lined with moist filter paper in the mid of October. After 24 hours at room temperature (24±2°C), the seeds were given cold shock for one week in a refrigerator (4°C) to break the dormancy, and then placed at room temperature. Germinated seeds were transplanted in plastic pots (10 cm diameter) containing sterilized mixture of sand and soil (1:1) and transferred in a net house under natural conditions. The annual grasses were harvested at the end of April and seeds were stored in the refrigerator at 4°C to maintain the viability of seeds.
Evaluation for leaf rust resistance: The seedlings of 59 accessions of Ae. tauschii, 17 of Ae. geniculata, 33 of Ae. neglecta, two of Ae. speltoides and one each of Ae. variables and Ae .triunciallis were grown in plastic pots (10 cm diameter). The pots were surrounded by highly susceptible wheat varieties (Balochi Local White, Morocco, Yecora, SA-42) which acted as spreader for the disease. Two pots of each accession with three seedlings per pot were artificially inoculated at the end of January with freshly collected uredospores of the Puccinia recondita tritici and also against prevalent leaf rust races present in this region. The data on rust severity were recorded at seedling, tillering and maturity stages according to the modified Cobb scale (Saari and Prescott, 1975).
Results
Maintenance of annual Aegilops species: Fungicide treated seeds did not show seed borne diseases like smuts and bunts. The seeds of annual grasses were harvested when the seed was of normal size, fully matured with moisture contents of 6±1 percent. The threshed seeds were treated with fungicide Vitavax-200 and sealed in aluminum foil packets and stored at a temperature of -21±1°C. To get better seed germination, the seeds were given cold shock treatment for one week at 4°C. This helped to break the seed dormancy.
Evaluation for leaf rust resistance: The frequency of immune and resistant accessions was fairly high in Ae. geniculata, Ae. variables and Ae. tauschii (Fig. 1) while low in the accessions of Ae. neglecta. Out of 59 accessions of Ae. tauschii, 36 (61%) accessions were immune, 3 (8.3%) accessions resistant, 8 (13.6%) accessions moderately resistant and 12 (20.3 percent) accessions were susceptible (Table 2). Among 33 accessions of Ae. neglecta, 11 (33.3%) accessions were found to be immune, only one (3.1%) accession resistant, 7 (21.2%) accessions moderately resistant and 14 (42.2%) accessions were susceptible (Table 3). From 17 accessions of Ae. geniculata, 13 (76.5%) accessions were immune, 2 (12%) accessions resistant and 2 (12 percent) accessions were found to be moderately resistant (Table 4). No accession of Ae. geniculata were found to be susceptible. All the tested accessions of Ae. variables, Ae. speltoides and Ae. triunciallis were immune to the prevalent races of leaf rust in this region.
Discussion
Maintenance of annual Aegilops species: Wild seeds need to be retained for more than one season or a year for further use, If necessary protection against heat, moisture and pest is not maintained they lose their viability. Moisture content of seed is one of the most important factors affecting seed longevity, the drier the seed the greater is longevity (Van der Maesen, 1984). It is well established fact that the seed with proper moisture germinate easily.
| Table 1: | The genomes and ploidy levels of different goat grass species |
| Table 2: | Different accessions of Aegilops tauschii, with their origin and reaction to leaf rust grown in the field |
| 0-Immune, 1-10 Resistant, 11-25 Moderately Resistant, Above 25- Susceptible Received from: *USDA, ARS, USA. **Plant Breeding Institute, Cambridge, U.K. | |
| Fig. 1: | Leaf rust resistance (%) in Aegilops species |
| Table 3: | Different accessions of Aegilops neglecta and reaction to leaf rust shown in the field |
0-Immune, 1-10 Resistant, 11-25 Moderately Resistant, Above 25- Susceptible Received from: USDA, ARS, USA. Plant Breeding Institute, Cambridge, U.K. | |
| Table 4: | Accessions of Aegilops geniculata, Ae. Speltoides, Ae. triuciallis and Ae. variables, with leaf rust resistance grown in the field |
Our earlier study showed that seeds harvested with proper moisture contents, have good germination (Unpublished data). In contrast to these seeds with more than 12 percent moisture contents may be attacked by fungi or insects (Van der Maesen, 1984), the results of this study are in agreement with the above findings. The results of present study showed that the use of Fungicide Vitavax-200 can be recommended for seed treatment as it did not alter the viability and longevity of the seeds (Anonymous, 1990). This type of Fungicide treatment to seeds is relatively cheap and poses less threat to the environment. In this investigation, dormancy of annual grasses was broken by removing the seed coat over the embryo with a needle and transferred the seeds just above freezing to room temperature daily the same was also recommended by Tosun et al. (1980). For long term storage of wild seeds were stored in air tight containers with 5±1 percent moisture content at -18°C or less as suggested by Roberts (1972). This type of storage provided good results in case of seed viability and longevity. All the above mentioned germplasm has been maintained and stored for further utilization as gene source for the improvement of salt and rust resistance of bread wheat.
Leaf rust Resistance: It was observed that Aegilops species have higher potential for leaf rust resistance (Kerber and Dyck, 1969, 1979). As Ae. tauschii (ancestor of wheat with diploid chromosomes) have also possessed resistance for leaf and stem rust so it can be used in the breeding program to evolve the leaf and stem rust resistant wheat varieties which is necessary for the climatic conditions of Pakistan. Ae. geniculata possesses resistance against powdery mildew (Gill et al., 1989), yellow rust, brown rust. powdery mildew and Karnal bunt (Dhaliwal et al., 1986; Warham et al., 1986). Khem et al. (1987) also confirmed the above findings as they screened 11 different Aegilops species (1763 accessions) against yellow rust, brown rust and Karnal bunt and 407 different accessions were identified to be resistant. It means that the wild relatives of wheat possessed a large gene pool which is useful to create genetic variation necessary for varietal improvement. Screening of the closely related genus i.e. Aegilops speltoides has also identified for having resistance to powdery mildew (Miller and Reader, 1987). In the present study, different accessions of Ae. speltoides, Ae. triunciallis and Ae. variables also showed good resistance for rust and can be used for wheat improvement.
Ae. geniculata (2n=4x=28, UU MoMo) is a species of particular interest for genetic and breeding research as donor of genes for complex disease resistance (Bochev et al., 1982). Farooq et al. (1990a) observed that crossability of Ae. geniculata with Triticum aestivum and T. turgidum is very high compared to that of Ae. tauschii and of Ae. neglecta. Since Ae. geniculata shares genome U with Ae. umbellulata, so it can be anticipated as a donor of agronomically important gene(s) to wheat.
Ae. variables (2n=4x=28, UUSVSV) is also an annual, bushy, tetraploid member of the section Polyeides of the genus Aegilops has resistance against leaf rust which is confirmed by our study. Beside the above resistant genes it also possesses pairing promoters genes which further increase the crossability to tetraploid and hexaploid wheat cultivars (Farooq et al., 1990b).
Specialized cytogenetic techniques are now available to make successful genetic transfer from distantly located genomes and it is anticipated that genes for resistance to leaf rust and salt tolerance could be transferred to wheat from resistant accessions of Ae. geniculata, Ae. tauschii., Ae. neglecta, Ae. triunciallis, Ae. speltoides and Ae. variables.