Department of Plant Pathology, University of the Free State, P.O. Box 339, Bloemfontein, South Africa

Background and objectives
Wild relatives of Triticum aestivum L. provide genetic variation for the improvement of disease resistance in bread wheat. In rust diseases prehaustorial resistance is a defence mechanism assumed to be long lasting due to the absence of compatibility between the pathogen and the host plant [1]. Identification of Triticum species displaying this resistance type, and the exploitation thereof in wheat breeding, would assist in testing the assumption of durability. The objective of this study was to determine microscopically the mechanism of resistance to Puccinia recondita f. sp. tritici in selected Triticum accessions.

Materials and methods
Penetration and establishment of the wheat leaf rust pathogen were studied in flag leaves of Triticum timopheevii, T. turgidum ssp. dicoccum, T. turgidum ssp. durum and T. turgidum ssp. compactum. These accessions were highly resistant to wheat leaf rust in preliminary studies. The T. aestivum wheats Thatcher (Tc) (susceptible control) and TcLrl9 (resistant line) were included in the experiment. For fluorescence microscopy, sections of flag leaves inoculated with pathotype UVPrtl3 of P. recondita f. sp. tritici were sampled 14 days after inoculation, cleared in ethanol:dichloromethane (3:1) and stained with Uvitex 2B. Observations were carried out at 1 OOX and 400X magnification using a Nikon Labophot epifluorescence microscope equipped with UV-1A and B-2A filters. Infection sites were classified as either.prestomatal exclusion, abortive penetration, early abortion or colony formation. Nonpenetrating appressoria and aborted substomatal vesicles were regarded as abortive penetration [2]. To confirm observations, upper and inner surfaces of epidermal tissue of T. timopheevii

  • and T. turgidum ssp. dicoccum were fixed, critical point dried in liquid C02, sputter coated with gold and viewed with a scanning electron microscope.

    Results and conclusions
    Using fluorescence microscopy, 78% of all infection sites observed in T. turgidum ssp. compactum, 74% in T. turgidum ssp. durum, 66% in T. turgidum ssp. dicoccum and 62% in T. timopheevii were classified as aborted penetrants. In T. turgidum ssp. compactum and ssp. dicoccum no differentiation beyond the substomatal vesicles was detected. Expression of resistance in these two accessions contrasted with Thatcher and TcLrl9 in which abortive penetration was lower (10% and 31 %, respectively) and colony formation more frequent (76% and 7%, respectively). Likewise, 12% of infection sites proceeded to colony stage in T. timopheevii. Scanning electron microscopy showed that primary hyphae, not detected with fluorescence microscopy, developed from substomatal vesicles in T. turgidum ssp. dicoccum. No secondary hyphae or haustorium mother cells were noted in T. turgidum ssp. dicoccum. Limited vesicle collapse occurred at the developmental stage sampled. Host cell necrosis was not apparent except in T. timopheevii. Observations with scanning electron microscopy, especially of inner epidermal surfaces, supplemented those obtained by fluorescence microscopy. Attempts should be made to transfer the adult-plant resistance in T. turgidum spp. compactum and spp. dicoccum to bread wheat.

    1. Niks RE, Dekens RG, 1991. Phytopathology 81, 847-851. 2. Parieviiet JE, Kievit C, 1986. Euphytica 35, 953-959.